Learning Python

Learning Python FOURTH EDITION Learning Python Mark Lutz Beijing • Cambridge • Farnham • Köln • Sebastopol • Taipei • Tokyo Learning Python, Fourth Edition by Mark Lutz Copyright © 2009 Mark Lutz. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles (http://my.safaribooksonline.com). For more information, contact our corporate/institutional sales department: (800) 998-9938 or [email protected]. Editor: Julie Steele Production Editor: Sumita Mukherji Copyeditor: Rachel Head Production Services: Newgen North America Indexer: John Bickelhaupt Cover Designer: Karen Montgomery Interior Designer: David Futato Illustrator: Robert Romano Printing History: March 1999: December 2003: October 2007: September 2009: First Edition. Second Edition. Third Edition. Fourth Edition. Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc. Learning Python, the image of a wood rat, and related trade dress are trademarks of O’Reilly Media, Inc. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and O’Reilly Media, Inc., was aware of a trademark claim, the designations have been printed in caps or initial caps. While every precaution has been taken in the preparation of this book, the publisher and author assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. ISBN: 978-0-596-15806-4 [M] 1252944666 To Vera. You are my life. Table of Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxxi Part I. Getting Started 1. A Python Q&A Session . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Why Do People Use Python? Software Quality Developer Productivity Is Python a “Scripting Language”? OK, but What’s the Downside? Who Uses Python Today? What Can I Do with Python? Systems Programming GUIs Internet Scripting Component Integration Database Programming Rapid Prototyping Numeric and Scientific Programming Gaming, Images, Serial Ports, XML, Robots, and More How Is Python Supported? What Are Python’s Technical Strengths? It’s Object-Oriented It’s Free It’s Portable It’s Powerful It’s Mixable It’s Easy to Use It’s Easy to Learn It’s Named After Monty Python How Does Python Stack Up to Language X? 3 4 5 5 7 7 9 9 9 10 10 11 11 11 12 12 13 13 13 14 15 16 16 17 17 17 vii Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 18 19 19 2. How Python Runs Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Introducing the Python Interpreter Program Execution The Programmer’s View Python’s View Execution Model Variations Python Implementation Alternatives Execution Optimization Tools Frozen Binaries Other Execution Options Future Possibilities? Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 23 24 24 26 29 29 30 32 33 33 34 34 34 3. How You Run Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 The Interactive Prompt Running Code Interactively Why the Interactive Prompt? Using the Interactive Prompt System Command Lines and Files A First Script Running Files with Command Lines Using Command Lines and Files Unix Executable Scripts (#!) Clicking File Icons Clicking Icons on Windows The input Trick Other Icon-Click Limitations Module Imports and Reloads The Grander Module Story: Attributes import and reload Usage Notes Using exec to Run Module Files The IDLE User Interface IDLE Basics Using IDLE Advanced IDLE Tools Other IDEs Other Launch Options viii | Table of Contents 35 37 38 39 41 42 43 44 46 47 47 49 50 51 53 56 57 58 58 60 62 63 64 Embedding Calls Frozen Binary Executables Text Editor Launch Options Still Other Launch Options Future Possibilities? Which Option Should I Use? Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers Test Your Knowledge: Part I Exercises 64 65 65 66 66 66 68 68 69 70 Part II. Types and Operations 4. Introducing Python Object Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Why Use Built-in Types? Python’s Core Data Types Numbers Strings Sequence Operations Immutability Type-Specific Methods Getting Help Other Ways to Code Strings Pattern Matching Lists Sequence Operations Type-Specific Operations Bounds Checking Nesting Comprehensions Dictionaries Mapping Operations Nesting Revisited Sorting Keys: for Loops Iteration and Optimization Missing Keys: if Tests Tuples Why Tuples? Files Other File-Like Tools Other Core Types How to Break Your Code’s Flexibility 76 77 78 80 80 82 82 84 85 85 86 86 87 87 88 88 90 90 91 93 94 95 96 97 97 99 99 100 Table of Contents | ix User-Defined Classes And Everything Else Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 101 102 103 103 104 5. Numeric Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Numeric Type Basics Numeric Literals Built-in Numeric Tools Python Expression Operators Numbers in Action Variables and Basic Expressions Numeric Display Formats Comparisons: Normal and Chained Division: Classic, Floor, and True Integer Precision Complex Numbers Hexadecimal, Octal, and Binary Notation Bitwise Operations Other Built-in Numeric Tools Other Numeric Types Decimal Type Fraction Type Sets Booleans Numeric Extensions Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 105 106 108 108 113 113 115 116 117 121 122 122 124 125 127 127 129 133 139 140 141 141 141 6. The Dynamic Typing Interlude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 The Case of the Missing Declaration Statements Variables, Objects, and References Types Live with Objects, Not Variables Objects Are Garbage-Collected Shared References Shared References and In-Place Changes Shared References and Equality Dynamic Typing Is Everywhere Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers x | Table of Contents 143 144 145 146 148 149 151 152 153 153 154 7. Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 String Literals Single- and Double-Quoted Strings Are the Same Escape Sequences Represent Special Bytes Raw Strings Suppress Escapes Triple Quotes Code Multiline Block Strings Strings in Action Basic Operations Indexing and Slicing String Conversion Tools Changing Strings String Methods String Method Examples: Changing Strings String Method Examples: Parsing Text Other Common String Methods in Action The Original string Module (Gone in 3.0) String Formatting Expressions Advanced String Formatting Expressions Dictionary-Based String Formatting Expressions String Formatting Method Calls The Basics Adding Keys, Attributes, and Offsets Adding Specific Formatting Comparison to the % Formatting Expression Why the New Format Method? General Type Categories Types Share Operation Sets by Categories Mutable Types Can Be Changed In-Place Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 157 158 158 161 162 163 164 165 169 171 172 174 176 177 178 179 181 182 183 184 184 185 187 190 193 194 194 195 195 196 8. Lists and Dictionaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 Lists Lists in Action Basic List Operations List Iteration and Comprehensions Indexing, Slicing, and Matrixes Changing Lists In-Place Dictionaries Dictionaries in Action Basic Dictionary Operations Changing Dictionaries In-Place 197 200 200 200 201 202 207 209 209 210 Table of Contents | xi More Dictionary Methods A Languages Table Dictionary Usage Notes Other Ways to Make Dictionaries Dictionary Changes in Python 3.0 Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 211 212 213 216 217 223 224 224 9. Tuples, Files, and Everything Else . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 Tuples Tuples in Action Why Lists and Tuples? Files Opening Files Using Files Files in Action Other File Tools Type Categories Revisited Object Flexibility References Versus Copies Comparisons, Equality, and Truth Python 3.0 Dictionary Comparisons The Meaning of True and False in Python Python’s Type Hierarchies Type Objects Other Types in Python Built-in Type Gotchas Assignment Creates References, Not Copies Repetition Adds One Level Deep Beware of Cyclic Data Structures Immutable Types Can’t Be Changed In-Place Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers Test Your Knowledge: Part II Exercises 225 227 229 229 230 231 232 238 239 241 241 244 246 246 248 249 250 251 251 252 252 253 253 254 254 255 Part III. Statements and Syntax 10. Introducing Python Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 Python Program Structure Revisited Python’s Statements xii | Table of Contents 261 262 A Tale of Two ifs What Python Adds What Python Removes Why Indentation Syntax? A Few Special Cases A Quick Example: Interactive Loops A Simple Interactive Loop Doing Math on User Inputs Handling Errors by Testing Inputs Handling Errors with try Statements Nesting Code Three Levels Deep Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 264 264 265 266 269 271 271 272 273 274 275 276 276 277 11. Assignments, Expressions, and Prints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Assignment Statements Assignment Statement Forms Sequence Assignments Extended Sequence Unpacking in Python 3.0 Multiple-Target Assignments Augmented Assignments Variable Name Rules Expression Statements Expression Statements and In-Place Changes Print Operations The Python 3.0 print Function The Python 2.6 print Statement Print Stream Redirection Version-Neutral Printing Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 279 280 281 284 288 289 292 295 296 297 298 300 302 306 308 308 308 12. if Tests and Syntax Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 if Statements General Format Basic Examples Multiway Branching Python Syntax Rules Block Delimiters: Indentation Rules Statement Delimiters: Lines and Continuations A Few Special Cases 311 311 312 312 314 315 317 318 Table of Contents | xiii Truth Tests The if/else Ternary Expression Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 320 321 324 324 324 13. while and for Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 while Loops General Format Examples break, continue, pass, and the Loop else General Loop Format pass continue break Loop else for Loops General Format Examples Loop Coding Techniques Counter Loops: while and range Nonexhaustive Traversals: range and Slices Changing Lists: range Parallel Traversals: zip and map Generating Both Offsets and Items: enumerate Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 327 328 328 329 329 330 331 331 332 334 334 335 341 342 343 344 345 348 349 349 350 14. Iterations and Comprehensions, Part 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 Iterators: A First Look The Iteration Protocol: File Iterators Manual Iteration: iter and next Other Built-in Type Iterators List Comprehensions: A First Look List Comprehension Basics Using List Comprehensions on Files Extended List Comprehension Syntax Other Iteration Contexts New Iterables in Python 3.0 The range Iterator The map, zip, and filter Iterators Multiple Versus Single Iterators xiv | Table of Contents 351 352 354 356 358 359 359 361 362 366 367 368 369 Dictionary View Iterators Other Iterator Topics Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 370 372 372 372 373 15. The Documentation Interlude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Python Documentation Sources # Comments The dir Function Docstrings: __doc__ PyDoc: The help Function PyDoc: HTML Reports The Standard Manual Set Web Resources Published Books Common Coding Gotchas Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers Test Your Knowledge: Part III Exercises 375 376 376 377 380 383 386 387 387 387 389 389 390 390 Part IV. Functions 16. Function Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 Why Use Functions? Coding Functions def Statements def Executes at Runtime A First Example: Definitions and Calls Definition Calls Polymorphism in Python A Second Example: Intersecting Sequences Definition Calls Polymorphism Revisited Local Variables Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 396 396 398 399 400 400 400 401 402 402 403 403 404 404 405 405 Table of Contents | xv 17. Scopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 Python Scope Basics Scope Rules Name Resolution: The LEGB Rule Scope Example The Built-in Scope The global Statement Minimize Global Variables Minimize Cross-File Changes Other Ways to Access Globals Scopes and Nested Functions Nested Scope Details Nested Scope Examples The nonlocal Statement nonlocal Basics nonlocal in Action Why nonlocal? Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 407 408 410 411 412 414 415 416 418 419 419 419 425 425 426 429 432 433 434 18. Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 Argument-Passing Basics Arguments and Shared References Avoiding Mutable Argument Changes Simulating Output Parameters Special Argument-Matching Modes The Basics Matching Syntax The Gritty Details Keyword and Default Examples Arbitrary Arguments Examples Python 3.0 Keyword-Only Arguments The min Wakeup Call! Full Credit Bonus Points The Punch Line... Generalized Set Functions Emulating the Python 3.0 print Function Using Keyword-Only Arguments Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers xvi | Table of Contents 435 436 438 439 440 441 442 443 444 446 450 453 454 455 456 456 457 459 460 461 462 19. Advanced Function Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463 Function Design Concepts Recursive Functions Summation with Recursion Coding Alternatives Loop Statements Versus Recursion Handling Arbitrary Structures Function Objects: Attributes and Annotations Indirect Function Calls Function Introspection Function Attributes Function Annotations in 3.0 Anonymous Functions: lambda lambda Basics Why Use lambda? How (Not) to Obfuscate Your Python Code Nested lambdas and Scopes Mapping Functions over Sequences: map Functional Programming Tools: filter and reduce Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 463 465 465 466 467 468 469 469 470 471 472 474 474 475 477 478 479 481 483 483 483 20. Iterations and Comprehensions, Part 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 List Comprehensions Revisited: Functional Tools List Comprehensions Versus map Adding Tests and Nested Loops: filter List Comprehensions and Matrixes Comprehending List Comprehensions Iterators Revisited: Generators Generator Functions: yield Versus return Generator Expressions: Iterators Meet Comprehensions Generator Functions Versus Generator Expressions Generators Are Single-Iterator Objects Emulating zip and map with Iteration Tools Value Generation in Built-in Types and Classes 3.0 Comprehension Syntax Summary Comprehending Set and Dictionary Comprehensions Extended Comprehension Syntax for Sets and Dictionaries Timing Iteration Alternatives Timing Module Timing Script Timing Results 485 486 487 489 490 492 492 497 498 499 500 506 507 507 508 509 509 510 511 Table of Contents | xvii Timing Module Alternatives Other Suggestions Function Gotchas Local Names Are Detected Statically Defaults and Mutable Objects Functions Without returns Enclosing Scope Loop Variables Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers Test Your Knowledge: Part IV Exercises 513 517 518 518 520 522 522 522 523 523 524 Part V. Modules 21. Modules: The Big Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529 Why Use Modules? Python Program Architecture How to Structure a Program Imports and Attributes Standard Library Modules How Imports Work 1. Find It 2. Compile It (Maybe) 3. Run It The Module Search Path Configuring the Search Path Search Path Variations The sys.path List Module File Selection Advanced Module Selection Concepts Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 529 530 531 531 533 533 534 534 535 535 537 538 538 539 540 541 541 542 22. Module Coding Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543 Module Creation Module Usage The import Statement The from Statement The from * Statement Imports Happen Only Once import and from Are Assignments xviii | Table of Contents 543 544 544 545 545 546 546 Cross-File Name Changes import and from Equivalence Potential Pitfalls of the from Statement Module Namespaces Files Generate Namespaces Attribute Name Qualification Imports Versus Scopes Namespace Nesting Reloading Modules reload Basics reload Example Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 547 548 548 550 550 552 552 553 554 555 556 558 558 558 23. Module Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561 Package Import Basics Packages and Search Path Settings Package __init__.py Files Package Import Example from Versus import with Packages Why Use Package Imports? A Tale of Three Systems Package Relative Imports Changes in Python 3.0 Relative Import Basics Why Relative Imports? The Scope of Relative Imports Module Lookup Rules Summary Relative Imports in Action Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 561 562 563 564 566 566 567 569 570 570 572 574 575 575 581 582 582 24. Advanced Module Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583 Data Hiding in Modules Minimizing from * Damage: _X and __all__ Enabling Future Language Features Mixed Usage Modes: __name__ and __main__ Unit Tests with __name__ Using Command-Line Arguments with __name__ Changing the Module Search Path The as Extension for import and from 583 584 584 585 586 587 590 591 Table of Contents | xix Modules Are Objects: Metaprograms Importing Modules by Name String Transitive Module Reloads Module Design Concepts Module Gotchas Statement Order Matters in Top-Level Code from Copies Names but Doesn’t Link from * Can Obscure the Meaning of Variables reload May Not Impact from Imports reload, from, and Interactive Testing Recursive from Imports May Not Work Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers Test Your Knowledge: Part V Exercises 591 594 595 598 599 599 600 601 601 602 603 604 604 605 605 Part VI. Classes and OOP 25. OOP: The Big Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 611 Why Use Classes? OOP from 30,000 Feet Attribute Inheritance Search Classes and Instances Class Method Calls Coding Class Trees OOP Is About Code Reuse Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 612 613 613 615 616 616 619 622 622 622 26. Class Coding Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 625 Classes Generate Multiple Instance Objects Class Objects Provide Default Behavior Instance Objects Are Concrete Items A First Example Classes Are Customized by Inheritance A Second Example Classes Are Attributes in Modules Classes Can Intercept Python Operators A Third Example Why Use Operator Overloading? The World’s Simplest Python Class xx | Table of Contents 625 626 626 627 629 630 631 633 634 636 636 Classes Versus Dictionaries Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 639 641 641 641 27. A More Realistic Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643 Step 1: Making Instances Coding Constructors Testing As You Go Using Code Two Ways Step 2: Adding Behavior Methods Coding Methods Step 3: Operator Overloading Providing Print Displays Step 4: Customizing Behavior by Subclassing Coding Subclasses Augmenting Methods: The Bad Way Augmenting Methods: The Good Way Polymorphism in Action Inherit, Customize, and Extend OOP: The Big Idea Step 5: Customizing Constructors, Too OOP Is Simpler Than You May Think Other Ways to Combine Classes Step 6: Using Introspection Tools Special Class Attributes A Generic Display Tool Instance Versus Class Attributes Name Considerations in Tool Classes Our Classes’ Final Form Step 7 (Final): Storing Objects in a Database Pickles and Shelves Storing Objects on a Shelve Database Exploring Shelves Interactively Updating Objects on a Shelve Future Directions Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 644 644 645 646 648 649 651 652 653 653 654 654 656 657 658 658 660 660 663 664 665 666 667 668 669 670 671 672 674 675 677 677 678 28. Class Coding Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681 The class Statement General Form 681 681 Table of Contents | xxi Example Methods Method Example Calling Superclass Constructors Other Method Call Possibilities Inheritance Attribute Tree Construction Specializing Inherited Methods Class Interface Techniques Abstract Superclasses Python 2.6 and 3.0 Abstract Superclasses Namespaces: The Whole Story Simple Names: Global Unless Assigned Attribute Names: Object Namespaces The “Zen” of Python Namespaces: Assignments Classify Names Namespace Dictionaries Namespace Links Documentation Strings Revisited Classes Versus Modules Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 682 684 685 686 686 687 687 687 689 690 692 693 693 693 694 696 699 701 703 703 703 704 29. Operator Overloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705 The Basics Constructors and Expressions: __init__ and __sub__ Common Operator Overloading Methods Indexing and Slicing: __getitem__ and __setitem__ Intercepting Slices Index Iteration: __getitem__ Iterator Objects: __iter__ and __next__ User-Defined Iterators Multiple Iterators on One Object Membership: __contains__, __iter__, and __getitem__ Attribute Reference: __getattr__ and __setattr__ Other Attribute Management Tools Emulating Privacy for Instance Attributes: Part 1 String Representation: __repr__ and __str__ Right-Side and In-Place Addition: __radd__ and __iadd__ In-Place Addition Call Expressions: __call__ Function Interfaces and Callback-Based Code Comparisons: __lt__, __gt__, and Others xxii | Table of Contents 705 706 706 708 708 710 711 712 714 716 718 719 720 721 723 725 725 727 728 The 2.6 __cmp__ Method (Removed in 3.0) Boolean Tests: __bool__ and __len__ Object Destruction: __del__ Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 729 730 732 733 734 734 30. Designing with Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 737 Python and OOP Overloading by Call Signatures (or Not) OOP and Inheritance: “Is-a” Relationships OOP and Composition: “Has-a” Relationships Stream Processors Revisited OOP and Delegation: “Wrapper” Objects Pseudoprivate Class Attributes Name Mangling Overview Why Use Pseudoprivate Attributes? Methods Are Objects: Bound or Unbound Unbound Methods are Functions in 3.0 Bound Methods and Other Callable Objects Multiple Inheritance: “Mix-in” Classes Coding Mix-in Display Classes Classes Are Objects: Generic Object Factories Why Factories? Other Design-Related Topics Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 737 738 739 740 742 745 747 748 748 750 752 754 756 757 768 769 770 770 770 771 31. Advanced Class Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773 Extending Built-in Types Extending Types by Embedding Extending Types by Subclassing The “New-Style” Class Model New-Style Class Changes Type Model Changes Diamond Inheritance Change New-Style Class Extensions Instance Slots Class Properties __getattribute__ and Descriptors Metaclasses Static and Class Methods 773 774 775 777 778 779 783 788 788 792 794 794 795 Table of Contents | xxiii Why the Special Methods? Static Methods in 2.6 and 3.0 Static Method Alternatives Using Static and Class Methods Counting Instances with Static Methods Counting Instances with Class Methods Decorators and Metaclasses: Part 1 Function Decorator Basics A First Function Decorator Example Class Decorators and Metaclasses For More Details Class Gotchas Changing Class Attributes Can Have Side Effects Changing Mutable Class Attributes Can Have Side Effects, Too Multiple Inheritance: Order Matters Methods, Classes, and Nested Scopes Delegation-Based Classes in 3.0: __getattr__ and built-ins “Overwrapping-itis” Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers Test Your Knowledge: Part VI Exercises 795 796 798 799 800 802 804 804 805 807 808 808 808 810 811 812 814 814 815 815 815 816 Part VII. Exceptions and Tools 32. Exception Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825 Why Use Exceptions? Exception Roles Exceptions: The Short Story Default Exception Handler Catching Exceptions Raising Exceptions User-Defined Exceptions Termination Actions Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 825 826 827 827 828 829 830 830 833 833 833 33. Exception Coding Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835 The try/except/else Statement try Statement Clauses The try else Clause xxiv | Table of Contents 835 837 839 Example: Default Behavior Example: Catching Built-in Exceptions The try/finally Statement Example: Coding Termination Actions with try/finally Unified try/except/finally Unified try Statement Syntax Combining finally and except by Nesting Unified try Example The raise Statement Propagating Exceptions with raise Python 3.0 Exception Chaining: raise from The assert Statement Example: Trapping Constraints (but Not Errors!) with/as Context Managers Basic Usage The Context Management Protocol Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 840 841 842 843 844 845 845 846 848 849 849 850 851 851 852 853 855 856 856 34. Exception Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 857 Exceptions: Back to the Future String Exceptions Are Right Out! Class-Based Exceptions Coding Exceptions Classes Why Exception Hierarchies? Built-in Exception Classes Built-in Exception Categories Default Printing and State Custom Print Displays Custom Data and Behavior Providing Exception Details Providing Exception Methods Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 858 858 859 859 861 864 865 866 867 868 868 869 870 871 871 35. Designing with Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873 Nesting Exception Handlers Example: Control-Flow Nesting Example: Syntactic Nesting Exception Idioms Exceptions Aren’t Always Errors 873 875 875 877 877 Table of Contents | xxv Functions Can Signal Conditions with raise Closing Files and Server Connections Debugging with Outer try Statements Running In-Process Tests More on sys.exc_info Exception Design Tips and Gotchas What Should Be Wrapped Catching Too Much: Avoid Empty except and Exception Catching Too Little: Use Class-Based Categories Core Language Summary The Python Toolset Development Tools for Larger Projects Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers Test Your Knowledge: Part VII Exercises 878 878 879 880 881 882 882 883 885 885 886 887 890 891 891 891 Part VIII. Advanced Topics 36. Unicode and Byte Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895 String Changes in 3.0 String Basics Character Encoding Schemes Python’s String Types Text and Binary Files Python 3.0 Strings in Action Literals and Basic Properties Conversions Coding Unicode Strings Coding ASCII Text Coding Non-ASCII Text Encoding and Decoding Non-ASCII text Other Unicode Coding Techniques Converting Encodings Coding Unicode Strings in Python 2.6 Source File Character Set Encoding Declarations Using 3.0 Bytes Objects Method Calls Sequence Operations Other Ways to Make bytes Objects Mixing String Types Using 3.0 (and 2.6) bytearray Objects xxvi | Table of Contents 896 897 897 899 900 902 902 903 904 905 905 906 907 909 910 912 913 913 914 915 916 917 Using Text and Binary Files Text File Basics Text and Binary Modes in 3.0 Type and Content Mismatches Using Unicode Files Reading and Writing Unicode in 3.0 Handling the BOM in 3.0 Unicode Files in 2.6 Other String Tool Changes in 3.0 The re Pattern Matching Module The struct Binary Data Module The pickle Object Serialization Module XML Parsing Tools Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 920 920 921 923 924 924 926 928 929 929 930 932 934 937 937 937 37. Managed Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 941 Why Manage Attributes? Inserting Code to Run on Attribute Access Properties The Basics A First Example Computed Attributes Coding Properties with Decorators Descriptors The Basics A First Example Computed Attributes Using State Information in Descriptors How Properties and Descriptors Relate __getattr__ and __getattribute__ The Basics A First Example Computed Attributes __getattr__ and __getattribute__ Compared Management Techniques Compared Intercepting Built-in Operation Attributes Delegation-Based Managers Revisited Example: Attribute Validations Using Properties to Validate Using Descriptors to Validate Using __getattr__ to Validate 941 942 943 943 944 945 946 947 948 950 952 953 955 956 957 959 961 962 963 966 970 973 973 975 977 Table of Contents | xxvii Using __getattribute__ to Validate Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 978 979 980 980 38. Decorators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 983 What’s a Decorator? Managing Calls and Instances Managing Functions and Classes Using and Defining Decorators Why Decorators? The Basics Function Decorators Class Decorators Decorator Nesting Decorator Arguments Decorators Manage Functions and Classes, Too Coding Function Decorators Tracing Calls State Information Retention Options Class Blunders I: Decorating Class Methods Timing Calls Adding Decorator Arguments Coding Class Decorators Singleton Classes Tracing Object Interfaces Class Blunders II: Retaining Multiple Instances Decorators Versus Manager Functions Why Decorators? (Revisited) Managing Functions and Classes Directly Example: “Private” and “Public” Attributes Implementing Private Attributes Implementation Details I Generalizing for Public Declarations, Too Implementation Details II Open Issues Python Isn’t About Control Example: Validating Function Arguments The Goal A Basic Range-Testing Decorator for Positional Arguments Generalizing for Keywords and Defaults, Too Implementation Details Open Issues xxviii | Table of Contents 983 984 984 984 985 986 986 990 993 994 995 996 996 997 1001 1006 1008 1011 1011 1013 1016 1018 1019 1021 1023 1023 1025 1026 1029 1030 1034 1034 1034 1035 1037 1040 1042 Decorator Arguments Versus Function Annotations Other Applications: Type Testing (If You Insist!) Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 1043 1045 1046 1047 1047 39. Metaclasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051 To Metaclass or Not to Metaclass Increasing Levels of Magic The Downside of “Helper” Functions Metaclasses Versus Class Decorators: Round 1 The Metaclass Model Classes Are Instances of type Metaclasses Are Subclasses of Type Class Statement Protocol Declaring Metaclasses Coding Metaclasses A Basic Metaclass Customizing Construction and Initialization Other Metaclass Coding Techniques Instances Versus Inheritance Example: Adding Methods to Classes Manual Augmentation Metaclass-Based Augmentation Metaclasses Versus Class Decorators: Round 2 Example: Applying Decorators to Methods Tracing with Decoration Manually Tracing with Metaclasses and Decorators Applying Any Decorator to Methods Metaclasses Versus Class Decorators: Round 3 Chapter Summary Test Your Knowledge: Quiz Test Your Knowledge: Answers 1052 1052 1054 1056 1058 1058 1061 1061 1062 1063 1064 1065 1065 1068 1070 1070 1071 1073 1076 1076 1077 1079 1080 1084 1084 1085 Part IX. Appendixes A. Installation and Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1089 B. Solutions to End-of-Part Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1101 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1139 Table of Contents | xxix Preface This book provides an introduction to the Python programming language. Python is a popular open source programming language used for both standalone programs and scripting applications in a wide variety of domains. It is free, portable, powerful, and remarkably easy and fun to use. Programmers from every corner of the software industry have found Python’s focus on developer productivity and software quality to be a strategic advantage in projects both large and small. Whether you are new to programming or are a professional developer, this book’s goal is to bring you quickly up to speed on the fundamentals of the core Python language. After reading this book, you will know enough about Python to apply it in whatever application domains you choose to explore. By design, this book is a tutorial that focuses on the core Python language itself, rather than specific applications of it. As such, it’s intended to serve as the first in a two-volume set: • Learning Python, this book, teaches Python itself. • Programming Python, among others, shows what you can do with Python after you’ve learned it. That is, applications-focused books such as Programming Python pick up where this book leaves off, exploring Python’s role in common domains such as the Web, graphical user interfaces (GUIs), and databases. In addition, the book Python Pocket Reference provides additional reference materials not included here, and it is designed to supplement this book. Because of this book’s foundations focus, though, it is able to present Python fundamentals with more depth than many programmers see when first learning the language. And because it’s based upon a three-day Python training class with quizzes and exercises throughout, this book serves as a self-paced introduction to the language. xxxi About This Fourth Edition This fourth edition of this book has changed in three ways. This edition: • Covers both Python 3.0 and Python 2.6—it emphasizes 3.0, but notes differences in 2.6 • Includes a set of new chapters mainly targeted at advanced core-language topics • Reorganizes some existing material and expands it with new examples for clarity As I write this edition in 2009, Python comes in two flavors—version 3.0 is an emerging and incompatible mutation of the language, and 2.6 retains backward compatibility with the vast body of existing Python code. Although Python 3 is viewed as the future of Python, Python 2 is still widely used and will be supported in parallel with Python 3 for years to come. While 3.0 is largely the same language, it runs almost no code written for prior releases (the mutation of print from statement to function alone, aesthetically sound as it may be, breaks nearly every Python program ever written). This split presents a bit of a dilemma for both programmers and book authors. While it would be easier for a book to pretend that Python 2 never existed and cover 3 only, this would not address the needs of the large Python user base that exists today. A vast amount of existing code was written for Python 2, and it won’t be going away any time soon. And while newcomers to the language can focus on Python 3, anyone who must use code written in the past needs to keep one foot in the Python 2 world today. Since it may be years before all third-party libraries and extensions are ported to Python 3, this fork might not be entirely temporary. Coverage for Both 3.0 and 2.6 To address this dichotomy and to meet the needs of all potential readers, this edition of this book has been updated to cover both Python 3.0 and Python 2.6 (and later releases in the 3.X and 2.X lines). It’s intended for programmers using Python 2, programmers using Python 3, and programmers stuck somewhere between the two. That is, you can use this book to learn either Python line. Although the focus here is on 3.0 primarily, 2.6 differences and tools are also noted along the way for programmers using older code. While the two versions are largely the same, they diverge in some important ways, and I’ll point these out along the way. For instance, I’ll use 3.0 print calls in most examples, but will describe the 2.6 print statement, too, so you can make sense of earlier code. I’ll also freely introduce new features, such as the nonlocal statement in 3.0 and the string format method in 2.6 and 3.0, and will point out when such extensions are not present in older Pythons. If you are learning Python for the first time and don’t need to use any legacy code, I encourage you to begin with Python 3.0; it cleans up some longstanding warts in the language, while retaining all the original core ideas and adding some nice new tools. xxxii | Preface Many popular Python libraries and tools will likely be available for Python 3.0 by the time you read these words, especially given the file I/O performance improvements expected in the upcoming 3.1 release. If you are using a system based on Python 2.X, however, you’ll find that this book addresses your concerns, too, and will help you migrate to 3.0 in the future. By proxy, this edition addresses other Python version 2 and 3 releases as well, though some older version 2.X code may not be able to run all the examples here. Although class decorators are available in both Python 2.6 and 3.0, for example, you cannot use them in an older Python 2.X that did not yet have this feature. See Tables P-1 and P-2 later in this Preface for summaries of 2.6 and 3.0 changes. Shortly before going to press, this book was also augmented with notes about prominent extensions in the upcoming Python 3.1 release— comma separators and automatic field numbering in string format method calls, multiple context manager syntax in with statements, new methods for numbers, and so on. Because Python 3.1 was targeted primarily at optimization, this book applies directly to this new release as well. In fact, because Python 3.1 supersedes 3.0, and because the latest Python is usually the best Python to fetch and use anyhow, in this book the term “Python 3.0” generally refers to the language variations introduced by Python 3.0 but that are present in the entire 3.X line. New Chapters Although the main purpose of this edition is to update the examples and material from the preceding edition for 3.0 and 2.6, I’ve also added five new chapters to address new topics and add context: • Chapter 27 is a new class tutorial, using a more realistic example to explore the basics of Python object-oriented programming (OOP). • Chapter 36 provides details on Unicode and byte strings and outlines string and file differences between 3.0 and 2.6. • Chapter 37 collects managed attribute tools such as properties and provides new coverage of descriptors. • Chapter 38 presents function and class decorators and works through comprehensive examples. • Chapter 39 covers metaclasses and compares and contrasts them with decorators. The first of these chapters provides a gradual, step-by-step tutorial for using classes and OOP in Python. It’s based upon a live demonstration I have been using in recent years in the training classes I teach, but has been honed here for use in a book. The chapter is designed to show OOP in a more realistic context than earlier examples and to Preface | xxxiii illustrate how class concepts come together into larger, working programs. I hope it works as well here as it has in live classes. The last four of these new chapters are collected in a new final part of the book, “Advanced Topics.” Although these are technically core language topics, not every Python programmer needs to delve into the details of Unicode text or metaclasses. Because of this, these four chapters have been separated out into this new part, and are officially optional reading. The details of Unicode and binary data strings, for example, have been moved to this final part because most programmers use simple ASCII strings and don’t need to know about these topics. Similarly, decorators and metaclasses are specialist topics that are usually of more interest to API builders than application programmers. If you do use such tools, though, or use code that does, these new advanced topic chapters should help you master the basics. In addition, these chapters’ examples include case studies that tie core language concepts together, and they are more substantial than those in most of the rest of the book. Because this new part is optional reading, it has end-of-chapter quizzes but no end-of-part exercises. Changes to Existing Material In addition, some material from the prior edition has been reorganized, or supplemented with new examples. Multiple inheritance, for instance, gets a new case study example that lists class trees in Chapter 30; new examples for generators that manually implement map and zip are provided in Chapter 20; static and class methods are illustrated by new code in Chapter 31; package relative imports are captured in action in Chapter 23; and the __contains__, __bool__, and __index__ operator overloading methods are illustrated by example now as well in Chapter 29, along with the new overloading protocols for slicing and comparison. This edition also incorporates some reorganization for clarity. For instance, to accommodate new material and topics, and to avoid chapter topic overload, five prior chapters have been split into two each here. The result is new standalone chapters on operator overloading, scopes and arguments, exception statement details, and comprehension and iteration topics. Some reordering has been done within the existing chapters as well, to improve topic flow. This edition also tries to minimize forward references with some reordering, though Python 3.0’s changes make this impossible in some cases: to understand printing and the string format method, you now must know keyword arguments for functions; to understand dictionary key lists and key tests, you must now know iteration; to use exec to run code, you need to be able to use file objects; and so on. A linear reading still probably makes the most sense, but some topics may require nonlinear jumps and random lookups. All told, there have been hundreds of changes in this edition. The next section’s tables alone document 27 additions and 57 changes in Python. In fact, it’s fair to say that this xxxiv | Preface edition is somewhat more advanced, because Python is somewhat more advanced. As for Python 3.0 itself, though, you’re probably better off discovering most of this book’s changes for yourself, rather than reading about them further in this Preface. Specific Language Extensions in 2.6 and 3.0 In general, Python 3.0 is a cleaner language, but it is also in some ways a more sophisticated language. In fact, some of its changes seem to assume you must already know Python in order to learn Python! The prior section outlined some of the more prominent circular knowledge dependencies in 3.0; as a random example, the rationale for wrapping dictionary views in a list call is incredibly subtle and requires substantial foreknowledge. Besides teaching Python fundamentals, this book serves to help bridge this knowledge gap. Table P-1 lists the most prominent new language features covered in this edition, along with the primary chapters in which they appear. Table P-1. Extensions in Python 2.6 and 3.0 Extension Covered in chapter(s) The print function in 3.0 11 The nonlocal x,y statement in 3.0 17 The str.format method in 2.6 and 3.0 7 String types in 3.0: str for Unicode text, bytes for binary data 7, 36 Text and binary file distinctions in 3.0 9, 36 Class decorators in 2.6 and 3.0: @private('age') 31, 38 New iterators in 3.0: range, map, zip 14, 20 Dictionary views in 3.0: D.keys, D.values, D.items 8, 14 Division operators in 3.0: remainders, / and // 5 Set literals in 3.0: {a, b, c} 5 Set comprehensions in 3.0: {x**2 for x in seq} 4, 5, 14, 20 Dictionary comprehensions in 3.0: {x: x**2 for x in seq} 4, 8, 14, 20 Binary digit-string support in 2.6 and 3.0: 0b0101, bin(I) 5 The fraction number type in 2.6 and 3.0: Fraction(1, 3) 5 Function annotations in 3.0: def f(a:99, b:str)->int 19 Keyword-only arguments in 3.0: def f(a, *b, c, **d) 18, 20 Extended sequence unpacking in 3.0: a, *b = seq 11, 13 Relative import syntax for packages enabled in 3.0: from . 23 Context managers enabled in 2.6 and 3.0: with/as 33, 35 Exception syntax changes in 3.0: raise, except/as, superclass 33, 34 Preface | xxxv Extension Covered in chapter(s) Exception chaining in 3.0: raise e2 from e1 33 Reserved word changes in 2.6 and 3.0 11 New-style class cutover in 3.0 31 Property decorators in 2.6 and 3.0: @property 37 Descriptor use in 2.6 and 3.0 31, 38 Metaclass use in 2.6 and 3.0 31, 39 Abstract base classes support in 2.6 and 3.0 28 Specific Language Removals in 3.0 In addition to extensions, a number of language tools have been removed in 3.0 in an effort to clean up its design. Table P-2 summarizes the changes that impact this book, covered in various chapters of this edition. Many of the removals listed in Table P-2 have direct replacements, some of which are also available in 2.6 to support future migration to 3.0. Table P-2. Removals in Python 3.0 that impact this book Removed Replacement Covered in chapter(s) reload(M) imp.reload(M) (or exec) 3, 22 apply(f, ps, ks) f(*ps, **ks) 18 `X` repr(X) 5 X <> Y X != Y 5 long int 5 9999L 9999 5 D.has_key(K) K in D (or D.get(key) != None) 8 raw_input input 3, 10 old input eval(input()) 3 xrange range 14 file open (and io module classes) 9 X.next X.__next__, called by next(X) 14, 20, 29 X.__getslice__ X.__getitem__ passed a slice object 7, 29 X.__setslice__ X.__setitem__ passed a slice object 7, 29 reduce functools.reduce (or loop code) 14, 19 execfile(filename) exec(open(filename).read()) 3 exec open(filename) exec(open(filename).read()) 3 0777 0o777 5 print x, y print(x, y) 11 xxxvi | Preface Removed Replacement Covered in chapter(s) print >> F, x, y print(x, y, file=F) 11 print x, y, print(x, y, end=' ') 11 u'ccc' 'ccc' 7, 36 'bbb' for byte strings b'bbb' 7, 9, 36 raise E, V raise E(V) 32, 33, 34 except E, X: except E as X: 32, 33, 34 def f((a, b)): def f(x): (a, b) = x 11, 18, 20 file.xreadlines for line in file: (or X=iter(file)) 13, 14 D.keys(), etc. as lists list(D.keys()) (dictionary views) 8, 14 map(), range(), etc. as lists list(map()), list(range()) (built-ins) 14 map(None, ...) zip (or manual code to pad results) 13, 20 X=D.keys(); X.sort() sorted(D) (or list(D.keys())) 4, 8, 14 cmp(x, y) (x > y) - (x < y) 29 X.__cmp__(y) __lt__, __gt__, __eq__, etc. 29 X.__nonzero__ X.__bool__ 29 X.__hex__, X.__oct__ X._index__ 29 Sort comparison functions Use key=transform or reverse=True 8 Dictionary <, >, <=, >= Compare sorted(D.items()) (or loop code) 8, 9 types.ListType list (types is for nonbuilt-in names only) 9 __metaclass__ = M class C(metaclass=M): 28, 31, 39 __builtin__ builtins (renamed) 17 Tkinter tkinter (renamed) 18, 19, 24, 29, 30 sys.exc_type, exc_value sys.exc_info()[0], [1] 34, 35 function.func_code function.__code__ 19, 38 __getattr__ run by built-ins Redefine __X__ methods in wrapper classes 30, 37, 38 -t, –tt command-line switches Inconsistent tabs/spaces use is always an error 10, 12 from ... *, within a function May only appear at the top level of a file 22 import mod, in same package from . import mod, package-relative form 23 class MyException: class MyException(Exception): 34 exceptions module Built-in scope, library manual 34 thread, Queue modules _thread, queue (both renamed) 17 anydbm module dbm (renamed) 27 cPickle module _pickle (renamed, used automatically) 9 os.popen2/3/4 subprocess.Popen (os.popen retained) 14 String-based exceptions Class-based exceptions (also required in 2.6) 32, 33, 34 Preface | xxxvii Removed Replacement Covered in chapter(s) String module functions String object methods 7 Unbound methods Functions (staticmethod to call via instance) 30, 31 Mixed type comparisons, sorts Nonnumeric mixed type comparisons are errors 5, 9 There are additional changes in Python 3.0 that are not listed in this table, simply because they don’t affect this book. Changes in the standard library, for instance, might have a larger impact on applications-focused books like Programming Python than they do here; although most standard library functionality is still present, Python 3.0 takes further liberties with renaming modules, grouping them into packages, and so on. For a more comprehensive list of changes in 3.0, see the “What’s New in Python 3.0” document in Python’s standard manual set. If you are migrating from Python 2.X to Python 3.X, be sure to also see the 2to3 automatic code conversion script that is available with Python 3.0. It can’t translate everything, but it does a reasonable job of converting the majority of 2.X code to run under 3.X. As I write this, a new 3to2 back-conversion project is also underway to translate Python 3.X code to run in 2.X environments. Either tool may prove useful if you must maintain code for both Python lines; see the Web for details. Because this fourth edition is mostly a fairly straightforward update for 3.0 with a handful of new chapters, and because it’s only been two years since the prior edition was published, the rest of this Preface is taken from the prior edition with only minor updating. About The Third Edition In the four years between the publication of the second and third editions of this book there were substantial changes in Python itself, and in the topics I presented in Python training sessions. The third edition reflected these changes, and also incorporated a handful of structural changes. The Third Edition’s Python Language Changes On the language front, the third edition was thoroughly updated to reflect Python 2.5 and all changes to the language since the publication of the second edition in late 2003. (The second edition was based largely on Python 2.2, with some 2.3 features grafted on at the end of the project.) In addition, discussions of anticipated changes in the upcoming Python 3.0 release were incorporated where appropriate. Here are some of the major language topics for which new or expanded coverage was provided (chapter numbers here have been updated to reflect the fourth edition): xxxviii | Preface • • • • • • • • • • • • The new B if A else C conditional expression (Chapter 19) with/as context managers (Chapter 33) try/except/finally unification (Chapter 33) Relative import syntax (Chapter 23) Generator expressions (Chapter 20) New generator function features (Chapter 20) Function decorators (Chapter 31) The set object type (Chapter 5) New built-in functions: sorted, sum, any, all, enumerate (Chapters 13 and 14) The decimal fixed-precision object type (Chapter 5) Files, list comprehensions, and iterators (Chapters 14 and 20) New development tools: Eclipse, distutils, unittest and doctest, IDLE enhancements, Shedskin, and so on (Chapters 2 and 35) Smaller language changes (for instance, the widespread use of True and False; the new sys.exc_info for fetching exception details; and the demise of string-based exceptions, string methods, and the apply and reduce built-ins) are discussed throughout the book. The third edition also expanded coverage of some of the features that were new in the second edition, including three-limit slices and the arbitrary arguments call syntax that subsumed apply. The Third Edition’s Python Training Changes Besides such language changes, the third edition was augmented with new topics and examples presented in my Python training sessions. Changes included (chapter numbers again updated to reflect those in the fourth edition): • • • • • A new chapter introducing built-in types (Chapter 4) A new chapter introducing statement syntax (Chapter 10) A new full chapter on dynamic typing, with enhanced coverage (Chapter 6) An expanded OOP introduction (Chapter 25) New examples for files, scopes, statement nesting, classes, exceptions, and more Many additions and changes were made with Python beginners in mind, and some topics were moved to appear at the places where they proved simplest to digest in training classes. List comprehensions and iterators, for example, now make their initial appearance in conjunction with the for loop statement, instead of later with functional tools. Preface | xxxix Coverage of many original core language topics also was substantially expanded in the third edition, with new discussions and examples added. Because this text has become something of a de facto standard resource for learning the core Python language, the presentation was made more complete and augmented with new use cases throughout. In addition, a new set of Python tips and tricks, gleaned from 10 years of teaching classes and 15 years of using Python for real work, was incorporated, and the exercises were updated and expanded to reflect current Python best practices, new language features, and common beginners’ mistakes witnessed firsthand in classes. Overall, the core language coverage was expanded. The Third Edition’s Structural Changes Because the material was more complete, it was split into bite-sized chunks. The core language material was organized into many multichapter parts to make it easier to tackle. Types and statements, for instance, are now two top-level parts, with one chapter for each major type and statement topic. Exercises and “gotchas” (common mistakes) were also moved from chapter ends to part ends, appearing at the end of the last chapter in each part. In the third edition, I also augmented the end-of-part exercises with end-of-chapter summaries and end-of-chapter quizzes to help you review chapters as you complete them. Each chapter concludes with a set of questions to help you review and test your understanding of the chapter’s material. Unlike the end-of-part exercises, whose solutions are presented in Appendix B, the solutions to the end-of-chapter quizzes appear immediately after the questions; I encourage you to look at the solutions even if you’re sure you’ve answered the questions correctly because the answers are a sort of review in themselves. Despite all the new topics, the book is still oriented toward Python newcomers and is designed to be a first Python text for programmers. Because it is largely based on timetested training experience and materials, it can still serve as a self-paced introductory Python class. The Third Edition’s Scope Changes As of its third edition, this book is intended as a tutorial on the core Python language, and nothing else. It’s about learning the language in an in-depth fashion, before applying it in application-level programming. The presentation here is bottom-up and gradual, but it provides a complete look at the entire language, in isolation from its application roles. For some, “learning Python” involves spending an hour or two going through a tutorial on the Web. This works for already advanced programmers, up to a point; Python is, after all, relatively simple in comparison to other languages. The problem with this fasttrack approach is that its practitioners eventually stumble onto unusual cases and get xl | Preface stuck—variables change out from under them, mutable default arguments mutate inexplicably, and so on. The goal here is instead to provide a solid grounding in Python fundamentals, so that even the unusual cases will make sense when they crop up. This scope is deliberate. By restricting our gaze to language fundamentals, we can investigate them here in more satisfying depth. Other texts, described ahead, pick up where this book leaves off and provide a more complete look at application-level topics and additional reference materials. The purpose of the book you are reading now is solely to teach Python itself so that you can apply it to whatever domain you happen to work in. About This Book This section underscores some important points about this book in general, regardless of its edition number. No book addresses every possible audience, so it’s important to understand a book’s goals up front. This Book’s Prerequisites There are no absolute prerequisites to speak of, really. Both true beginners and crusty programming veterans have used this book successfully. If you are motivated to learn Python, this text will probably work for you. In general, though, I have found that any exposure to programming or scripting before this book can be helpful, even if not required for every reader. This book is designed to be an introductory-level Python text for programmers.* It may not be an ideal text for someone who has never touched a computer before (for instance, we’re not going to spend any time exploring what a computer is), but I haven’t made many assumptions about your programming background or education. On the other hand, I won’t insult readers by assuming they are “dummies,” either, whatever that means—it’s easy to do useful things in Python, and this book will show you how. The text occasionally contrasts Python with languages such as C, C++, Java, and Pascal, but you can safely ignore these comparisons if you haven’t used such languages in the past. This Book’s Scope and Other Books Although this book covers all the essentials of the Python language, I’ve kept its scope narrow in the interests of speed and size. To keep things simple, this book focuses on core concepts, uses small and self-contained examples to illustrate points, and * And by “programmers,” I mean anyone who has written a single line of code in any programming or scripting language in the past. If this doesn’t include you, you will probably find this book useful anyhow, but be aware that it will spend more time teaching Python than programming fundamentals. Preface | xli sometimes omits the small details that are readily available in reference manuals. Because of that, this book is probably best described as an introduction and a steppingstone to more advanced and complete texts. For example, we won’t talk much about Python/C integration—a complex topic that is nevertheless central to many Python-based systems. We also won’t talk much about Python’s history or development processes. And popular Python applications such as GUIs, system tools, and network scripting get only a short glance, if they are mentioned at all. Naturally, this scope misses some of the big picture. By and large, Python is about raising the quality bar a few notches in the scripting world. Some of its ideas require more context than can be provided here, and I’d be remiss if I didn’t recommend further study after you finish this book. I hope that most readers of this book will eventually go on to gain a more complete understanding of applicationlevel programming from other texts. Because of its beginner’s focus, Learning Python is designed to be naturally complemented by O’Reilly’s other Python books. For instance, Programming Python, another book I authored, provides larger and more complete examples, along with tutorials on application programming techniques, and was explicitly designed to be a follow-up text to the one you are reading now. Roughly, the current editions of Learning Python and Programming Python reflect the two halves of their author’s training materials—the core language, and application programming. In addition, O’Reilly’s Python Pocket Reference serves as a quick reference supplement for looking up some of the finer details skipped here. Other follow-up books can also provide references, additional examples, or details about using Python in specific domains such as the Web and GUIs. For instance, O’Reilly’s Python in a Nutshell and Sams’s Python Essential Reference serve as useful references, and O’Reilly’s Python Cookbook offers a library of self-contained examples for people already familiar with application programming techniques. Because reading books is such a subjective experience, I encourage you to browse on your own to find advanced texts that suit your needs. Regardless of which books you choose, though, keep in mind that the rest of the Python story requires studying examples that are more realistic than there is space for here. Having said that, I think you’ll find this book to be a good first text on Python, despite its limited scope (and perhaps because of it). You’ll learn everything you need to get started writing useful standalone Python programs and scripts. By the time you’ve finished this book, you will have learned not only the language itself, but also how to apply it well to your day-to-day tasks. And you’ll be equipped to tackle more advanced topics and examples as they come your way. xlii | Preface This Book’s Style and Structure This book is based on training materials developed for a three-day hands-on Python course. You’ll find quizzes at the end of each chapter, and exercises at the end of the last chapter of each part. Solutions to chapter quizzes appear in the chapters themselves, and solutions to part exercises show up in Appendix B. The quizzes are designed to review material, while the exercises are designed to get you coding right away and are usually one of the highlights of the course. I strongly recommend working through the quizzes and exercises along the way, not only to gain Python programming experience, but also because some of the exercises raise issues not covered elsewhere in the book. The solutions in the chapters and in Appendix B should help you if you get stuck (and you are encouraged to peek at the answers as much and as often as you like). The overall structure of this book is also derived from class materials. Because this text is designed to introduce language basics quickly, I’ve organized the presentation by major language features, not examples. We’ll take a bottom-up approach here: from built-in object types, to statements, to program units, and so on. Each chapter is fairly self-contained, but later chapters draw upon ideas introduced in earlier ones (e.g., by the time we get to classes, I’ll assume you know how to write functions), so a linear reading makes the most sense for most readers. In general terms, this book presents the Python language in a linear fashion. It is organized with one part per major language feature—types, functions, and so forth—and most of the examples are small and self-contained (some might also call the examples in this text artificial, but they illustrate the points it aims to make). More specifically, here is what you will find: Part I, Getting Started We begin with a general overview of Python that answers commonly asked initial questions—why people use the language, what it’s useful for, and so on. The first chapter introduces the major ideas underlying the technology to give you some background context. Then the technical material of the book begins, as we explore the ways that both we and Python run programs. The goal of this part of the book is to give you just enough information to be able to follow along with later examples and exercises. Part II, Types and Operations Next, we begin our tour of the Python language, studying Python’s major built-in object types in depth: numbers, lists, dictionaries, and so on. You can get a lot done in Python with these tools alone. This is the most substantial part of the book because we lay groundwork here for later chapters. We’ll also look at dynamic typing and its references—keys to using Python well—in this part. Preface | xliii Part III, Statements and Syntax The next part moves on to introduce Python’s statements—the code you type to create and process objects in Python. It also presents Python’s general syntax model. Although this part focuses on syntax, it also introduces some related tools, such as the PyDoc system, and explores coding alternatives. Part IV, Functions This part begins our look at Python’s higher-level program structure tools. Functions turn out to be a simple way to package code for reuse and avoid code redundancy. In this part, we will explore Python’s scoping rules, argument-passing techniques, and more. Part V, Modules Python modules let you organize statements and functions into larger components, and this part illustrates how to create, use, and reload modules. We’ll also look at some more advanced topics here, such as module packages, module reloading, and the __name__ variable. Part VI, Classes and OOP Here, we explore Python’s object-oriented programming tool, the class—an optional but powerful way to structure code for customization and reuse. As you’ll see, classes mostly reuse ideas we will have covered by this point in the book, and OOP in Python is mostly about looking up names in linked objects. As you’ll also see, OOP is optional in Python, but it can shave development time substantially, especially for long-term strategic project development. Part VII, Exceptions and Tools We conclude the language fundamentals coverage in this text with a look at Python’s exception handling model and statements, plus a brief overview of development tools that will become more useful when you start writing larger programs (debugging and testing tools, for instance). Although exceptions are a fairly lightweight tool, this part appears after the discussion of classes because exceptions should now all be classes. Part VIII, Advanced Topics (new in the fourth edition) In the final part, we explore some advanced topics. Here, we study Unicode and byte strings, managed attribute tools like properties and descriptors, function and class decorators, and metaclasses. These chapters are all optional reading, because not all programmers need to understand the subjects they address. On the other hand, readers who must process internationalized text or binary data, or are responsible for developing APIs for other programmers to use, should find something of interest in this part. Part IX, Appendixes The book wraps up with a pair of appendixes that give platform-specific tips for using Python on various computers (Appendix A) and provide solutions to the endof-part exercises (Appendix B). Solutions to end-of-chapter quizzes appear in the chapters themselves. xliv | Preface Note that the index and table of contents can be used to hunt for details, but there are no reference appendixes in this book (this book is a tutorial, not a reference). As mentioned earlier, you can consult Python Pocket Reference, as well as other books, and the free Python reference manuals maintained at http://www.python.org for syntax and built-in tool details. Book Updates Improvements happen (and so do mis^H^H^H typos). Updates, supplements, and corrections for this book will be maintained (or referenced) on the Web at one of the following sites: http://www.oreilly.com/catalog/9780596158064 (O’Reilly’s web page for the book) http://www.rmi.net/~lutz (the author’s site) http://www.rmi.net/~lutz/about-lp.html (the author’s web page for the book) The last of these three URLs points to a web page for this book where I will post updates, but be sure to search the Web if this link becomes invalid. If I could become more clairvoyant, I would, but the Web changes faster than printed books. About the Programs in This Book This fourth edition of this book, and all the program examples in it, is based on Python version 3.0. In addition, most of its examples run under Python 2.6, as described in the text, and notes for Python 2.6 readers are mixed in along the way. Because this text focuses on the core language, however, you can be fairly sure that most of what it has to say won’t change very much in future releases of Python. Most of this book applies to earlier Python versions, too, except when it does not; naturally, if you try using extensions added after the release you’ve got, all bets are off. As a rule of thumb, the latest Python is the best Python. Because this book focuses on the core language, most of it also applies to Jython, the Java-based Python language implementation, as well as other Python implementations described in Chapter 2. Source code for the book’s examples, as well as exercise solutions, can be fetched from the book’s website at http://www.oreilly.com/catalog/9780596158064/. So, how do you run the examples? We’ll study startup details in Chapter 3, so please stay tuned for information on this front. Using Code Examples This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, Preface | xlv writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Learning Python, Fourth Edition, by Mark Lutz. Copyright 2009 Mark Lutz, 978-0-596-15806-4.” If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at [email protected]. Font Conventions This book uses the following typographical conventions: Italic Used for email addresses, URLs, filenames, pathnames, and emphasizing new terms when they are first introduced Constant width Used for the contents of files and the output from commands, and to designate modules, methods, statements, and commands Constant width bold Used in code sections to show commands or text that would be typed by the user, and, occasionally, to highlight portions of code Constant width italic Used for replaceables and some comments in code sections <Constant width> Indicates a syntactic unit that should be replaced with real code Indicates a tip, suggestion, or general note relating to the nearby text. Indicates a warning or caution relating to the nearby text. xlvi | Preface Notes specific to this book: In this book’s examples, the % character at the start of a system command line stands for the system’s prompt, whatever that may be on your machine (e.g., C:\Python30> in a DOS window). Don’t type the % character (or the system prompt it sometimes stands for) yourself. Similarly, in interpreter interaction listings, do not type the >>> and ... characters shown at the start of lines—these are prompts that Python displays. Type just the text after these prompts. To help you remember this, user inputs are shown in bold font in this book. Also, you normally don’t need to type text that starts with a # in listings; as you’ll learn, these are comments, not executable code. Safari® Books Online Safari Books Online is an on-demand digital library that lets you easily search over 7,500 technology and creative reference books and videos to find the answers you need quickly. With a subscription, you can read any page and watch any video from our library online. Read books on your cell phone and mobile devices. Access new titles before they are available for print, and get exclusive access to manuscripts in development and post feedback for the authors. Copy and paste code samples, organize your favorites, download chapters, bookmark key sections, create notes, print out pages, and benefit from tons of other time-saving features. O’Reilly Media has uploaded this book to the Safari Books Online service. To have full digital access to this book and others on similar topics from O’Reilly and other publishers, sign up for free at http://my.safaribooksonline.com. How to Contact Us Please address comments and questions concerning this book to the publisher: O’Reilly Media, Inc. 1005 Gravenstein Highway North Sebastopol, CA 95472 800-998-9938 (in the United States or Canada) 707-829-0515 (international or local) 707-829-0104 (fax) We will also maintain a web page for this book, where we list errata, examples, and any additional information. You can access this page at: http://www.oreilly.com/catalog/9780596158064/ Preface | xlvii To comment or ask technical questions about this book, send email to: [email protected] For more information about our books, conferences, Resource Centers, and the O’Reilly Network, see our website at: http://www.oreilly.com For book updates, be sure to also see the other links mentioned earlier in this Preface. Acknowledgments As I write this fourth edition of this book in 2009, I can’t help but be in a sort of “mission accomplished” state of mind. I have now been using and promoting Python for 17 years, and have been teaching it for 12 years. Despite the passage of time and events, I am still constantly amazed at how successful Python has been over the years. It has grown in ways that most of us could not possibly have imagined in 1992. So, at the risk of sounding like a hopelessly self-absorbed author, you’ll have to pardon a few words of reminiscing, congratulations, and thanks here. It’s been the proverbial long and winding road. Looking back today, when I first discovered Python in 1992, I had no idea what an impact it would have on the next 17 years of my life. Two years after writing the first edition of Programming Python in 1995, I began traveling around the country and the world teaching Python to beginners and experts. Since finishing the first edition of Learning Python in 1999, I’ve been an independent Python trainer and writer, thanks largely to Python’s exponential growth in popularity. As I write these words in mid-2009, I have written 12 Python books (4 editions of 3). I have also been teaching Python for more than a decade; have taught some 225 Python training sessions in the U.S., Europe, Canada, and Mexico; and have met over 3,000 students along the way. Besides racking up frequent flyer miles, these classes helped me refine this text as well as my other Python books. Over the years, teaching honed the books, and vice versa. In fact, the book you’re reading is derived almost entirely from my classes. Because of this, I’d like to thank all the students who have participated in my courses during the last 12 years. Along with changes in Python itself, your feedback played a huge role in shaping this text. (There’s nothing quite as instructive as watching 3,000 students repeat the same beginner’s mistakes!) This edition owes its changes primarily to classes held after 2003, though every class held since 1997 has in some way helped refine this book. I’d especially like to single out clients who hosted classes in Dublin, Mexico City, Barcelona, London, Edmonton, and Puerto Rico; better perks would be hard to imagine. I’d also like to express my gratitude to everyone who played a part in producing this book. To the editors who worked on this project: Julie Steele on this edition, Tatiana xlviii | Preface Apandi on the prior edition, and many others on earlier editions. To Doug Hellmann and Jesse Noller for taking part in the technical review of this book. And to O’Reilly for giving me a chance to work on those 12 book projects—it’s been net fun (and only feels a little like the movie Groundhog Day). I want to thank my original coauthor David Ascher as well for his work on the first two editions of this book. David contributed the “Outer Layers” part in prior editions, which we unfortunately had to trim to make room for new core language materials in the third edition. To compensate, I added a handful of more advanced programs as a self-study final exercise in the third edition, and added both new advanced examples and a new complete part for advanced topics in the fourth edition. Also see the prior notes in this Preface about follow-up application-level texts you may want to consult once you’ve learned the fundamentals here. For creating such an enjoyable and useful language, I owe additional thanks to Guido van Rossum and the rest of the Python community. Like most open source systems, Python is the product of many heroic efforts. After 17 years of programming Python, I still find it to be seriously fun. It’s been my privilege to watch Python grow from a new kid on the scripting languages block to a widely used tool, deployed in some fashion by almost every organization writing software. That has been an exciting endeavor to be a part of, and I’d like to thank and congratulate the entire Python community for a job well done. I also want to thank my original editor at O’Reilly, the late Frank Willison. This book was largely Frank’s idea, and it reflects the contagious vision he had. In looking back, Frank had a profound impact on both my own career and that of Python itself. It is not an exaggeration to say that Frank was responsible for much of the fun and success of Python when it was new. We still miss him. Finally, a few personal notes of thanks. To OQO for the best toys so far (while they lasted). To the late Carl Sagan for inspiring an 18-year-old kid from Wisconsin. To my Mom, for courage. And to all the large corporations I’ve come across over the years, for reminding me how lucky I have been to be self-employed for the last decade! To my children, Mike, Sammy, and Roxy, for whatever futures you will choose to make. You were children when I began with Python, and you seem to have somehow grown up along the way; I’m proud of you. Life may compel us down paths all our own, but there will always be a path home. And most of all, to Vera, my best friend, my girlfriend, and my wife. The best day of my life was the day I finally found you. I don’t know what the next 50 years hold, but I do know that I want to spend all of them holding you. —Mark Lutz Sarasota, Florida July 2009 Preface | xlix PART I Getting Started CHAPTER 1 A Python Q&A Session If you’ve bought this book, you may already know what Python is and why it’s an important tool to learn. If you don’t, you probably won’t be sold on Python until you’ve learned the language by reading the rest of this book and have done a project or two. But before we jump into details, the first few pages of this book will briefly introduce some of the main reasons behind Python’s popularity. To begin sculpting a definition of Python, this chapter takes the form of a question-and-answer session, which poses some of the most common questions asked by beginners. Why Do People Use Python? Because there are many programming languages available today, this is the usual first question of newcomers. Given that there are roughly 1 million Python users out there at the moment, there really is no way to answer this question with complete accuracy; the choice of development tools is sometimes based on unique constraints or personal preference. But after teaching Python to roughly 225 groups and over 3,000 students during the last 12 years, some common themes have emerged. The primary factors cited by Python users seem to be these: Software quality For many, Python’s focus on readability, coherence, and software quality in general sets it apart from other tools in the scripting world. Python code is designed to be readable, and hence reusable and maintainable—much more so than traditional scripting languages. The uniformity of Python code makes it easy to understand, even if you did not write it. In addition, Python has deep support for more advanced software reuse mechanisms, such as object-oriented programming (OOP). Developer productivity Python boosts developer productivity many times beyond compiled or statically typed languages such as C, C++, and Java. Python code is typically one-third to one-fifth the size of equivalent C++ or Java code. That means there is less to type, 3 less to debug, and less to maintain after the fact. Python programs also run immediately, without the lengthy compile and link steps required by some other tools, further boosting programmer speed. Program portability Most Python programs run unchanged on all major computer platforms. Porting Python code between Linux and Windows, for example, is usually just a matter of copying a script’s code between machines. Moreover, Python offers multiple options for coding portable graphical user interfaces, database access programs, webbased systems, and more. Even operating system interfaces, including program launches and directory processing, are as portable in Python as they can possibly be. Support libraries Python comes with a large collection of prebuilt and portable functionality, known as the standard library. This library supports an array of application-level programming tasks, from text pattern matching to network scripting. In addition, Python can be extended with both homegrown libraries and a vast collection of third-party application support software. Python’s third-party domain offers tools for website construction, numeric programming, serial port access, game development, and much more. The NumPy extension, for instance, has been described as a free and more powerful equivalent to the Matlab numeric programming system. Component integration Python scripts can easily communicate with other parts of an application, using a variety of integration mechanisms. Such integrations allow Python to be used as a product customization and extension tool. Today, Python code can invoke C and C++ libraries, can be called from C and C++ programs, can integrate with Java and .NET components, can communicate over frameworks such as COM, can interface with devices over serial ports, and can interact over networks with interfaces like SOAP, XML-RPC, and CORBA. It is not a standalone tool. Enjoyment Because of Python’s ease of use and built-in toolset, it can make the act of programming more pleasure than chore. Although this may be an intangible benefit, its effect on productivity is an important asset. Of these factors, the first two (quality and productivity) are probably the most compelling benefits to most Python users. Software Quality By design, Python implements a deliberately simple and readable syntax and a highly coherent programming model. As a slogan at a recent Python conference attests, the net result is that Python seems to “fit your brain”—that is, features of the language interact in consistent and limited ways and follow naturally from a small set of core 4 | Chapter 1: A Python Q&A Session concepts. This makes the language easier to learn, understand, and remember. In practice, Python programmers do not need to constantly refer to manuals when reading or writing code; it’s a consistently designed system that many find yields surprisingly regular-looking code. By philosophy, Python adopts a somewhat minimalist approach. This means that although there are usually multiple ways to accomplish a coding task, there is usually just one obvious way, a few less obvious alternatives, and a small set of coherent interactions everywhere in the language. Moreover, Python doesn’t make arbitrary decisions for you; when interactions are ambiguous, explicit intervention is preferred over “magic.” In the Python way of thinking, explicit is better than implicit, and simple is better than complex.* Beyond such design themes, Python includes tools such as modules and OOP that naturally promote code reusability. And because Python is focused on quality, so too, naturally, are Python programmers. Developer Productivity During the great Internet boom of the mid-to-late 1990s, it was difficult to find enough programmers to implement software projects; developers were asked to implement systems as fast as the Internet evolved. Today, in an era of layoffs and economic recession, the picture has shifted. Programming staffs are often now asked to accomplish the same tasks with even fewer people. In both of these scenarios, Python has shined as a tool that allows programmers to get more done with less effort. It is deliberately optimized for speed of development—its simple syntax, dynamic typing, lack of compile steps, and built-in toolset allow programmers to develop programs in a fraction of the time needed when using some other tools. The net effect is that Python typically boosts developer productivity many times beyond the levels supported by traditional languages. That’s good news in both boom and bust times, and everywhere the software industry goes in between. Is Python a “Scripting Language”? Python is a general-purpose programming language that is often applied in scripting roles. It is commonly defined as an object-oriented scripting language—a definition that blends support for OOP with an overall orientation toward scripting roles. In fact, people often use the word “script” instead of “program” to describe a Python code file. In this book, the terms “script” and “program” are used interchangeably, with a slight * For a more complete look at the Python philosophy, type the command import this at any Python interactive prompt (you’ll see how in Chapter 2). This invokes an “Easter egg” hidden in Python—a collection of design principles underlying Python. The acronym EIBTI is now fashionable jargon for the “explicit is better than implicit” rule. Is Python a “Scripting Language”? | 5 preference for “script” to describe a simpler top-level file and “program” to refer to a more sophisticated multifile application. Because the term “scripting language” has so many different meanings to different observers, some would prefer that it not be applied to Python at all. In fact, people tend to make three very different associations, some of which are more useful than others, when they hear Python labeled as such: Shell tools Sometimes when people hear Python described as a scripting language, they think it means that Python is a tool for coding operating-system-oriented scripts. Such programs are often launched from console command lines and perform tasks such as processing text files and launching other programs. Python programs can and do serve such roles, but this is just one of dozens of common Python application domains. It is not just a better shell-script language. Control language To others, scripting refers to a “glue” layer used to control and direct (i.e., script) other application components. Python programs are indeed often deployed in the context of larger applications. For instance, to test hardware devices, Python programs may call out to components that give low-level access to a device. Similarly, programs may run bits of Python code at strategic points to support end-user product customization without the need to ship and recompile the entire system’s source code. Python’s simplicity makes it a naturally flexible control tool. Technically, though, this is also just a common Python role; many (perhaps most) Python programmers code standalone scripts without ever using or knowing about any integrated components. It is not just a control language. Ease of use Probably the best way to think of the term “scripting language” is that it refers to a simple language used for quickly coding tasks. This is especially true when the term is applied to Python, which allows much faster program development than compiled languages like C++. Its rapid development cycle fosters an exploratory, incremental mode of programming that has to be experienced to be appreciated. Don’t be fooled, though—Python is not just for simple tasks. Rather, it makes tasks simple by its ease of use and flexibility. Python has a simple feature set, but it allows programs to scale up in sophistication as needed. Because of that, it is commonly used for quick tactical tasks and longer-term strategic development. So, is Python a scripting language or not? It depends on whom you ask. In general, the term “scripting” is probably best used to describe the rapid and flexible mode of development that Python supports, rather than a particular application domain. 6 | Chapter 1: A Python Q&A Session OK, but What’s the Downside? After using it for 17 years and teaching it for 12, the only downside to Python I’ve found is that, as currently implemented, its execution speed may not always be as fast as that of compiled languages such as C and C++. We’ll talk about implementation concepts in detail later in this book. In short, the standard implementations of Python today compile (i.e., translate) source code statements to an intermediate format known as byte code and then interpret the byte code. Byte code provides portability, as it is a platform-independent format. However, because Python is not compiled all the way down to binary machine code (e.g., instructions for an Intel chip), some programs will run more slowly in Python than in a fully compiled language like C. Whether you will ever care about the execution speed difference depends on what kinds of programs you write. Python has been optimized numerous times, and Python code runs fast enough by itself in most application domains. Furthermore, whenever you do something “real” in a Python script, like processing a file or constructing a graphical user interface (GUI), your program will actually run at C speed, since such tasks are immediately dispatched to compiled C code inside the Python interpreter. More fundamentally, Python’s speed-of-development gain is often far more important than any speed-of-execution loss, especially given modern computer speeds. Even at today’s CPU speeds, though, there still are some domains that do require optimal execution speeds. Numeric programming and animation, for example, often need at least their core number-crunching components to run at C speed (or better). If you work in such a domain, you can still use Python—simply split off the parts of the application that require optimal speed into compiled extensions, and link those into your system for use in Python scripts. We won’t talk about extensions much in this text, but this is really just an instance of the Python-as-control-language role we discussed earlier. A prime example of this dual language strategy is the NumPy numeric programming extension for Python; by combining compiled and optimized numeric extension libraries with the Python language, NumPy turns Python into a numeric programming tool that is efficient and easy to use. You may never need to code such extensions in your own Python work, but they provide a powerful optimization mechanism if you ever do. Who Uses Python Today? At this writing, the best estimate anyone can seem to make of the size of the Python user base is that there are roughly 1 million Python users around the world today (plus or minus a few). This estimate is based on various statistics, like download rates and developer surveys. Because Python is open source, a more exact count is difficult— there are no license registrations to tally. Moreover, Python is automatically included Who Uses Python Today? | 7 with Linux distributions, Macintosh computers, and some products and hardware, further clouding the user-base picture. In general, though, Python enjoys a large user base and a very active developer community. Because Python has been around for some 19 years and has been widely used, it is also very stable and robust. Besides being employed by individual users, Python is also being applied in real revenue-generating products by real companies. For instance: • Google makes extensive use of Python in its web search systems, and employs Python’s creator. • The YouTube video sharing service is largely written in Python. • The popular BitTorrent peer-to-peer file sharing system is a Python program. • Google’s popular App Engine web development framework uses Python as its application language. • EVE Online, a Massively Multiplayer Online Game (MMOG), makes extensive use of Python. • Maya, a powerful integrated 3D modeling and animation system, provides a Python scripting API. • Intel, Cisco, Hewlett-Packard, Seagate, Qualcomm, and IBM use Python for hardware testing. • Industrial Light & Magic, Pixar, and others use Python in the production of animated movies. • JPMorgan Chase, UBS, Getco, and Citadel apply Python for financial market forecasting. • NASA, Los Alamos, Fermilab, JPL, and others use Python for scientific programming tasks. • iRobot uses Python to develop commercial robotic devices. • ESRI uses Python as an end-user customization tool for its popular GIS mapping products. • The NSA uses Python for cryptography and intelligence analysis. • The IronPort email server product uses more than 1 million lines of Python code to do its job. • The One Laptop Per Child (OLPC) project builds its user interface and activity model in Python. And so on. Probably the only common thread amongst the companies using Python today is that Python is used all over the map, in terms of application domains. Its general-purpose nature makes it applicable to almost all fields, not just one. In fact, it’s safe to say that virtually every substantial organization writing software is using Python, whether for short-term tactical tasks, such as testing and administration, or for longterm strategic product development. Python has proven to work well in both modes. 8 | Chapter 1: A Python Q&A Session For more details on companies using Python today, see Python’s website at http://www .python.org. What Can I Do with Python? In addition to being a well-designed programming language, Python is useful for accomplishing real-world tasks—the sorts of things developers do day in and day out. It’s commonly used in a variety of domains, as a tool for scripting other components and implementing standalone programs. In fact, as a general-purpose language, Python’s roles are virtually unlimited: you can use it for everything from website development and gaming to robotics and spacecraft control. However, the most common Python roles currently seem to fall into a few broad categories. The next few sections describe some of Python’s most common applications today, as well as tools used in each domain. We won’t be able to explore the tools mentioned here in any depth—if you are interested in any of these topics, see the Python website or other resources for more details. Systems Programming Python’s built-in interfaces to operating-system services make it ideal for writing portable, maintainable system-administration tools and utilities (sometimes called shell tools). Python programs can search files and directory trees, launch other programs, do parallel processing with processes and threads, and so on. Python’s standard library comes with POSIX bindings and support for all the usual OS tools: environment variables, files, sockets, pipes, processes, multiple threads, regular expression pattern matching, command-line arguments, standard stream interfaces, shell-command launchers, filename expansion, and more. In addition, the bulk of Python’s system interfaces are designed to be portable; for example, a script that copies directory trees typically runs unchanged on all major Python platforms. The Stackless Python system, used by EVE Online, also offers advanced solutions to multiprocessing requirements. GUIs Python’s simplicity and rapid turnaround also make it a good match for graphical user interface programming. Python comes with a standard object-oriented interface to the Tk GUI API called tkinter (Tkinter in 2.6) that allows Python programs to implement portable GUIs with a native look and feel. Python/tkinter GUIs run unchanged on Microsoft Windows, X Windows (on Unix and Linux), and the Mac OS (both Classic and OS X). A free extension package, PMW, adds advanced widgets to the tkinter toolkit. In addition, the wxPython GUI API, based on a C++ library, offers an alternative toolkit for constructing portable GUIs in Python. What Can I Do with Python? | 9 Higher-level toolkits such as PythonCard and Dabo are built on top of base APIs such as wxPython and tkinter. With the proper library, you can also use GUI support in other toolkits in Python, such as Qt with PyQt, GTK with PyGTK, MFC with PyWin32, .NET with IronPython, and Swing with Jython (the Java version of Python, described in Chapter 2) or JPype. For applications that run in web browsers or have simple interface requirements, both Jython and Python web frameworks and serverside CGI scripts, described in the next section, provide additional user interface options. Internet Scripting Python comes with standard Internet modules that allow Python programs to perform a wide variety of networking tasks, in client and server modes. Scripts can communicate over sockets; extract form information sent to server-side CGI scripts; transfer files by FTP; parse, generate, and analyze XML files; send, receive, compose, and parse email; fetch web pages by URLs; parse the HTML and XML of fetched web pages; communicate over XML-RPC, SOAP, and Telnet; and more. Python’s libraries make these tasks remarkably simple. In addition, a large collection of third-party tools are available on the Web for doing Internet programming in Python. For instance, the HTMLGen system generates HTML files from Python class-based descriptions, the mod_python package runs Python efficiently within the Apache web server and supports server-side templating with its Python Server Pages, and the Jython system provides for seamless Python/Java integration and supports coding of server-side applets that run on clients. In addition, full-blown web development framework packages for Python, such as Django, TurboGears, web2py, Pylons, Zope, and WebWare, support quick construction of full-featured and production-quality websites with Python. Many of these include features such as object-relational mappers, a Model/View/Controller architecture, server-side scripting and templating, and AJAX support, to provide complete and enterprise-level web development solutions. Component Integration We discussed the component integration role earlier when describing Python as a control language. Python’s ability to be extended by and embedded in C and C++ systems makes it useful as a flexible glue language for scripting the behavior of other systems and components. For instance, integrating a C library into Python enables Python to test and launch the library’s components, and embedding Python in a product enables onsite customizations to be coded without having to recompile the entire product (or ship its source code at all). 10 | Chapter 1: A Python Q&A Session Tools such as the SWIG and SIP code generators can automate much of the work needed to link compiled components into Python for use in scripts, and the Cython system allows coders to mix Python and C-like code. Larger frameworks, such as Python’s COM support on Windows, the Jython Java-based implementation, the IronPython .NET-based implementation, and various CORBA toolkits for Python, provide alternative ways to script components. On Windows, for example, Python scripts can use frameworks to script Word and Excel. Database Programming For traditional database demands, there are Python interfaces to all commonly used relational database systems—Sybase, Oracle, Informix, ODBC, MySQL, PostgreSQL, SQLite, and more. The Python world has also defined a portable database API for accessing SQL database systems from Python scripts, which looks the same on a variety of underlying database systems. For instance, because the vendor interfaces implement the portable API, a script written to work with the free MySQL system will work largely unchanged on other systems (such as Oracle); all you have to do is replace the underlying vendor interface. Python’s standard pickle module provides a simple object persistence system—it allows programs to easily save and restore entire Python objects to files and file-like objects. On the Web, you’ll also find a third-party open source system named ZODB that provides a complete object-oriented database system for Python scripts, and others (such as SQLObject and SQLAlchemy) that map relational tables onto Python’s class model. Furthermore, as of Python 2.5, the in-process SQLite embedded SQL database engine is a standard part of Python itself. Rapid Prototyping To Python programs, components written in Python and C look the same. Because of this, it’s possible to prototype systems in Python initially, and then move selected components to a compiled language such as C or C++ for delivery. Unlike some prototyping tools, Python doesn’t require a complete rewrite once the prototype has solidified. Parts of the system that don’t require the efficiency of a language such as C++ can remain coded in Python for ease of maintenance and use. Numeric and Scientific Programming The NumPy numeric programming extension for Python mentioned earlier includes such advanced tools as an array object, interfaces to standard mathematical libraries, and much more. By integrating Python with numeric routines coded in a compiled language for speed, NumPy turns Python into a sophisticated yet easy-to-use numeric programming tool that can often replace existing code written in traditional compiled languages such as FORTRAN or C++. Additional numeric tools for Python support What Can I Do with Python? | 11 animation, 3D visualization, parallel processing, and so on. The popular SciPy and ScientificPython extensions, for example, provide additional libraries of scientific programming tools and use NumPy code. Gaming, Images, Serial Ports, XML, Robots, and More Python is commonly applied in more domains than can be mentioned here. For example, you can do: • Game programming and multimedia in Python with the pygame system • Serial port communication on Windows, Linux, and more with the PySerial extension • Image processing with PIL, PyOpenGL, Blender, Maya, and others • Robot control programming with the PyRo toolkit • XML parsing with the xml library package, the xmlrpclib module, and third-party extensions • Artificial intelligence programming with neural network simulators and expert system shells • Natural language analysis with the NLTK package You can even play solitaire with the PySol program. You’ll find support for many such fields at the PyPI websites, and via web searches (search Google or http://www.python .org for links). Many of these specific domains are largely just instances of Python’s component integration role in action again. Adding it as a frontend to libraries of components written in a compiled language such as C makes Python useful for scripting in a wide variety of domains. As a general-purpose language that supports integration, Python is widely applicable. How Is Python Supported? As a popular open source system, Python enjoys a large and active development community that responds to issues and develops enhancements with a speed that many commercial software developers would find remarkable (if not downright shocking). Python developers coordinate work online with a source-control system. Changes follow a formal PEP (Python Enhancement Proposal) protocol and must be accompanied by extensions to Python’s extensive regression testing system. In fact, modifying Python today is roughly as involved as changing commercial software—a far cry from Python’s early days, when an email to its creator would suffice, but a good thing given its current large user base. 12 | Chapter 1: A Python Q&A Session The PSF (Python Software Foundation), a formal nonprofit group, organizes conferences and deals with intellectual property issues. Numerous Python conferences are held around the world; O’Reilly’s OSCON and the PSF’s PyCon are the largest. The former of these addresses multiple open source projects, and the latter is a Python-only event that has experienced strong growth in recent years. Attendance at PyCon 2008 nearly doubled from the prior year, growing from 586 attendees in 2007 to over 1,000 in 2008. This was on the heels of a 40% attendance increase in 2007, from 410 in 2006. PyCon 2009 had 943 attendees, a slight decrease from 2008, but a still very strong showing during a global recession. What Are Python’s Technical Strengths? Naturally, this is a developer’s question. If you don’t already have a programming background, the language in the next few sections may be a bit baffling—don’t worry, we’ll explore all of these terms in more detail as we proceed through this book. For developers, though, here is a quick introduction to some of Python’s top technical features. It’s Object-Oriented Python is an object-oriented language, from the ground up. Its class model supports advanced notions such as polymorphism, operator overloading, and multiple inheritance; yet, in the context of Python’s simple syntax and typing, OOP is remarkably easy to apply. In fact, if you don’t understand these terms, you’ll find they are much easier to learn with Python than with just about any other OOP language available. Besides serving as a powerful code structuring and reuse device, Python’s OOP nature makes it ideal as a scripting tool for object-oriented systems languages such as C++ and Java. For example, with the appropriate glue code, Python programs can subclass (specialize) classes implemented in C++, Java, and C#. Of equal significance, OOP is an option in Python; you can go far without having to become an object guru all at once. Much like C++, Python supports both procedural and object-oriented programming modes. Its object-oriented tools can be applied if and when constraints allow. This is especially useful in tactical development modes, which preclude design phases. It’s Free Python is completely free to use and distribute. As with other open source software, such as Tcl, Perl, Linux, and Apache, you can fetch the entire Python system’s source code for free on the Internet. There are no restrictions on copying it, embedding it in your systems, or shipping it with your products. In fact, you can even sell Python’s source code, if you are so inclined. What Are Python’s Technical Strengths? | 13 But don’t get the wrong idea: “free” doesn’t mean “unsupported.” On the contrary, the Python online community responds to user queries with a speed that most commercial software help desks would do well to try to emulate. Moreover, because Python comes with complete source code, it empowers developers, leading to the creation of a large team of implementation experts. Although studying or changing a programming language’s implementation isn’t everyone’s idea of fun, it’s comforting to know that you can do so if you need to. You’re not dependent on the whims of a commercial vendor; the ultimate documentation source is at your disposal. As mentioned earlier, Python development is performed by a community that largely coordinates its efforts over the Internet. It consists of Python’s creator—Guido van Rossum, the officially anointed Benevolent Dictator for Life (BDFL) of Python—plus a supporting cast of thousands. Language changes must follow a formal enhancement procedure and be scrutinized by both other developers and the BDFL. Happily, this tends to make Python more conservative with changes than some other languages. It’s Portable The standard implementation of Python is written in portable ANSI C, and it compiles and runs on virtually every major platform currently in use. For example, Python programs run today on everything from PDAs to supercomputers. As a partial list, Python is available on: • • • • • • • • • • Linux and Unix systems Microsoft Windows and DOS (all modern flavors) Mac OS (both OS X and Classic) BeOS, OS/2, VMS, and QNX Real-time systems such as VxWorks Cray supercomputers and IBM mainframes PDAs running Palm OS, PocketPC, and Linux Cell phones running Symbian OS and Windows Mobile Gaming consoles and iPods And more Like the language interpreter itself, the standard library modules that ship with Python are implemented to be as portable across platform boundaries as possible. Further, Python programs are automatically compiled to portable byte code, which runs the same on any platform with a compatible version of Python installed (more on this in the next chapter). 14 | Chapter 1: A Python Q&A Session What that means is that Python programs using the core language and standard libraries run the same on Linux, Windows, and most other systems with a Python interpreter. Most Python ports also contain platform-specific extensions (e.g., COM support on Windows), but the core Python language and libraries work the same everywhere. As mentioned earlier, Python also includes an interface to the Tk GUI toolkit called tkinter (Tkinter in 2.6), which allows Python programs to implement full-featured graphical user interfaces that run on all major GUI platforms without program changes. It’s Powerful From a features perspective, Python is something of a hybrid. Its toolset places it between traditional scripting languages (such as Tcl, Scheme, and Perl) and systems development languages (such as C, C++, and Java). Python provides all the simplicity and ease of use of a scripting language, along with more advanced software-engineering tools typically found in compiled languages. Unlike some scripting languages, this combination makes Python useful for large-scale development projects. As a preview, here are some of the main things you’ll find in Python’s toolbox: Dynamic typing Python keeps track of the kinds of objects your program uses when it runs; it doesn’t require complicated type and size declarations in your code. In fact, as you’ll see in Chapter 6, there is no such thing as a type or variable declaration anywhere in Python. Because Python code does not constrain data types, it is also usually automatically applicable to a whole range of objects. Automatic memory management Python automatically allocates objects and reclaims (“garbage collects”) them when they are no longer used, and most can grow and shrink on demand. As you’ll learn, Python keeps track of low-level memory details so you don’t have to. Programming-in-the-large support For building larger systems, Python includes tools such as modules, classes, and exceptions. These tools allow you to organize systems into components, use OOP to reuse and customize code, and handle events and errors gracefully. Built-in object types Python provides commonly used data structures such as lists, dictionaries, and strings as intrinsic parts of the language; as you’ll see, they’re both flexible and easy to use. For instance, built-in objects can grow and shrink on demand, can be arbitrarily nested to represent complex information, and more. Built-in tools To process all those object types, Python comes with powerful and standard operations, including concatenation (joining collections), slicing (extracting sections), sorting, mapping, and more. What Are Python’s Technical Strengths? | 15 Library utilities For more specific tasks, Python also comes with a large collection of precoded library tools that support everything from regular expression matching to networking. Once you learn the language itself, Python’s library tools are where much of the application-level action occurs. Third-party utilities Because Python is open source, developers are encouraged to contribute precoded tools that support tasks beyond those supported by its built-ins; on the Web, you’ll find free support for COM, imaging, CORBA ORBs, XML, database access, and much more. Despite the array of tools in Python, it retains a remarkably simple syntax and design. The result is a powerful programming tool with all the usability of a scripting language. It’s Mixable Python programs can easily be “glued” to components written in other languages in a variety of ways. For example, Python’s C API lets C programs call and be called by Python programs flexibly. That means you can add functionality to the Python system as needed, and use Python programs within other environments or systems. Mixing Python with libraries coded in languages such as C or C++, for instance, makes it an easy-to-use frontend language and customization tool. As mentioned earlier, this also makes Python good at rapid prototyping; systems may be implemented in Python first, to leverage its speed of development, and later moved to C for delivery, one piece at a time, according to performance demands. It’s Easy to Use To run a Python program, you simply type it and run it. There are no intermediate compile and link steps, like there are for languages such as C or C++. Python executes programs immediately, which makes for an interactive programming experience and rapid turnaround after program changes—in many cases, you can witness the effect of a program change as fast as you can type it. Of course, development cycle turnaround is only one aspect of Python’s ease of use. It also provides a deliberately simple syntax and powerful built-in tools. In fact, some have gone so far as to call Python “executable pseudocode.” Because it eliminates much of the complexity in other tools, Python programs are simpler, smaller, and more flexible than equivalent programs in languages like C, C++, and Java. 16 | Chapter 1: A Python Q&A Session It’s Easy to Learn This brings us to a key point of this book: compared to other programming languages, the core Python language is remarkably easy to learn. In fact, you can expect to be coding significant Python programs in a matter of days (or perhaps in just hours, if you’re already an experienced programmer). That’s good news for professional developers seeking to learn the language to use on the job, as well as for end users of systems that expose a Python layer for customization or control. Today, many systems rely on the fact that end users can quickly learn enough Python to tailor their Python customizations’ code onsite, with little or no support. Although Python does have advanced programming tools, its core language will still seem simple to beginners and gurus alike. It’s Named After Monty Python OK, this isn’t quite a technical strength, but it does seem to be a surprisingly well-kept secret that I wish to expose up front. Despite all the reptile icons in the Python world, the truth is that Python creator Guido van Rossum named it after the BBC comedy series Monty Python’s Flying Circus. He is a big fan of Monty Python, as are many software developers (indeed, there seems to almost be a symmetry between the two fields). This legacy inevitably adds a humorous quality to Python code examples. For instance, the traditional “foo” and “bar” for generic variable names become “spam” and “eggs” in the Python world. The occasional “Brian,” “ni,” and “shrubbery” likewise owe their appearances to this namesake. It even impacts the Python community at large: talks at Python conferences are regularly billed as “The Spanish Inquisition.” All of this is, of course, very funny if you are familiar with the show, but less so otherwise. You don’t need to be familiar with the series to make sense of examples that borrow references to Monty Python (including many you will see in this book), but at least you now know their root. How Does Python Stack Up to Language X? Finally, to place it in the context of what you may already know, people sometimes compare Python to languages such as Perl, Tcl, and Java. We talked about performance earlier, so here we’ll focus on functionality. While other languages are also useful tools to know and use, many people find that Python: How Does Python Stack Up to Language X? | 17 • Is more powerful than Tcl. Python’s support for “programming in the large” makes it applicable to the development of larger systems. • Has a cleaner syntax and simpler design than Perl, which makes it more readable and maintainable and helps reduce program bugs. • Is simpler and easier to use than Java. Python is a scripting language, but Java inherits much of the complexity and syntax of systems languages such as C++. • Is simpler and easier to use than C++, but it doesn’t often compete with C++; as a scripting language, Python typically serves different roles. • Is both more powerful and more cross-platform than Visual Basic. Its open source nature also means it is not controlled by a single company. • Is more readable and general-purpose than PHP. Python is sometimes used to construct websites, but it’s also widely used in nearly every other computer domain, from robotics to movie animation. • Is more mature and has a more readable syntax than Ruby. Unlike Ruby and Java, OOP is an option in Python—Python does not impose OOP on users or projects to which it may not apply. • Has the dynamic flavor of languages like SmallTalk and Lisp, but also has a simple, traditional syntax accessible to developers as well as end users of customizable systems. Especially for programs that do more than scan text files, and that might have to be read in the future by others (or by you!), many people find that Python fits the bill better than any other scripting or programming language available today. Furthermore, unless your application requires peak performance, Python is often a viable alternative to systems development languages such as C, C++, and Java: Python code will be much less difficult to write, debug, and maintain. Of course, your author has been a card-carrying Python evangelist since 1992, so take these comments as you may. They do, however, reflect the common experience of many developers who have taken time to explore what Python has to offer. Chapter Summary And that concludes the hype portion of this book. In this chapter, we’ve explored some of the reasons that people pick Python for their programming tasks. We’ve also seen how it is applied and looked at a representative sample of who is using it today. My goal is to teach Python, though, not to sell it. The best way to judge a language is to see it in action, so the rest of this book focuses entirely on the language details we’ve glossed over here. The next two chapters begin our technical introduction to the language. In them, we’ll explore ways to run Python programs, peek at Python’s byte code execution model, and introduce the basics of module files for saving code. The goal will be to give you 18 | Chapter 1: A Python Q&A Session just enough information to run the examples and exercises in the rest of the book. You won’t really start programming per se until Chapter 4, but make sure you have a handle on the startup details before moving on. Test Your Knowledge: Quiz In this edition of the book, we will be closing each chapter with a quick pop quiz about the material presented therein to help you review the key concepts. The answers for these quizzes appear immediately after the questions, and you are encouraged to read the answers once you’ve taken a crack at the questions yourself. In addition to these end-of-chapter quizzes, you’ll find lab exercises at the end of each part of the book, designed to help you start coding Python on your own. For now, here’s your first test. Good luck! 1. 2. 3. 4. 5. 6. 7. What are the six main reasons that people choose to use Python? Name four notable companies or organizations using Python today. Why might you not want to use Python in an application? What can you do with Python? What’s the significance of the Python import this statement? Why does “spam” show up in so many Python examples in books and on the Web? What is your favorite color? Test Your Knowledge: Answers How did you do? Here are the answers I came up with, though there may be multiple solutions to some quiz questions. Again, even if you’re sure you got a question right, I encourage you to look at these answers for additional context. See the chapter’s text for more details if any of these responses don’t make sense to you. 1. Software quality, developer productivity, program portability, support libraries, component integration, and simple enjoyment. Of these, the quality and productivity themes seem to be the main reasons that people choose to use Python. 2. Google, Industrial Light & Magic, EVE Online, Jet Propulsion Labs, Maya, ESRI, and many more. Almost every organization doing software development uses Python in some fashion, whether for long-term strategic product development or for short-term tactical tasks such as testing and system administration. 3. Python’s downside is performance: it won’t run as quickly as fully compiled languages like C and C++. On the other hand, it’s quick enough for most applications, and typical Python code runs at close to C speed anyhow because it invokes Test Your Knowledge: Answers | 19 4. 5. 6. 7. linked-in C code in the interpreter. If speed is critical, compiled extensions are available for number-crunching parts of an application. You can use Python for nearly anything you can do with a computer, from website development and gaming to robotics and spacecraft control. import this triggers an Easter egg inside Python that displays some of the design philosophies underlying the language. You’ll learn how to run this statement in the next chapter. “Spam” is a reference from a famous Monty Python skit in which people trying to order food in a cafeteria are drowned out by a chorus of Vikings singing about spam. Oh, and it’s also a common variable name in Python scripts.... Blue. No, yellow! Python Is Engineering, Not Art When Python first emerged on the software scene in the early 1990s, it spawned what is now something of a classic conflict between its proponents and those of another popular scripting language, Perl. Personally, I think the debate is tired and unwarranted today—developers are smart enough to draw their own conclusions. Still, this is one of the most common topics I’m asked about on the training road, so it seems fitting to say a few words about it here. The short story is this: you can do everything in Python that you can in Perl, but you can read your code after you do it. That’s it—their domains largely overlap, but Python is more focused on producing readable code. For many, the enhanced readability of Python translates to better code reusability and maintainability, making Python a better choice for programs that will not be written once and thrown away. Perl code is easy to write, but difficult to read. Given that most software has a lifespan much longer than its initial creation, many see Python as a more effective tool. The somewhat longer story reflects the backgrounds of the designers of the two languages and underscores some of the main reasons people choose to use Python. Python’s creator is a mathematician by training; as such, he produced a language with a high degree of uniformity—its syntax and toolset are remarkably coherent. Moreover, like math, Python’s design is orthogonal—most of the language follows from a small set of core concepts. For instance, once one grasps Python’s flavor of polymorphism, the rest is largely just details. By contrast, the creator of the Perl language is a linguist, and its design reflects this heritage. There are many ways to accomplish the same tasks in Perl, and language constructs interact in context-sensitive and sometimes quite subtle ways—much like natural language. As the well-known Perl motto states, “There’s more than one way to do it.” Given this design, both the Perl language and its user community have historically encouraged freedom of expression when writing code. One person’s Perl code can be radically different from another’s. In fact, writing unique, tricky code is often a source of pride among Perl users. 20 | Chapter 1: A Python Q&A Session But as anyone who has done any substantial code maintenance should be able to attest, freedom of expression is great for art, but lousy for engineering. In engineering, we need a minimal feature set and predictability. In engineering, freedom of expression can lead to maintenance nightmares. As more than one Perl user has confided to me, the result of too much freedom is often code that is much easier to rewrite from scratch than to modify. Consider this: when people create a painting or a sculpture, they do so for themselves for purely aesthetic purposes. The possibility of someone else having to change that painting or sculpture later does not enter into it. This is a critical difference between art and engineering. When people write software, they are not writing it for themselves. In fact, they are not even writing primarily for the computer. Rather, good programmers know that code is written for the next human being who has to read it in order to maintain or reuse it. If that person cannot understand the code, it’s all but useless in a realistic development scenario. This is where many people find that Python most clearly differentiates itself from scripting languages like Perl. Because Python’s syntax model almost forces users to write readable code, Python programs lend themselves more directly to the full software development cycle. And because Python emphasizes ideas such as limited interactions, code uniformity and regularity, and feature consistency, it more directly fosters code that can be used long after it is first written. In the long run, Python’s focus on code quality in itself boosts programmer productivity, as well as programmer satisfaction. Python programmers can be creative, too, of course, and as we’ll see, the language does offer multiple solutions for some tasks. At its core, though, Python encourages good engineering in ways that other scripting languages often do not. At least, that’s the common consensus among many people who have adopted Python. You should always judge such claims for yourself, of course, by learning what Python has to offer. To help you get started, let’s move on to the next chapter. Test Your Knowledge: Answers | 21 CHAPTER 2 How Python Runs Programs This chapter and the next take a quick look at program execution—how you launch code, and how Python runs it. In this chapter, we’ll study the Python interpreter. Chapter 3 will then show you how to get your own programs up and running. Startup details are inherently platform-specific, and some of the material in these two chapters may not apply to the platform you work on, so you should feel free to skip parts not relevant to your intended use. Likewise, more advanced readers who have used similar tools in the past and prefer to get to the meat of the language quickly may want to file some of this chapter away as “for future reference.” For the rest of you, let’s learn how to run some code. Introducing the Python Interpreter So far, I’ve mostly been talking about Python as a programming language. But, as currently implemented, it’s also a software package called an interpreter. An interpreter is a kind of program that executes other programs. When you write a Python program, the Python interpreter reads your program and carries out the instructions it contains. In effect, the interpreter is a layer of software logic between your code and the computer hardware on your machine. When the Python package is installed on your machine, it generates a number of components—minimally, an interpreter and a support library. Depending on how you use it, the Python interpreter may take the form of an executable program, or a set of libraries linked into another program. Depending on which flavor of Python you run, the interpreter itself may be implemented as a C program, a set of Java classes, or something else. Whatever form it takes, the Python code you write must always be run by this interpreter. And to enable that, you must install a Python interpreter on your computer. Python installation details vary by platform and are covered in more depth in Appendix A. In short: 23 • Windows users fetch and run a self-installing executable file that puts Python on their machines. Simply double-click and say Yes or Next at all prompts. • Linux and Mac OS X users probably already have a usable Python preinstalled on their computers—it’s a standard component on these platforms today. • Some Linux and Mac OS X users (and most Unix users) compile Python from its full source code distribution package. • Linux users can also find RPM files, and Mac OS X users can find various Macspecific installation packages. • Other platforms have installation techniques relevant to those platforms. For instance, Python is available on cell phones, game consoles, and iPods, but installation details vary widely. Python itself may be fetched from the downloads page on the website, http://www .python.org. It may also be found through various other distribution channels. Keep in mind that you should always check to see whether Python is already present before installing it. If you’re working on Windows, you’ll usually find Python in the Start menu, as captured in Figure 2-1 (these menu options are discussed in the next chapter). On Unix and Linux, Python probably lives in your /usr directory tree. Because installation details are so platform-specific, we’ll finesse the rest of this story here. For more details on the installation process, consult Appendix A. For the purposes of this chapter and the next, I’ll assume that you’ve got Python ready to go. Program Execution What it means to write and run a Python script depends on whether you look at these tasks as a programmer, or as a Python interpreter. Both views offer important perspectives on Python programming. The Programmer’s View In its simplest form, a Python program is just a text file containing Python statements. For example, the following file, named script0.py, is one of the simplest Python scripts I could dream up, but it passes for a fully functional Python program: print('hello world') print(2 ** 100) This file contains two Python print statements, which simply print a string (the text in quotes) and a numeric expression result (2 to the power 100) to the output stream. Don’t worry about the syntax of this code yet—for this chapter, we’re interested only in getting it to run. I’ll explain the print statement, and why you can raise 2 to the power 100 in Python without overflowing, in the next parts of this book. 24 | Chapter 2: How Python Runs Programs Figure 2-1. When installed on Windows, this is how Python shows up in your Start button menu. This can vary a bit from release to release, but IDLE starts a development GUI, and Python starts a simple interactive session. Also here are the standard manuals and the PyDoc documentation engine (Module Docs). You can create such a file of statements with any text editor you like. By convention, Python program files are given names that end in .py; technically, this naming scheme is required only for files that are “imported,” as shown later in this book, but most Python files have .py names for consistency. After you’ve typed these statements into a text file, you must tell Python to execute the file—which simply means to run all the statements in the file from top to bottom, one after another. As you’ll see in the next chapter, you can launch Python program files Program Execution | 25 by shell command lines, by clicking their icons, from within IDEs, and with other standard techniques. If all goes well, when you execute the file, you’ll see the results of the two print statements show up somewhere on your computer—by default, usually in the same window you were in when you ran the program: hello world 1267650600228229401496703205376 For example, here’s what happened when I ran this script from a DOS command line on a Windows laptop (typically called a Command Prompt window, found in the Accessories program menu), to make sure it didn’t have any silly typos: C:\temp> python script0.py hello world 1267650600228229401496703205376 We’ve just run a Python script that prints a string and a number. We probably won’t win any programming awards with this code, but it’s enough to capture the basics of program execution. Python’s View The brief description in the prior section is fairly standard for scripting languages, and it’s usually all that most Python programmers need to know. You type code into text files, and you run those files through the interpreter. Under the hood, though, a bit more happens when you tell Python to “go.” Although knowledge of Python internals is not strictly required for Python programming, a basic understanding of the runtime structure of Python can help you grasp the bigger picture of program execution. When you instruct Python to run your script, there are a few steps that Python carries out before your code actually starts crunching away. Specifically, it’s first compiled to something called “byte code” and then routed to something called a “virtual machine.” Byte code compilation Internally, and almost completely hidden from you, when you execute a program Python first compiles your source code (the statements in your file) into a format known as byte code. Compilation is simply a translation step, and byte code is a lower-level, platform-independent representation of your source code. Roughly, Python translates each of your source statements into a group of byte code instructions by decomposing them into individual steps. This byte code translation is performed to speed execution—byte code can be run much more quickly than the original source code statements in your text file. You’ll notice that the prior paragraph said that this is almost completely hidden from you. If the Python process has write access on your machine, it will store the byte code of your programs in files that end with a .pyc extension (“.pyc” means compiled “.py” source). You will see these files show up on your computer after you’ve run a few 26 | Chapter 2: How Python Runs Programs programs alongside the corresponding source code files (that is, in the same directories). Python saves byte code like this as a startup speed optimization. The next time you run your program, Python will load the .pyc files and skip the compilation step, as long as you haven’t changed your source code since the byte code was last saved. Python automatically checks the timestamps of source and byte code files to know when it must recompile—if you resave your source code, byte code is automatically re-created the next time your program is run. If Python cannot write the byte code files to your machine, your program still works— the byte code is generated in memory and simply discarded on program exit.* However, because .pyc files speed startup time, you’ll want to make sure they are written for larger programs. Byte code files are also one way to ship Python programs—Python is happy to run a program if all it can find are .pyc files, even if the original .py source files are absent. (See “Frozen Binaries” on page 32 for another shipping option.) The Python Virtual Machine (PVM) Once your program has been compiled to byte code (or the byte code has been loaded from existing .pyc files), it is shipped off for execution to something generally known as the Python Virtual Machine (PVM, for the more acronym-inclined among you). The PVM sounds more impressive than it is; really, it’s not a separate program, and it need not be installed by itself. In fact, the PVM is just a big loop that iterates through your byte code instructions, one by one, to carry out their operations. The PVM is the runtime engine of Python; it’s always present as part of the Python system, and it’s the component that truly runs your scripts. Technically, it’s just the last step of what is called the “Python interpreter.” Figure 2-2 illustrates the runtime structure described here. Keep in mind that all of this complexity is deliberately hidden from Python programmers. Byte code compilation is automatic, and the PVM is just part of the Python system that you have installed on your machine. Again, programmers simply code and run files of statements. Performance implications Readers with a background in fully compiled languages such as C and C++ might notice a few differences in the Python model. For one thing, there is usually no build or “make” step in Python work: code runs immediately after it is written. For another, Python byte code is not binary machine code (e.g., instructions for an Intel chip). Byte code is a Python-specific representation. * And, strictly speaking, byte code is saved only for files that are imported, not for the top-level file of a program. We’ll explore imports in Chapter 3, and again in Part V. Byte code is also never saved for code typed at the interactive prompt, which is described in Chapter 3. Program Execution | 27 Figure 2-2. Python’s traditional runtime execution model: source code you type is translated to byte code, which is then run by the Python Virtual Machine. Your code is automatically compiled, but then it is interpreted. This is why some Python code may not run as fast as C or C++ code, as described in Chapter 1—the PVM loop, not the CPU chip, still must interpret the byte code, and byte code instructions require more work than CPU instructions. On the other hand, unlike in classic interpreters, there is still an internal compile step—Python does not need to reanalyze and reparse each source statement repeatedly. The net effect is that pure Python code runs at speeds somewhere between those of a traditional compiled language and a traditional interpreted language. See Chapter 1 for more on Python performance tradeoffs. Development implications Another ramification of Python’s execution model is that there is really no distinction between the development and execution environments. That is, the systems that compile and execute your source code are really one and the same. This similarity may have a bit more significance to readers with a background in traditional compiled languages, but in Python, the compiler is always present at runtime and is part of the system that runs programs. This makes for a much more rapid development cycle. There is no need to precompile and link before execution may begin; simply type and run the code. This also adds a much more dynamic flavor to the language—it is possible, and often very convenient, for Python programs to construct and execute other Python programs at runtime. The eval and exec built-ins, for instance, accept and run strings containing Python program code. This structure is also why Python lends itself to product customization—because Python code can be changed on the fly, users can modify the Python parts of a system onsite without needing to have or compile the entire system’s code. At a more fundamental level, keep in mind that all we really have in Python is runtime— there is no initial compile-time phase at all, and everything happens as the program is running. This even includes operations such as the creation of functions and classes and the linkage of modules. Such events occur before execution in more static languages, but happen as programs execute in Python. As we’ll see, the net effect makes for a much more dynamic programming experience than that to which some readers may be accustomed. 28 | Chapter 2: How Python Runs Programs Execution Model Variations Before moving on, I should point out that the internal execution flow described in the prior section reflects the standard implementation of Python today but is not really a requirement of the Python language itself. Because of that, the execution model is prone to changing with time. In fact, there are already a few systems that modify the picture in Figure 2-2 somewhat. Let’s take a few moments to explore the most prominent of these variations. Python Implementation Alternatives Really, as this book is being written, there are three primary implementations of the Python language—CPython, Jython, and IronPython—along with a handful of secondary implementations such as Stackless Python. In brief, CPython is the standard implementation; all the others have very specific purposes and roles. All implement the same Python language but execute programs in different ways. CPython The original, and standard, implementation of Python is usually called CPython, when you want to contrast it with the other two. Its name comes from the fact that it is coded in portable ANSI C language code. This is the Python that you fetch from http://www .python.org, get with the ActivePython distribution, and have automatically on most Linux and Mac OS X machines. If you’ve found a preinstalled version of Python on your machine, it’s probably CPython, unless your company is using Python in very specialized ways. Unless you want to script Java or .NET applications with Python, you probably want to use the standard CPython system. Because it is the reference implementation of the language, it tends to run the fastest, be the most complete, and be more robust than the alternative systems. Figure 2-2 reflects CPython’s runtime architecture. Jython The Jython system (originally known as JPython) is an alternative implementation of the Python language, targeted for integration with the Java programming language. Jython consists of Java classes that compile Python source code to Java byte code and then route the resulting byte code to the Java Virtual Machine (JVM). Programmers still code Python statements in .py text files as usual; the Jython system essentially just replaces the rightmost two bubbles in Figure 2-2 with Java-based equivalents. Jython’s goal is to allow Python code to script Java applications, much as CPython allows Python to script C and C++ components. Its integration with Java is remarkably seamless. Because Python code is translated to Java byte code, it looks and feels like a true Java program at runtime. Jython scripts can serve as web applets and servlets, build Java-based GUIs, and so on. Moreover, Jython includes integration support that allows Execution Model Variations | 29 Python code to import and use Java classes as though they were coded in Python. Because Jython is slower and less robust than CPython, though, it is usually seen as a tool of interest primarily to Java developers looking for a scripting language to be a frontend to Java code. IronPython A third implementation of Python, and newer than both CPython and Jython, IronPython is designed to allow Python programs to integrate with applications coded to work with Microsoft’s .NET Framework for Windows, as well as the Mono open source equivalent for Linux. .NET and its C# programming language runtime system are designed to be a language-neutral object communication layer, in the spirit of Microsoft’s earlier COM model. IronPython allows Python programs to act as both client and server components, accessible from other .NET languages. By implementation, IronPython is very much like Jython (and, in fact, was developed by the same creator)—it replaces the last two bubbles in Figure 2-2 with equivalents for execution in the .NET environment. Also, like Jython, IronPython has a special focus—it is primarily of interest to developers integrating Python with .NET components. Because it is being developed by Microsoft, though, IronPython might also be able to leverage some important optimization tools for better performance. IronPython’s scope is still evolving as I write this; for more details, consult the Python online resources or search the Web.† Execution Optimization Tools CPython, Jython, and IronPython all implement the Python language in similar ways: by compiling source code to byte code and executing the byte code on an appropriate virtual machine. Still other systems, including the Psyco just-in-time compiler and the Shedskin C++ translator, instead attempt to optimize the basic execution model. These systems are not required knowledge at this point in your Python career, but a quick look at their place in the execution model might help demystify the model in general. The Psyco just-in-time compiler The Psyco system is not another Python implementation, but rather a component that extends the byte code execution model to make programs run faster. In terms of Figure 2-2, Psyco is an enhancement to the PVM that collects and uses type information while the program runs to translate portions of the program’s byte code all the way down to real binary machine code for faster execution. Psyco accomplishes this † Jython and IronPython are completely independent implementations of Python that compile Python source for different runtime architectures. It is also possible to access Java and .NET software from standard CPython programs: JPype and Python for .NET systems, for example, allow CPython code to call out to Java and .NET components. 30 | Chapter 2: How Python Runs Programs translation without requiring changes to the code or a separate compilation step during development. Roughly, while your program runs, Psyco collects information about the kinds of objects being passed around; that information can be used to generate highly efficient machine code tailored for those object types. Once generated, the machine code then replaces the corresponding part of the original byte code to speed your program’s overall execution. The net effect is that, with Psyco, your program becomes much quicker over time and as it is running. In ideal cases, some Python code may become as fast as compiled C code under Psyco. Because this translation from byte code happens at program runtime, Psyco is generally known as a just-in-time (JIT) compiler. Psyco is actually a bit different from the JIT compilers some readers may have seen for the Java language, though. Really, Psyco is a specializing JIT compiler—it generates machine code tailored to the data types that your program actually uses. For example, if a part of your program uses different data types at different times, Psyco may generate a different version of machine code to support each different type combination. Psyco has been shown to speed Python code dramatically. According to its web page, Psyco provides “2x to 100x speed-ups, typically 4x, with an unmodified Python interpreter and unmodified source code, just a dynamically loadable C extension module.” Of equal significance, the largest speedups are realized for algorithmic code written in pure Python—exactly the sort of code you might normally migrate to C to optimize. With Psyco, such migrations become even less important. Psyco is not yet a standard part of Python; you will have to fetch and install it separately. It is also still something of a research project, so you’ll have to track its evolution online. In fact, at this writing, although Psyco can still be fetched and installed by itself, it appears that much of the system may eventually be absorbed into the newer “PyPy” project—an attempt to reimplement Python’s PVM in Python code, to better support optimizations like Psyco. Perhaps the largest downside of Psyco is that it currently only generates machine code for Intel x86 architecture chips, though this includes Windows and Linux boxes and recent Macs. For more details on the Psyco extension, and other JIT efforts that may arise, consult http://www.python.org; you can also check out Psyco’s home page, which currently resides at http://psyco.sourceforge.net. The Shedskin C++ translator Shedskin is an emerging system that takes a different approach to Python program execution—it attempts to translate Python source code to C++ code, which your computer’s C++ compiler then compiles to machine code. As such, it represents a platformneutral approach to running Python code. Shedskin is still somewhat experimental as I write these words, and it limits Python programs to an implicit statically typed constraint that is technically not normal Python, so we won’t go into further detail here. Execution Model Variations | 31 Initial results, though, show that it has the potential to outperform both standard Python and the Psyco extension in terms of execution speed, and it is a promising project. Search the Web for details on the project’s current status. Frozen Binaries Sometimes when people ask for a “real” Python compiler, what they’re really seeking is simply a way to generate standalone binary executables from their Python programs. This is more a packaging and shipping idea than an execution-flow concept, but it’s somewhat related. With the help of third-party tools that you can fetch off the Web, it is possible to turn your Python programs into true executables, known as frozen binaries in the Python world. Frozen binaries bundle together the byte code of your program files, along with the PVM (interpreter) and any Python support files your program needs, into a single package. There are some variations on this theme, but the end result can be a single binary executable program (e.g., an .exe file on Windows) that can easily be shipped to customers. In Figure 2-2, it is as though the byte code and PVM are merged into a single component—a frozen binary file. Today, three primary systems are capable of generating frozen binaries: py2exe (for Windows), PyInstaller (which is similar to py2exe but also works on Linux and Unix and is capable of generating self-installing binaries), and freeze (the original). You may have to fetch these tools separately from Python itself, but they are available free of charge. They are also constantly evolving, so consult http://www.python.org or your favorite web search engine for more on these tools. To give you an idea of the scope of these systems, py2exe can freeze standalone programs that use the tkinter, PMW, wxPython, and PyGTK GUI libraries; programs that use the pygame game programming toolkit; win32com client programs; and more. Frozen binaries are not the same as the output of a true compiler—they run byte code through a virtual machine. Hence, apart from a possible startup improvement, frozen binaries run at the same speed as the original source files. Frozen binaries are not small (they contain a PVM), but by current standards they are not unusually large either. Because Python is embedded in the frozen binary, though, it does not have to be installed on the receiving end to run your program. Moreover, because your code is embedded in the frozen binary, it is more effectively hidden from recipients. This single file-packaging scheme is especially appealing to developers of commercial software. For instance, a Python-coded user interface program based on the tkinter toolkit can be frozen into an executable file and shipped as a self-contained program on a CD or on the Web. End users do not need to install (or even have to know about) Python to run the shipped program. 32 | Chapter 2: How Python Runs Programs Other Execution Options Still other schemes for running Python programs have more focused goals: • The Stackless Python system is a standard CPython implementation variant that does not save state on the C language call stack. This makes Python more easy to port to small stack architectures, provides efficient multiprocessing options, and fosters novel programming structures such as coroutines. • The Cython system (based on work done by the Pyrex project) is a hybrid language that combines Python code with the ability to call C functions and use C type declarations for variables, parameters, and class attributes. Cython code can be compiled to C code that uses the Python/C API, which may then be compiled completely. Though not completely compatible with standard Python, Cython can be useful both for wrapping external C libraries and for coding efficient C extensions for Python. For more details on these systems, search the Web for recent links. Future Possibilities? Finally, note that the runtime execution model sketched here is really an artifact of the current implementation of Python, not of the language itself. For instance, it’s not impossible that a full, traditional compiler for translating Python source code to machine code may appear during the shelf life of this book (although one has not in nearly two decades!). New byte code formats and implementation variants may also be adopted in the future. For instance: • The Parrot project aims to provide a common byte code format, virtual machine, and optimization techniques for a variety of programming languages (see http:// www.python.org). Python’s own PVM runs Python code more efficiently than Parrot, but it’s unclear how Parrot will evolve. • The PyPy project is an attempt to reimplement the PVM in Python itself to enable new implementation techniques. Its goal is to produce a fast and flexible implementation of Python. • The Google-sponsored Unladen Swallow project aims to make standard Python faster by a factor of at least 5, and fast enough to replace the C language in many contexts. It is an optimization branch of CPython, intended to be fully compatible and significantly faster. This project also hopes to remove the Python multithreading Global Interpreter Lock (GIL), which prevents pure Python threads from truly overlapping in time. This is currently an emerging project being developed as open source by Google engineers; it is initially targeting Python 2.6, though 3.0 may acquire its changes too. Search Google for up-to-date details. Although such future implementation schemes may alter the runtime structure of Python somewhat, it seems likely that the byte code compiler will still be the standard for Execution Model Variations | 33 some time to come. The portability and runtime flexibility of byte code are important features of many Python systems. Moreover, adding type constraint declarations to support static compilation would break the flexibility, conciseness, simplicity, and overall spirit of Python coding. Due to Python’s highly dynamic nature, any future implementation will likely retain many artifacts of the current PVM. Chapter Summary This chapter introduced the execution model of Python (how Python runs your programs) and explored some common variations on that model (just-in-time compilers and the like). Although you don’t really need to come to grips with Python internals to write Python scripts, a passing acquaintance with this chapter’s topics will help you truly understand how your programs run once you start coding them. In the next chapter, you’ll start actually running some code of your own. First, though, here’s the usual chapter quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. What is the Python interpreter? What is source code? What is byte code? What is the PVM? Name two variations on Python’s standard execution model. How are CPython, Jython, and IronPython different? Test Your Knowledge: Answers 1. The Python interpreter is a program that runs the Python programs you write. 2. Source code is the statements you write for your program—it consists of text in text files that normally end with a .py extension. 3. Byte code is the lower-level form of your program after Python compiles it. Python automatically stores byte code in files with a .pyc extension. 4. The PVM is the Python Virtual Machine—the runtime engine of Python that interprets your compiled byte code. 5. Psyco, Shedskin, and frozen binaries are all variations on the execution model. 6. CPython is the standard implementation of the language. Jython and IronPython implement Python programs for use in Java and .NET environments, respectively; they are alternative compilers for Python. 34 | Chapter 2: How Python Runs Programs CHAPTER 3 How You Run Programs OK, it’s time to start running some code. Now that you have a handle on program execution, you’re finally ready to start some real Python programming. At this point, I’ll assume that you have Python installed on your computer; if not, see the prior chapter and Appendix A for installation and configuration hints. There are a variety of ways to tell Python to execute the code you type. This chapter discusses all the program launching techniques in common use today. Along the way, you’ll learn how to type code interactively and how to save it in files to be run with system command lines, icon clicks, module imports and reloads, exec calls, menu options in GUIs such as IDLE, and more. If you just want to find out how to run a Python program quickly, you may be tempted to read the parts of this chapter that pertain only to your platform and move on to Chapter 4. But don’t skip the material on module imports, as that’s essential to understanding Python’s program architecture. I also encourage you to at least skim the sections on IDLE and other IDEs, so you’ll know what tools are available for when you start developing more sophisticated Python programs. The Interactive Prompt Perhaps the simplest way to run Python programs is to type them at Python’s interactive command line, sometimes called the interactive prompt. There are a variety of ways to start this command line: in an IDE, from a system console, and so on. Assuming the interpreter is installed as an executable program on your system, the most platformneutral way to start an interactive interpreter session is usually just to type python at your operating system’s prompt, without any arguments. For example: 35 % python Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ... Type "help", "copyright", "credits" or "license" for more information. >>> Typing the word “python” at your system shell prompt like this begins an interactive Python session; the “%” character at the start of this listing stands for a generic system prompt in this book—it’s not input that you type yourself. The notion of a system shell prompt is generic, but exactly how you access it varies by platform: • On Windows, you can type python in a DOS console window (a.k.a. the Command Prompt, usually found in the Accessories section of the Start→Programs menu) or in the Start→Run... dialog box. • On Unix, Linux, and Mac OS X, you might type this command in a shell or terminal window (e.g., in an xterm or console running a shell such as ksh or csh). • Other systems may use similar or platform-specific devices. On handheld devices, for example, you generally click the Python icon in the home or application window to launch an interactive session. If you have not set your shell’s PATH environment variable to include Python’s install directory, you may need to replace the word “python” with the full path to the Python executable on your machine. On Unix, Linux, and similar, /usr/local/bin/python or /usr/bin/python will often suffice. On Windows, try typing C:\Python30\python (for version 3.0): C:\misc> c:\python30\python Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ... Type "help", "copyright", "credits" or "license" for more information. >>> Alternatively, you can run a change-directory command to go to Python’s install directory before typing “python”—try the cd c:\python30 command on Windows, for example: C:\misc> cd C:\Python30 C:\Python30> python Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ... Type "help", "copyright", "credits" or "license" for more information. >>> On Windows, besides typing python in a shell window, you can also begin similar interactive sessions by starting IDLE’s main window (discussed later) or by selecting the “Python (command line)” menu option from the Start button menu for Python, as shown in Figure 2-1 back in Chapter 2. Both spawn a Python interactive prompt with equivalent functionality; typing a shell command isn’t necessary. 36 | Chapter 3: How You Run Programs Running Code Interactively However it’s started, the Python interactive session begins by printing two lines of informational text (which I’ll omit from most of this book’s examples to save space), then prompts for input with >>> when it’s waiting for you to type a new Python statement or expression. When working interactively, the results of your code are displayed after the >>> lines after you press the Enter key. For instance, here are the results of two Python print statements (print is really a function call in Python 3.0, but not in 2.6, so the parentheses here are required in 3.0 only): % python >>> print('Hello world!') Hello world! >>> print(2 ** 8) 256 Again, you don’t need to worry about the details of the print statements shown here yet; we’ll start digging into syntax in the next chapter. In short, they print a Python string and an integer, as shown by the output lines that appear after each >>> input line (2 ** 8 means 2 raised to the power 8 in Python). When coding interactively like this, you can type as many Python commands as you like; each is run immediately after it’s entered. Moreover, because the interactive session automatically prints the results of expressions you type, you don’t usually need to say “print” explicitly at this prompt: >>> lumberjack = 'okay' >>> lumberjack 'okay' >>> 2 ** 8 256 >>> % <== Use Ctrl-D (on Unix) or Ctrl-Z (on Windows) to exit Here, the fist line saves a value by assigning it to a variable, and the last two lines typed are expressions (lumberjack and 2 ** 8)—their results are displayed automatically. To exit an interactive session like this one and return to your system shell prompt, type Ctrl-D on Unix-like machines; on MS-DOS and Windows systems, type Ctrl-Z to exit. In the IDLE GUI discussed later, either type Ctrl-D or simply close the window. Now, we didn’t do much in this session’s code—just typed some Python print and assignment statements, along with a few expressions, which we’ll study in detail later. The main thing to notice is that the interpreter executes the code entered on each line immediately, when the Enter key is pressed. The Interactive Prompt | 37 For example, when we typed the first print statement at the >>> prompt, the output (a Python string) was echoed back right away. There was no need to create a source-code file, and no need to run the code through a compiler and linker first, as you’d normally do when using a language such as C or C++. As you’ll see in later chapters, you can also run multiline statements at the interactive prompt; such a statement runs immediately after you’ve entered all of its lines and pressed Enter twice to add a blank line. Why the Interactive Prompt? The interactive prompt runs code and echoes results as you go, but it doesn’t save your code in a file. Although this means you won’t do the bulk of your coding in interactive sessions, the interactive prompt turns out to be a great place to both experiment with the language and test program files on the fly. Experimenting Because code is executed immediately, the interactive prompt is a perfect place to experiment with the language and will be used often in this book to demonstrate smaller examples. In fact, this is the first rule of thumb to remember: if you’re ever in doubt about how a piece of Python code works, fire up the interactive command line and try it out to see what happens. For instance, suppose you’re reading a Python program’s code and you come across an expression like 'Spam!' * 8 whose meaning you don’t understand. At this point, you can spend 10 minutes wading through manuals and books to try to figure out what the code does, or you can simply run it interactively: >>> 'Spam!' * 8 'Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam!' <== Learning by trying The immediate feedback you receive at the interactive prompt is often the quickest way to deduce what a piece of code does. Here, it’s clear that it does string repetition: in Python * means multiply for numbers, but repeat for strings—it’s like concatenating a string to itself repeatedly (more on strings in Chapter 4). Chances are good that you won’t break anything by experimenting this way—at least, not yet. To do real damage, like deleting files and running shell commands, you must really try, by importing modules explicitly (you also need to know more about Python’s system interfaces in general before you will become that dangerous!). Straight Python code is almost always safe to run. For instance, watch what happens when you make a mistake at the interactive prompt: 38 | Chapter 3: How You Run Programs >>> X Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'X' is not defined <== Making mistakes In Python, using a variable before it has been assigned a value is always an error (otherwise, if names were filled in with defaults, some errors might go undetected). We’ll learn more about that later; the important point here is that you don’t crash Python or your computer when you make a mistake this way. Instead, you get a meaningful error message pointing out the mistake and the line of code that made it, and you can continue on in your session or script. In fact, once you get comfortable with Python, its error messages may often provide as much debugging support as you’ll need (you’ll read more on debugging in the sidebar “Debugging Python Code” on page 67). Testing Besides serving as a tool for experimenting while you’re learning the language, the interactive interpreter is also an ideal place to test code you’ve written in files. You can import your module files interactively and run tests on the tools they define by typing calls at the interactive prompt. For instance, of the following tests a function in a precoded module that ships with Python in its standard library (it prints the name of the directory you’re currently working in), but you can do the same once you start writing module files of your own: >>> import os >>> os.getcwd() 'c:\\Python30' <== Testing on the fly More generally, the interactive prompt is a place to test program components, regardless of their source—you can import and test functions and classes in your Python files, type calls to linked-in C functions, exercise Java classes under Jython, and more. Partly because of its interactive nature, Python supports an experimental and exploratory programming style you’ll find convenient when getting started. Using the Interactive Prompt Although the interactive prompt is simple to use, there are a few tips that beginners should keep in mind. I’m including lists of common mistakes like this in this chapter for reference, but they might also spare you from a few headaches if you read them up front: • Type Python commands only. First of all, remember that you can only type Python code at the Python prompt, not system commands. There are ways to run system commands from within Python code (e.g., with os.system), but they are not as direct as simply typing the commands themselves. The Interactive Prompt | 39 • print statements are required only in files. Because the interactive interpreter automatically prints the results of expressions, you do not need to type complete print statements interactively. This is a nice feature, but it tends to confuse users when they move on to writing code in files: within a code file, you must use print statements to see your output because expression results are not automatically echoed. Remember, you must say print in files, but not interactively. • Don’t indent at the interactive prompt (yet). When typing Python programs, either interactively or into a text file, be sure to start all your unnested statements in column 1 (that is, all the way to the left). If you don’t, Python may print a “SyntaxError” message, because blank space to the left of your code is taken to be indentation that groups nested statements. Until Chapter 10, all statements you write will be unnested, so this includes everything for now. This seems to be a recurring confusion in introductory Python classes. Remember, a leading space generates an error message. • Watch out for prompt changes for compound statements. We won’t meet compound (multiline) statements until Chapter 4, and not in earnest until Chapter 10, but as a preview, you should know that when typing lines 2 and beyond of a compound statement interactively, the prompt may change. In the simple shell window interface, the interactive prompt changes to ... instead of >>> for lines 2 and beyond; in the IDLE interface, lines after the first are automatically indented. You’ll see why this matters in Chapter 10. For now, if you happen to come across a ... prompt or a blank line when entering your code, it probably means that you’ve somehow confused interactive Python into thinking you’re typing a multiline statement. Try hitting the Enter key or a Ctrl-C combination to get back to the main prompt. The >>> and ... prompt strings can also be changed (they are available in the built-in module sys), but I’ll assume they have not been in the book’s example listings. • Terminate compound statements at the interactive prompt with a blank line. At the interactive prompt, inserting a blank line (by hitting the Enter key at the start of a line) is necessary to tell interactive Python that you’re done typing the multiline statement. That is, you must press Enter twice to make a compound statement run. By contrast, blank lines are not required in files and are simply ignored if present. If you don’t press Enter twice at the end of a compound statement when working interactively, you’ll appear to be stuck in a limbo state, because the interactive interpreter will do nothing at all—it’s waiting for you to press Enter again! • The interactive prompt runs one statement at a time. At the interactive prompt, you must run one statement to completion before typing another. This is natural for simple statements, because pressing the Enter key runs the statement entered. For compound statements, though, remember that you must submit a blank line to terminate the statement and make it run before you can type the next statement. 40 | Chapter 3: How You Run Programs Entering multiline statements At the risk of repeating myself, I received emails from readers who’d gotten burned by the last two points as I was updating this chapter, so it probably merits emphasis. I’ll introduce multiline (a.k.a. compound) statements in the next chapter, and we’ll explore their syntax more formally later in this book. Because their behavior differs slightly in files and at the interactive prompt, though, two cautions are in order here. First, be sure to terminate multiline compound statements like for loops and if tests at the interactive prompt with a blank line. You must press the Enter key twice, to terminate the whole multiline statement and then make it run. For example (pun not intended...): >>> for x in 'spam': ... print(x) ... <== Press Enter twice here to make this loop run You don’t need the blank line after compound statements in a script file, though; this is required only at the interactive prompt. In a file, blank lines are not required and are simply ignored when present; at the interactive prompt, they terminate multiline statements. Also bear in mind that the interactive prompt runs just one statement at a time: you must press Enter twice to run a loop or other multiline statement before you can type the next statement: >>> for x in 'spam': ... print(x) ... print('done') File "<stdin>", line 3 print('done') ^ SyntaxError: invalid syntax <== Need to press Enter twice before a new statement This means you can’t cut and paste multiple lines of code into the interactive prompt, unless the code includes blank lines after each compound statement. Such code is better run in a file—the next section’s topic. System Command Lines and Files Although the interactive prompt is great for experimenting and testing, it has one big disadvantage: programs you type there go away as soon as the Python interpreter executes them. Because the code you type interactively is never stored in a file, you can’t run it again without retyping it from scratch. Cut-and-paste and command recall can help some here, but not much, especially when you start writing larger programs. To cut and paste code from an interactive session, you would have to edit out Python prompts, program outputs, and so on—not exactly a modern software development methodology! System Command Lines and Files | 41 To save programs permanently, you need to write your code in files, which are usually known as modules. Modules are simply text files containing Python statements. Once coded, you can ask the Python interpreter to execute the statements in such a file any number of times, and in a variety of ways—by system command lines, by file icon clicks, by options in the IDLE user interface, and more. Regardless of how it is run, Python executes all the code in a module file from top to bottom each time you run the file. Terminology in this domain can vary somewhat. For instance, module files are often referred to as programs in Python—that is, a program is considered to be a series of precoded statements stored in a file for repeated execution. Module files that are run directly are also sometimes called scripts—an informal term usually meaning a top-level program file. Some reserve the term “module” for a file imported from another file. (More on the meaning of “top-level” and imports in a few moments.) Whatever you call them, the next few sections explore ways to run code typed into module files. In this section, you’ll learn how to run files in the most basic way: by listing their names in a python command line entered at your computer’s system prompt. Though it might seem primitive to some, for many programmers a system shell command-line window, together with a text editor window, constitutes as much of an integrated development environment as they will ever need. A First Script Let’s get started. Open your favorite text editor (e.g., vi, Notepad, or the IDLE editor), and type the following statements into a new text file named script1.py: # A first Python script import sys print(sys.platform) print(2 ** 100) x = 'Spam!' print(x * 8) # Load a library module # Raise 2 to a power # String repetition This file is our first official Python script (not counting the two-liner in Chapter 2). You shouldn’t worry too much about this file’s code, but as a brief description, this file: • Imports a Python module (libraries of additional tools), to fetch the name of the platform • Runs three print function calls, to display the script’s results • Uses a variable named x, created when it’s assigned, to hold onto a string object • Applies various object operations that we’ll begin studying in the next chapter The sys.platform here is just a string that identifies the kind of computer you’re working on; it lives in a standard Python module called sys, which you must import to load (again, more on imports later). 42 | Chapter 3: How You Run Programs For color, I’ve also added some formal Python comments here—the text after the # characters. Comments can show up on lines by themselves, or to the right of code on a line. The text after a # is simply ignored as a human-readable comment and is not considered part of the statement’s syntax. If you’re copying this code, you can ignore the comments as well. In this book, we usually use a different formatting style to make comments more visually distinctive, but they’ll appear as normal text in your code. Again, don’t focus on the syntax of the code in this file for now; we’ll learn about all of it later. The main point to notice is that you’ve typed this code into a file, rather than at the interactive prompt. In the process, you’ve coded a fully functional Python script. Notice that the module file is called script1.py. As for all top-level files, it could also be called simply script, but files of code you want to import into a client have to end with a .py suffix. We’ll study imports later in this chapter. Because you may want to import them in the future, it’s a good idea to use .py suffixes for most Python files that you code. Also, some text editors detect Python files by their .py suffix; if the suffix is not present, you may not get features like syntax colorization and automatic indentation. Running Files with Command Lines Once you’ve saved this text file, you can ask Python to run it by listing its full filename as the first argument to a python command, typed at the system shell prompt: % python script1.py win32 1267650600228229401496703205376 Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam! Again, you can type such a system shell command in whatever your system provides for command-line entry—a Windows Command Prompt window, an xterm window, or similar. Remember to replace “python” with a full directory path, as before, if your PATH setting is not configured. If all works as planned, this shell command makes Python run the code in this file line by line, and you will see the output of the script’s three print statements—the name of the underlying platform, 2 raised to the power 100, and the result of the same string repetition expression we saw earlier (again, more on the last two of these in Chapter 4). If all didn’t work as planned, you’ll get an error message—make sure you’ve entered the code in your file exactly as shown, and try again. We’ll talk about debugging options in the sidebar “Debugging Python Code” on page 67, but at this point in the book your best bet is probably rote imitation. Because this scheme uses shell command lines to start Python programs, all the usual shell syntax applies. For instance, you can route the output of a Python script to a file to save it for later use or inspection by using special shell syntax: % python script1.py > saveit.txt System Command Lines and Files | 43 In this case, the three output lines shown in the prior run are stored in the file saveit.txt instead of being printed. This is generally known as stream redirection; it works for input and output text and is available on Windows and Unix-like systems. It also has little to do with Python (Python simply supports it), so we will skip further details on shell redirection syntax here. If you are working on a Windows platform, this example works the same, but the system prompt is normally different: C:\Python30> python script1.py win32 1267650600228229401496703205376 Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam! As usual, be sure to type the full path to Python if you haven’t set your PATH environment variable to include this path or run a change-directory command to go to the path: D:\temp> C:\python30\python script1.py win32 1267650600228229401496703205376 Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam! On all recent versions of Windows, you can also type just the name of your script, and omit the name of Python itself. Because newer Windows systems use the Windows Registry to find a program with which to run a file, you don’t need to name “python” on the command line explicitly to run a .py file. The prior command, for example, could be simplified to this on most Windows machines: D:\temp> script1.py Finally, remember to give the full path to your script file if it lives in a different directory from the one in which you are working. For example, the following system command line, run from D:\other, assumes Python is in your system path but runs a file located elsewhere: D:\other> python c:\code\otherscript.py If your PATH doesn’t include Python’s directory, and neither Python nor your script file is in the directory you’re working in, use full paths for both: D:\other> C:\Python30\python c:\code\otherscript.py Using Command Lines and Files Running program files from system command lines is also a fairly straightforward launch option, especially if you are familiar with command lines in general from prior work. For newcomers, though, here are a few pointers about common beginner traps that might help you avoid some frustration: 44 | Chapter 3: How You Run Programs • Beware of automatic extensions on Windows. If you use the Notepad program to code program files on Windows, be careful to pick the type All Files when it comes time to save your file, and give the file a .py suffix explicitly. Otherwise, Notepad will save your file with a .txt extension (e.g., as script1.py.txt), making it difficult to run in some launching schemes. Worse, Windows hides file extensions by default, so unless you have changed your view options you may not even notice that you’ve coded a text file and not a Python file. The file’s icon may give this away—if it doesn’t have a snake on it, you may have trouble. Uncolored code in IDLE and files that open to edit instead of run when clicked are other symptoms of this problem. Microsoft Word similarly adds a .doc extension by default; much worse, it adds formatting characters that are not legal Python syntax. As a rule of thumb, always pick All Files when saving under Windows, or use a more programmer-friendly text editor such as IDLE. IDLE does not even add a .py suffix automatically—a feature programmers tend to like, but users do not. • Use file extensions and directory paths at system prompts, but not for imports. Don’t forget to type the full name of your file in system command lines— that is, use python script1.py rather than python script1. By contrast, Python’s import statements, which we’ll meet later in this chapter, omit both the .py file suffix and the directory path (e.g., import script1). This may seem trivial, but confusing these two is a common mistake. At the system prompt, you are in a system shell, not Python, so Python’s module file search rules do not apply. Because of that, you must include both the .py extension and, if necessary, the full directory path leading to the file you wish to run. For instance, to run a file that resides in a different directory from the one in which you are working, you would typically list its full path (e.g., python d:\tests\spam.py). Within Python code, however, you can just say import spam and rely on the Python module search path to locate your file, as described later. • Use print statements in files. Yes, we’ve already been over this, but it is such a common mistake that it’s worth repeating at least once here. Unlike in interactive coding, you generally must use print statements to see output from program files. If you don’t see any output, make sure you’ve said “print” in your file. Again, though, print statements are not required in an interactive session, since Python automatically echoes expression results; prints don’t hurt here, but are superfluous extra typing. System Command Lines and Files | 45 Unix Executable Scripts (#!) If you are going to use Python on a Unix, Linux, or Unix-like system, you can also turn files of Python code into executable programs, much as you would for programs coded in a shell language such as csh or ksh. Such files are usually called executable scripts. In simple terms, Unix-style executable scripts are just normal text files containing Python statements, but with two special properties: • Their first line is special. Scripts usually start with a line that begins with the characters #! (often called “hash bang”), followed by the path to the Python interpreter on your machine. • They usually have executable privileges. Script files are usually marked as executable to tell the operating system that they may be run as top-level programs. On Unix systems, a command such as chmod +x file.py usually does the trick. Let’s look at an example for Unix-like systems. Use your text editor again to create a file of Python code called brian: #!/usr/local/bin/python print('The Bright Side ' + 'of Life...') # + means concatenate for strings The special line at the top of the file tells the system where the Python interpreter lives. Technically, the first line is a Python comment. As mentioned earlier, all comments in Python programs start with a # and span to the end of the line; they are a place to insert extra information for human readers of your code. But when a comment such as the first line in this file appears, it’s special because the operating system uses it to find an interpreter for running the program code in the rest of the file. Also, note that this file is called simply brian, without the .py suffix used for the module file earlier. Adding a .py to the name wouldn’t hurt (and might help you remember that this is a Python program file), but because you don’t plan on letting other modules import the code in this file, the name of the file is irrelevant. If you give the file executable privileges with a chmod +x brian shell command, you can run it from the operating system shell as though it were a binary program: % brian The Bright Side of Life... A note for Windows users: the method described here is a Unix trick, and it may not work on your platform. Not to worry; just use the basic command-line technique explored earlier. List the file’s name on an explicit python command line:* * As we discussed when exploring command lines, modern Windows versions also let you type just the name of a .py file at the system command line—they use the Registry to determine that the file should be opened with Python (e.g., typing brian.py is equivalent to typing python brian.py). This command-line mode is similar in spirit to the Unix #!, though it is system-wide on Windows, not per-file. Note that some programs may actually interpret and use a first #! line on Windows much like on Unix, but the DOS system shell on Windows simply ignores it. 46 | Chapter 3: How You Run Programs C:\misc> python brian The Bright Side of Life... In this case, you don’t need the special #! comment at the top (although Python just ignores it if it’s present), and the file doesn’t need to be given executable privileges. In fact, if you want to run files portably between Unix and Microsoft Windows, your life will probably be simpler if you always use the basic command-line approach, not Unixstyle scripts, to launch programs. The Unix env Lookup Trick On some Unix systems, you can avoid hardcoding the path to the Python interpreter by writing the special first-line comment like this: #!/usr/bin/env python ...script goes here... When coded this way, the env program locates the Python interpreter according to your system search path settings (i.e., in most Unix shells, by looking in all the directories listed in the PATH environment variable). This scheme can be more portable, as you don’t need to hardcode a Python install path in the first line of all your scripts. Provided you have access to env everywhere, your scripts will run no matter where Python lives on your system—you need only change the PATH environment variable settings across platforms, not in the first line in all your scripts. Of course, this assumes that env lives in the same place everywhere (on some machines, it may be in /sbin, /bin, or elsewhere); if not, all portability bets are off! Clicking File Icons On Windows, the Registry makes opening files with icon clicks easy. Python automatically registers itself to be the program that opens Python program files when they are clicked. Because of that, it is possible to launch the Python programs you write by simply clicking (or double-clicking) on their file icons with your mouse cursor. On non-Windows systems, you will probably be able to perform a similar trick, but the icons, file explorer, navigation schemes, and more may differ slightly. On some Unix systems, for instance, you may need to register the .py extension with your file explorer GUI, make your script executable using the #! trick discussed in the previous section, or associate the file MIME type with an application or command by editing files, installing programs, or using other tools. See your file explorer’s documentation for more details if clicks do not work correctly right off the bat. Clicking Icons on Windows To illustrate, let’s keep using the script we wrote earlier, script1.py, repeated here to minimize page flipping: Clicking File Icons | 47 # A first Python script import sys print(sys.platform) print(2 ** 100) x = 'Spam!' print(x * 8) # Load a library module # Raise 2 to a power # String repetition As we’ve seen, you can always run this file from a system command line: C:\misc> c:\python30\python script1.py win32 1267650600228229401496703205376 However, icon clicks allow you to run the file without any typing at all. If you find this file’s icon—for instance, by selecting Computer (or My Computer in XP) in your Start menu and working your way down on the C drive on Windows—you will get the file explorer picture captured in Figure 3-1 (Windows Vista is being used here). Python source files show up with white backgrounds on Windows, and byte code files show up with black backgrounds. You will normally want to click (or otherwise run) the source code file, in order to pick up your most recent changes. To launch the file here, simply click on the icon for script1.py. Figure 3-1. On Windows, Python program files show up as icons in file explorer windows and can automatically be run with a double-click of the mouse (though you might not see printed output or error messages this way). 48 | Chapter 3: How You Run Programs The input Trick Unfortunately, on Windows, the result of clicking on a file icon may not be incredibly satisfying. In fact, as it is, this example script generates a perplexing “flash” when clicked—not exactly the sort of feedback that budding Python programmers usually hope for! This is not a bug, but has to do with the way the Windows version of Python handles printed output. By default, Python generates a pop-up black DOS console window to serve as a clicked file’s input and output. If a script just prints and exits, well, it just prints and exits— the console window appears, and text is printed there, but the console window closes and disappears on program exit. Unless you are very fast, or your machine is very slow, you won’t get to see your output at all. Although this is normal behavior, it’s probably not what you had in mind. Luckily, it’s easy to work around this. If you need your script’s output to stick around when you launch it with an icon click, simply put a call to the built-in input function at the very bottom of the script (raw_input in 2.6: see the note ahead). For example: # A first Python script import sys print(sys.platform) print(2 ** 100) x = 'Spam!' print(x * 8) input() # Load a library module # Raise 2 to a power # String repetition # <== ADDED In general, input reads the next line of standard input, waiting if there is none yet available. The net effect in this context will be to pause the script, thereby keeping the output window shown in Figure 3-2 open until you press the Enter key. Figure 3-2. When you click a program’s icon on Windows, you will be able to see its printed output if you include an input call at the very end of the script. But you only need to do so in this context! Clicking File Icons | 49 Now that I’ve shown you this trick, keep in mind that it is usually only required for Windows, and then only if your script prints text and exits and only if you will launch the script by clicking its file icon. You should add this call to the bottom of your toplevel files if and only if all of these three conditions apply. There is no reason to add this call in any other contexts (unless you’re unreasonably fond of pressing your computer’s Enter key!).† That may sound obvious, but it’s another common mistake in live classes. Before we move ahead, note that the input call applied here is the input counterpart of using the print statement for outputs. It is the simplest way to read user input, and it is more general than this example implies. For instance, input: • Optionally accepts a string that will be printed as a prompt (e.g., input('Press Enter to exit')) • Returns to your script a line of text read as a string (e.g., nextinput = input()) • Supports input stream redirections at the system shell level (e.g., python spam.py < input.txt), just as the print statement does for output We’ll use input in more advanced ways later in this text; for instance, Chapter 10 will apply it in an interactive loop. Version skew note: If you are working in Python 2.6 or earlier, use raw_input() instead of input() in this code. The former was renamed to the latter in Python 3.0. Technically, 2.6 has an input too, but it also evaluates strings as though they are program code typed into a script, and so will not work in this context (an empty string is an error). Python 3.0’s input (and 2.6’s raw_input) simply returns the entered text as a string, unevaluated. To simulate 2.6’s input in 3.0, use eval(input()). Other Icon-Click Limitations Even with the input trick, clicking file icons is not without its perils. You also may not get to see Python error messages. If your script generates an error, the error message text is written to the pop-up console window—which then immediately disappears! Worse, adding an input call to your file will not help this time because your script will likely abort long before it reaches this call. In other words, you won’t be able to tell what went wrong. † It is also possible to completely suppress the pop-up DOS console window for clicked files on Windows. Files whose names end in a .pyw extension will display only windows constructed by your script, not the default DOS console window. .pyw files are simply .py source files that have this special operational behavior on Windows. They are mostly used for Python-coded user interfaces that build windows of their own, often in conjunction with various techniques for saving printed output and errors to files. 50 | Chapter 3: How You Run Programs Because of these limitations, it is probably best to view icon clicks as a way to launch programs after they have been debugged or have been instrumented to write their output to a file. Especially when starting out, use other techniques—such as system command lines and IDLE (discussed further in the section “The IDLE User Interface” on page 58)—so that you can see generated error messages and view your normal output without resorting to coding tricks. When we discuss exceptions later in this book, you’ll also learn that it is possible to intercept and recover from errors so that they do not terminate your programs. Watch for the discussion of the try statement later in this book for an alternative way to keep the console window from closing on errors. Module Imports and Reloads So far, I’ve been talking about “importing modules” without really explaining what this term means. We’ll study modules and larger program architecture in depth in Part V, but because imports are also a way to launch programs, this section will introduce enough module basics to get you started. In simple terms, every file of Python source code whose name ends in a .py extension is a module. Other files can access the items a module defines by importing that module; import operations essentially load another file and grant access to that file’s contents. The contents of a module are made available to the outside world through its attributes (a term I’ll define in the next section). This module-based services model turns out to be the core idea behind program architecture in Python. Larger programs usually take the form of multiple module files, which import tools from other module files. One of the modules is designated as the main or top-level file, and this is the one launched to start the entire program. We’ll delve into such architectural issues in more detail later in this book. This chapter is mostly interested in the fact that import operations run the code in a file that is being loaded as a final step. Because of this, importing a file is yet another way to launch it. For instance, if you start an interactive session (from a system command line, from the Start menu, from IDLE, or otherwise), you can run the script1.py file you created earlier with a simple import (be sure to delete the input line you added in the prior section first, or you’ll need to press Enter for no reason): C:\misc> c:\python30\python >>> import script1 win32 1267650600228229401496703205376 Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam! Module Imports and Reloads | 51 This works, but only once per session (really, process) by default. After the first import, later imports do nothing, even if you change and save the module’s source file again in another window: >>> import script1 >>> import script1 This is by design; imports are too expensive an operation to repeat more than once per file, per program run. As you’ll learn in Chapter 21, imports must find files, compile them to byte code, and run the code. If you really want to force Python to run the file again in the same session without stopping and restarting the session, you need to instead call the reload function available in the imp standard library module (this function is also a simple built-in in Python 2.6, but not in 3.0): >>> from imp import reload # Must load from module in 3.0 >>> reload(script1) win32 65536 Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam! <module 'script1' from 'script1.py'> >>> The from statement here simply copies a name out of a module (more on this soon). The reload function itself loads and runs the current version of your file’s code, picking up changes if you’ve changed and saved it in another window. This allows you to edit and pick up new code on the fly within the current Python interactive session. In this session, for example, the second print statement in script1.py was changed in another window to print 2 ** 16 between the time of the first import and the reload call. The reload function expects the name of an already loaded module object, so you have to have successfully imported a module once before you reload it. Notice that reload also expects parentheses around the module object name, whereas import does not. reload is a function that is called, and import is a statement. That’s why you must pass the module name to reload as an argument in parentheses, and that’s why you get back an extra output line when reloading. The last output line is just the display representation of the reload call’s return value, a Python module object. We’ll learn more about using functions in general in Chapter 16. 52 | Chapter 3: How You Run Programs Version skew note: Python 3.0 moved the reload built-in function to the imp standard library module. It still reloads files as before, but you must import it in order to use it. In 3.0, run an import imp and use imp.reload(M), or run a from imp import reload and use reload(M), as shown here. We’ll discuss import and from statements in the next section, and more formally later in this book. If you are working in Python 2.6 (or 2.X in general), reload is available as a built-in function, so no import is required. In Python 2.6, reload is available in both forms—built-in and module function—to aid the transition to 3.0. In other words, reloading is still available in 3.0, but an extra line of code is required to fetch the reload call. The move in 3.0 was likely motivated in part by some well-known issues involving reload and from statements that we’ll encounter in the next section. In short, names loaded with a from are not directly updated by a reload, but names accessed with an import statement are. If your names don’t seem to change after a reload, try using import and module.attribute name references instead. The Grander Module Story: Attributes Imports and reloads provide a natural program launch option because import operations execute files as a last step. In the broader scheme of things, though, modules serve the role of libraries of tools, as you’ll learn in Part V. More generally, a module is mostly just a package of variable names, known as a namespace. The names within that package are called attributes—an attribute is simply a variable name that is attached to a specific object (like a module). In typical use, importers gain access to all the names assigned at the top level of a module’s file. These names are usually assigned to tools exported by the module— functions, classes, variables, and so on—that are intended to be used in other files and other programs. Externally, a module file’s names can be fetched with two Python statements, import and from, as well as the reload call. To illustrate, use a text editor to create a one-line Python module file called myfile.py with the following contents: title = "The Meaning of Life" This may be one of the world’s simplest Python modules (it contains a single assignment statement), but it’s enough to illustrate the point. When this file is imported, its code is run to generate the module’s attribute. The assignment statement creates a module attribute named title. Module Imports and Reloads | 53 You can access this module’s title attribute in other components in two different ways. First, you can load the module as a whole with an import statement, and then qualify the module name with the attribute name to fetch it: % python >>> import myfile >>> print(myfile.title) The Meaning of Life # Start Python # Run file; load module as a whole # Use its attribute names: '.' to qualify In general, the dot expression syntax object.attribute lets you fetch any attribute attached to any object, and this is a very common operation in Python code. Here, we’ve used it to access the string variable title inside the module myfile—in other words, myfile.title. Alternatively, you can fetch (really, copy) names out of a module with from statements: % python >>> from myfile import title >>> print(title) The Meaning of Life # Start Python # Run file; copy its names # Use name directly: no need to qualify As you’ll see in more detail later, from is just like an import, with an extra assignment to names in the importing component. Technically, from copies a module’s attributes, such that they become simple variables in the recipient—thus, you can simply refer to the imported string this time as title (a variable) instead of myfile.title (an attribute reference).‡ Whether you use import or from to invoke an import operation, the statements in the module file myfile.py are executed, and the importing component (here, the interactive prompt) gains access to names assigned at the top level of the file. There’s only one such name in this simple example—the variable title, assigned to a string—but the concept will be more useful when you start defining objects such as functions and classes in your modules: such objects become reusable software components that can be accessed by name from one or more client modules. In practice, module files usually define more than one name to be used in and outside the files. Here’s an example that defines three: a = 'dead' b = 'parrot' c = 'sketch' print(a, b, c) # Define three attributes # Exported to other files # Also used in this file This file, threenames.py, assigns three variables, and so generates three attributes for the outside world. It also uses its own three variables in a print statement, as we see when we run this as a top-level file: ‡ Notice that import and from both list the name of the module file as simply myfile without its .py suffix. As you’ll learn in Part V, when Python looks for the actual file, it knows to include the suffix in its search procedure. Again, you must include the .py suffix in system shell command lines, but not in import statements. 54 | Chapter 3: How You Run Programs % python threenames.py dead parrot sketch All of this file’s code runs as usual the first time it is imported elsewhere (by either an import or from). Clients of this file that use import get a module with attributes, while clients that use from get copies of the file’s names: % python >>> import threenames dead parrot sketch >>> >>> threenames.b, threenames.c ('parrot', 'sketch') >>> >>> from threenames import a, b, c >>> b, c ('parrot', 'sketch') # Grab the whole module # Copy multiple names The results here are printed in parentheses because they are really tuples (a kind of object covered in the next part of this book); you can safely ignore them for now. Once you start coding modules with multiple names like this, the built-in dir function starts to come in handy—you can use it to fetch a list of the names available inside a module. The following returns a Python list of strings (we’ll start studying lists in the next chapter): >>> dir(threenames) ['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'a', 'b', 'c'] I ran this on Python 3.0 and 2.6; older Pythons may return fewer names. When the dir function is called with the name of an imported module passed in parentheses like this, it returns all the attributes inside that module. Some of the names it returns are names you get “for free”: names with leading and trailing double underscores are builtin names that are always predefined by Python and that have special meaning to the interpreter. The variables our code defined by assignment—a, b, and c—show up last in the dir result. Modules and namespaces Module imports are a way to run files of code, but, as we’ll discuss later in the book, modules are also the largest program structure in Python programs. In general, Python programs are composed of multiple module files, linked together by import statements. Each module file is a self-contained package of variables—that is, a namespace. One module file cannot see the names defined in another file unless it explicitly imports that other file, so modules serve to minimize name collisions in your code—because each file is a self-contained namespace, the names in one file cannot clash with those in another, even if they are spelled the same way. Module Imports and Reloads | 55 In fact, as you’ll see, modules are one of a handful of ways that Python goes to great lengths to package your variables into compartments to avoid name clashes. We’ll discuss modules and other namespace constructs (including classes and function scopes) further later in the book. For now, modules will come in handy as a way to run your code many times without having to retype it. import versus from: I should point out that the from statement in a sense defeats the namespace partitioning purpose of modules—because the from copies variables from one file to another, it can cause same-named variables in the importing file to be overwritten (and won’t warn you if it does). This essentially collapses namespaces together, at least in terms of the copied variables. Because of this, some recommend using import instead of from. I won’t go that far, though; not only does from involve less typing, but its purported problem is rarely an issue in practice. Besides, this is something you control by listing the variables you want in the from; as long as you understand that they’ll be assigned values, this is no more dangerous than coding assignment statements—another feature you’ll probably want to use! import and reload Usage Notes For some reason, once people find out about running files using import and reload, many tend to focus on this alone and forget about other launch options that always run the current version of the code (e.g., icon clicks, IDLE menu options, and system command lines). This approach can quickly lead to confusion, though—you need to remember when you’ve imported to know if you can reload, you need to remember to use parentheses when you call reload (only), and you need to remember to use reload in the first place to get the current version of your code to run. Moreover, reloads aren’t transitive—reloading a module reloads that module only, not any modules it may import—so you sometimes have to reload multiple files. Because of these complications (and others we’ll explore later, including the reload/ from issue mentioned in a prior note in this chapter), it’s generally a good idea to avoid the temptation to launch by imports and reloads for now. The IDLE Run→Run Module menu option described in the next section, for example, provides a simpler and less error-prone way to run your files, and always runs the current version of your code. System shell command lines offer similar benefits. You don’t need to use reload if you use these techniques. In addition, you may run into trouble if you use modules in unusual ways at this point in the book. For instance, if you want to import a module file that is stored in a directory other than the one you’re working in, you’ll have to skip ahead to Chapter 21 and learn about the module search path. 56 | Chapter 3: How You Run Programs For now, if you must import, try to keep all your files in the directory you are working in to avoid complications.§ That said, imports and reloads have proven to be a popular testing technique in Python classes, and you may prefer using this approach too. As usual, though, if you find yourself running into a wall, stop running into a wall! Using exec to Run Module Files In fact, there are more ways to run code stored in module files than have yet been exposed here. For instance, the exec(open('module.py').read()) built-in function call is another way to launch files from the interactive prompt without having to import and later reload. Each exec runs the current version of the file, without requiring later reloads (script1.py is as we left it after a reload in the prior section): C:\misc> c:\python30\python >>> exec(open('script1.py').read()) win32 65536 Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam! ...change script1.py in a text edit window... >>> exec(open('script1.py').read()) win32 4294967296 Spam!Spam!Spam!Spam!Spam!Spam!Spam!Spam! The exec call has an effect similar to an import, but it doesn’t technically import the module—by default, each time you call exec this way it runs the file anew, as though you had pasted it in at the place where exec is called. Because of that, exec does not require module reloads after file changes—it skips the normal module import logic. On the downside, because it works as if pasting code into the place where it is called, exec, like the from statement mentioned earlier, has the potential to silently overwrite variables you may currently be using. For example, our script1.py assigns to a variable named x. If that name is also being used in the place where exec is called, the name’s value is replaced: >>> x = 999 >>> exec(open('script1.py').read()) ...same outout... >>> x 'Spam!' # Code run in this namespace by default # Its assignments can overwrite names here § If you’re burning with curiosity, the short story is that Python searches for imported modules in every directory listed in sys.path—a Python list of directory name strings in the sys module, which is initialized from a PYTHONPATH environment variable, plus a set of standard directories. If you want to import from a directory other than the one you are working in, that directory must generally be listed in your PYTHONPATH setting. For more details, see Chapter 21. Using exec to Run Module Files | 57 By contrast, the basic import statement runs the file only once per process, and it makes the file a separate module namespace so that its assignments will not change variables in your scope. The price you pay for the namespace partitioning of modules is the need to reload after changes. Version skew note: Python 2.6 also includes an execfile('module.py') built-in function, in addition to allowing the form exec(open('module.py')), which both automatically read the file’s content. Both of these are equivalent to the exec(open('module.py').read()) form, which is more complex but runs in both 2.6 and 3.0. Unfortunately, neither of these two simpler 2.6 forms is available in 3.0, which means you must understand both files and their read methods to fully understand this technique today (alas, this seems to be a case of aesthetics trouncing practicality in 3.0). In fact, the exec form in 3.0 involves so much typing that the best advice may simply be not to do it—it’s usually best to launch files by typing system shell command lines or by using the IDLE menu options described in the next section. For more on the 3.0 exec form, see Chapter 9. The IDLE User Interface So far, we’ve seen how to run Python code with the interactive prompt, system command lines, icon clicks, and module imports and exec calls. If you’re looking for something a bit more visual, IDLE provides a graphical user interface for doing Python development, and it’s a standard and free part of the Python system. It is usually referred to as an integrated development environment (IDE), because it binds together various development tasks into a single view.‖ In short, IDLE is a GUI that lets you edit, run, browse, and debug Python programs, all from a single interface. Moreover, because IDLE is a Python program that uses the tkinter GUI toolkit (known as Tkinter in 2.6), it runs portably on most Python platforms, including Microsoft Windows, X Windows (for Linux, Unix, and Unix-like platforms), and the Mac OS (both Classic and OS X). For many, IDLE represents an easy-to-use alternative to typing command lines, and a less problem-prone alternative to clicking on icons. IDLE Basics Let’s jump right into an example. IDLE is easy to start under Windows—it has an entry in the Start button menu for Python (see Figure 2-1, shown previously), and it can also be selected by right-clicking on a Python program icon. On some Unix-like systems, ‖ IDLE is officially a corruption of IDE, but it’s really named in honor of Monty Python member Eric Idle. 58 | Chapter 3: How You Run Programs you may need to launch IDLE’s top-level script from a command line, or by clicking on the icon for the idle.pyw or idle.py file located in the idlelib subdirectory of Python’s Lib directory. On Windows, IDLE is a Python script that currently lives in C:\Python30\Lib\idlelib (or C:Python26\Lib\idlelib in Python 2.6).# Figure 3-3 shows the scene after starting IDLE on Windows. The Python shell window that opens initially is the main window, which runs an interactive session (notice the >>> prompt). This works like all interactive sessions—code you type here is run immediately after you type it—and serves as a testing tool. Figure 3-3. The main Python shell window of the IDLE development GUI, shown here running on Windows. Use the File menu to begin (New Window) or change (Open...) a source file; use the text edit window’s Run menu to run the code in that window (Run Module). #IDLE is a Python program that uses the standard library’s tkinter GUI toolkit (a.k.a. Tkinter in Python 2.6) to build the IDLE GUI. This makes IDLE portable, but it also means that you’ll need to have tkinter support in your Python to use IDLE. The Windows version of Python has this by default, but some Linux and Unix users may need to install the appropriate tkinter support (a yum tkinter command may suffice on some Linux distributions, but see the installation hints in Appendix A for details). Mac OS X may have everything you need preinstalled, too; look for an idle command or script on your machine. The IDLE User Interface | 59 IDLE uses familiar menus with keyboard shortcuts for most of its operations. To make (or edit) a source code file under IDLE, open a text edit window: in the main window, select the File pull-down menu, and pick New Window (or Open... to open a text edit window displaying an existing file for editing). Although it may not show up fully in this book’s graphics, IDLE uses syntax-directed colorization for the code typed in both the main window and all text edit windows— keywords are one color, literals are another, and so on. This helps give you a better picture of the components in your code (and can even help you spot mistakes— run-on strings are all one color, for example). To run a file of code that you are editing in IDLE, select the file’s text edit window, open that window’s Run pull-down menu, and choose the Run Module option listed there (or use the equivalent keyboard shortcut, given in the menu). Python will let you know that you need to save your file first if you’ve changed it since it was opened or last saved and forgot to save your changes—a common mistake when you’re knee deep in coding. When run this way, the output of your script and any error messages it may generate show up back in the main interactive window (the Python shell window). In Figure 3-3, for example, the three lines after the “RESTART” line near the middle of the window reflect an execution of our script1.py file opened in a separate edit window. The “RESTART” message tells us that the user-code process was restarted to run the edited script and serves to separate script output (it does not appear if IDLE is started without a user-code subprocess—more on this mode in a moment). IDLE hint of the day: If you want to repeat prior commands in IDLE’s main interactive window, you can use the Alt-P key combination to scroll backward through the command history, and Alt-N to scroll forward (on some Macs, try Ctrl-P and Ctrl-N instead). Your prior commands will be recalled and displayed, and may be edited and rerun. You can also recall commands by positioning the cursor on them, or use cut-and-paste operations, but these techniques tend to involve more work. Outside IDLE, you may be able to recall commands in an interactive session with the arrow keys on Windows. Using IDLE IDLE is free, easy to use, portable, and automatically available on most platforms. I generally recommend it to Python newcomers because it sugarcoats some of the details and does not assume prior experience with system command lines. However, it is somewhat limited compared to more advanced commercial IDEs. To help you avoid some common pitfalls, here is a list of issues that IDLE beginners should bear in mind: • You must add “.py” explicitly when saving your files. I mentioned this when talking about files in general, but it’s a common IDLE stumbling block, especially 60 | Chapter 3: How You Run Programs • • • • • for Windows users. IDLE does not automatically add a .py extension to filenames when files are saved. Be careful to type the .py extension yourself when saving a file for the first time. If you don’t, while you will be able to run your file from IDLE (and system command lines), you will not be able to import it either interactively or from other modules. Run scripts by selecting Run→Run Module in text edit windows, not by interactive imports and reloads. Earlier in this chapter, we saw that it’s possible to run a file by importing it interactively. However, this scheme can grow complex because it requires you to manually reload files after changes. By contrast, using the Run→Run Module menu option in IDLE always runs the most current version of your file, just like running it using a system shell command line. IDLE also prompts you to save your file first, if needed (another common mistake outside IDLE). You need to reload only modules being tested interactively. Like system shell command lines, IDLE’s Run→Run Module menu option always runs the current version of both the top-level file and any modules it imports. Because of this, Run→Run Module eliminates common confusions surrounding imports. You only need to reload modules that you are importing and testing interactively in IDLE. If you choose to use the import and reload technique instead of Run→Run Module, remember that you can use the Alt-P/Alt-N key combinations to recall prior commands. You can customize IDLE. To change the text fonts and colors in IDLE, select the Configure option in the Options menu of any IDLE window. You can also customize key combination actions, indentation settings, and more; see IDLE’s Help pull-down menu for more hints. There is currently no clear-screen option in IDLE. This seems to be a frequent request (perhaps because it’s an option available in similar IDEs), and it might be added eventually. Today, though, there is no way to clear the interactive window’s text. If you want the window’s text to go away, you can either press and hold the Enter key, or type a Python loop to print a series of blank lines (nobody really uses the latter technique, of course, but it sounds more high-tech than pressing the Enter key!). tkinter GUI and threaded programs may not work well with IDLE. Because IDLE is a Python/tkinter program, it can hang if you use it to run certain types of advanced Python/tkinter programs. This has become less of an issue in more recent versions of IDLE that run user code in one process and the IDLE GUI itself in another, but some programs (especially those that use multithreading) might still hang the GUI. Your code may not exhibit such problems, but as a rule of thumb, it’s always safe to use IDLE to edit GUI programs but launch them using other options, such as icon clicks or system command lines. When in doubt, if your code fails in IDLE, try it outside the GUI. The IDLE User Interface | 61 • If connection errors arise, try starting IDLE in single-process mode. Because IDLE requires communication between its separate user and GUI processes, it can sometimes have trouble starting up on certain platforms (notably, it fails to start occasionally on some Windows machines, due to firewall software that blocks connections). If you run into such connection errors, it’s always possible to start IDLE with a system command line that forces it to run in single-process mode without a user-code subprocess and therefore avoids communication issues: its -n command-line flag forces this mode. On Windows, for example, start a Command Prompt window and run the system command line idle.py -n from within the directory C:\Python30\Lib\idlelib (cd there first if needed). • Beware of some IDLE usability features. IDLE does much to make life easier for beginners, but some of its tricks won’t apply outside the IDLE GUI. For instance, IDLE runs your scripts in its own interactive namespace, so variables in your code show up automatically in the IDLE interactive session—you don’t always need to run import commands to access names at the top level of files you’ve already run. This can be handy, but it can also be confusing, because outside the IDLE environment names must always be imported from files to be used. IDLE also automatically changes both to the directory of a file just run and adds its directory to the module import search path—a handy feature that allows you to import files there without search path settings, but also something that won’t work the same when you run files outside IDLE. It’s OK to use such features, but don’t forget that they are IDLE behavior, not Python behavior. Advanced IDLE Tools Besides the basic edit and run functions, IDLE provides more advanced features, including a point-and-click program debugger and an object browser. The IDLE debugger is enabled via the Debug menu and the object browser via the File menu. The browser allows you to navigate through the module search path to files and objects in files; clicking on a file or object opens the corresponding source in a text edit window. IDLE debugging is initiated by selecting the Debug→Debugger menu option in the main window and then starting your script by selecting the Run→Run Module option in the text edit window; once the debugger is enabled, you can set breakpoints in your code that stop its execution by right-clicking on lines in the text edit windows, show variable values, and so on. You can also watch program execution when debugging—the current line of code is noted as you step through your code. For simpler debugging operations, you can also right-click with your mouse on the text of an error message to quickly jump to the line of code where the error occurred—a trick that makes it simple and fast to repair and run again. In addition, IDLE’s text editor offers a large collection of programmer-friendly tools, including automatic indentation, advanced text and file search operations, and more. Because IDLE uses 62 | Chapter 3: How You Run Programs intuitive GUI interactions, you should experiment with the system live to get a feel for its other tools. Other IDEs Because IDLE is free, portable, and a standard part of Python, it’s a nice first development tool to become familiar with if you want to use an IDE at all. Again, I recommend that you use IDLE for this book’s exercises if you’re just starting out, unless you are already familiar with and prefer a command-line-based development mode. There are, however, a handful of alternative IDEs for Python developers, some of which are substantially more powerful and robust than IDLE. Here are some of the most commonly used IDEs: Eclipse and PyDev Eclipse is an advanced open source IDE GUI. Originally developed as a Java IDE, Eclipse also supports Python development when you install the PyDev (or a similar) plug-in. Eclipse is a popular and powerful option for Python development, and it goes well beyond IDLE’s feature set. It includes support for code completion, syntax highlighting, syntax analysis, refactoring, debugging, and more. Its downsides are that it is a large system to install and may require shareware extensions for some features (this may vary over time). Still, when you are ready to graduate from IDLE, the Eclipse/PyDev combination is worth your attention. Komodo A full-featured development environment GUI for Python (and other languages), Komodo includes standard syntax-coloring, text-editing, debugging, and other features. In addition, Komodo offers many advanced features that IDLE does not, including project files, source-control integration, regular-expression debugging, and a drag-and-drop GUI builder that generates Python/tkinter code to implement the GUIs you design interactively. At this writing, Komodo is not free; it is available at http://www.activestate.com. NetBeans IDE for Python NetBeans is a powerful open-source development environment GUI with support for many advanced features for Python developers: code completion, automatic indentation and code colorization, editor hints, code folding, refactoring, debugging, code coverage and testing, projects, and more. It may be used to develop both CPython and Jython code. Like Eclipse, NetBeans requires installation steps beyond those of the included IDLE GUI, but it is seen by many as more than worth the effort. Search the Web for the latest information and links. PythonWin PythonWin is a free Windows-only IDE for Python that ships as part of ActiveState’s ActivePython distribution (and may also be fetched separately from http:// www.python.org resources). It is roughly like IDLE, with a handful of useful Windows-specific extensions added; for example, PythonWin has support for Other IDEs | 63 COM objects. Today, IDLE is probably more advanced than PythonWin (for instance, IDLE’s dual-process architecture often prevents it from hanging). However, PythonWin still offers tools for Windows developers that IDLE does not. See http: //www.activestate.com for more information. Others There are roughly half a dozen other widely used IDEs that I’m aware of (including the commercial Wing IDE and PythonCard) but do not have space to do justice to here, and more will probably appear over time. In fact, almost every programmerfriendly text editor has some sort of support for Python development these days, whether it be preinstalled or fetched separately. Emacs and Vim, for instance, have substantial Python support. I won’t try to document all such options here; for more information, see the resources available at http://www.python.org or search the Web for “Python IDE.” You might also try running a web search for “Python editors”—today, this leads you to a wiki page that maintains information about many IDE and text-editor options for Python programming. Other Launch Options At this point, we’ve seen how to run code typed interactively, and how to launch code saved in files in a variety of ways—system command lines, imports and execs, GUIs like IDLE, and more. That covers most of the cases you’ll see in this book. There are additional ways to run Python code, though, most of which have special or narrow roles. The next few sections take a quick look at some of these. Embedding Calls In some specialized domains, Python code may be run automatically by an enclosing system. In such cases, we say that the Python programs are embedded in (i.e., run by) another program. The Python code itself may be entered into a text file, stored in a database, fetched from an HTML page, parsed from an XML document, and so on. But from an operational perspective, another system—not you—may tell Python to run the code you’ve created. Such an embedded execution mode is commonly used to support end-user customization—a game program, for instance, might allow for play modifications by running user-accessible embedded Python code at strategic points in time. Users can modify this type of system by providing or changing Python code. Because Python code is interpreted, there is no need to recompile the entire system to incorporate the change (see Chapter 2 for more on how Python code is run). 64 | Chapter 3: How You Run Programs In this mode, the enclosing system that runs your code might be written in C, C++, or even Java when the Jython system is used. As an example, it’s possible to create and run strings of Python code from a C program by calling functions in the Python runtime API (a set of services exported by the libraries created when Python is compiled on your machine): #include <Python.h> ... Py_Initialize(); PyRun_SimpleString("x = 'brave ' + 'sir robin'"); // This is C, not Python // But it runs Python code In this C code snippet, a program coded in the C language embeds the Python interpreter by linking in its libraries, and passes it a Python assignment statement string to run. C programs may also gain access to Python modules and objects and process or execute them using other Python API tools. This book isn’t about Python/C integration, but you should be aware that, depending on how your organization plans to use Python, you may or may not be the one who actually starts the Python programs you create. Regardless, you can usually still use the interactive and file-based launching techniques described here to test code in isolation from those enclosing systems that may eventually use it.* Frozen Binary Executables Frozen binary executables, described in Chapter 2, are packages that combine your program’s byte code and the Python interpreter into a single executable program. This approach enables Python programs to be launched in the same ways that you would launch any other executable program (icon clicks, command lines, etc.). While this option works well for delivery of products, it is not really intended for use during program development; you normally freeze just before shipping (after development is finished). See the prior chapter for more on this option. Text Editor Launch Options As mentioned previously, although they’re not full-blown IDE GUIs, most programmer-friendly text editors have support for editing, and possibly running, Python programs. Such support may be built in or fetchable on the Web. For instance, if you are familiar with the Emacs text editor, you can do all your Python editing and launching from inside that text editor. See the text editor resources page at http://www.python .org/editors for more details, or search the Web for the phrase “Python editors.” * See Programming Python (O’Reilly) for more details on embedding Python in C/C++. The embedding API can call Python functions directly, load modules, and more. Also, note that the Jython system allows Java programs to invoke Python code using a Java-based API (a Python interpreter class). Other Launch Options | 65 Still Other Launch Options Depending on your platform, there may be additional ways that you can start Python programs. For instance, on some Macintosh systems you may be able to drag Python program file icons onto the Python interpreter icon to make them execute, and on Windows you can always start Python scripts with the Run... option in the Start menu. Additionally, the Python standard library has utilities that allow Python programs to be started by other Python programs in separate processes (e.g., os.popen, os.system), and Python scripts might also be spawned in larger contexts like the Web (for instance, a web page might invoke a script on a server); however, these are beyond the scope of the present chapter. Future Possibilities? This chapter reflects current practice, but much of the material is both platform- and time-specific. Indeed, many of the execution and launch details presented arose during the shelf life of this book’s various editions. As with program execution options, it’s not impossible that new program launch options may arise over time. New operating systems, and new versions of existing systems, may also provide execution techniques beyond those outlined here. In general, because Python keeps pace with such changes, you should be able to launch Python programs in whatever way makes sense for the machines you use, both now and in the future—be that by drawing on tablet PCs or PDAs, grabbing icons in a virtual reality, or shouting a script’s name over your coworkers’ conversations. Implementation changes may also impact launch schemes somewhat (e.g., a full compiler could produce normal executables that are launched much like frozen binaries today). If I knew what the future truly held, though, I would probably be talking to a stockbroker instead of writing these words! Which Option Should I Use? With all these options, one question naturally arises: which one is best for me? In general, you should give the IDLE interface a try if you are just getting started with Python. It provides a user-friendly GUI environment and hides some of the underlying configuration details. It also comes with a platform-neutral text editor for coding your scripts, and it’s a standard and free part of the Python system. If, on the other hand, you are an experienced programmer, you might be more comfortable with simply the text editor of your choice in one window, and another window for launching the programs you edit via system command lines and icon clicks (in fact, this is how I develop Python programs, but I have a Unix-biased past). Because the choice of development environments is very subjective, I can’t offer much more in the 66 | Chapter 3: How You Run Programs way of universal guidelines; in general, whatever environment you like to use will be the best for you to use. Debugging Python Code Naturally, none of my readers or students ever have bugs in their code (insert smiley here), but for less fortunate friends of yours who may, here’s a quick look at the strategies commonly used by real-world Python programmers to debug code: • Do nothing. By this, I don’t mean that Python programmers don’t debug their code—but when you make a mistake in a Python program, you get a very useful and readable error message (you’ll get to see some soon, if you haven’t already). If you already know Python, and especially for your own code, this is often enough—read the error message, and go fix the tagged line and file. For many, this is debugging in Python. It may not always be ideal for larger system you didn’t write, though. • Insert print statements. Probably the main way that Python programmers debug their code (and the way that I debug Python code) is to insert print statements and run again. Because Python runs immediately after changes, this is usually the quickest way to get more information than error messages provide. The print statements don’t have to be sophisticated—a simple “I am here” or display of variable values is usually enough to provide the context you need. Just remember to delete or comment out (i.e., add a # before) the debugging prints before you ship your code! • Use IDE GUI debuggers. For larger systems you didn’t write, and for beginners who want to trace code in more detail, most Python development GUIs have some sort of point-and-click debugging support. IDLE has a debugger too, but it doesn’t appear to be used very often in practice—perhaps because it has no command line, or perhaps because adding print statements is usually quicker than setting up a GUI debugging session. To learn more, see IDLE’s Help, or simply try it on your own; its basic interface is described in the section “Advanced IDLE Tools” on page 62. Other IDEs, such as Eclipse, NetBeans, Komodo, and Wing IDE, offer advanced point-and-click debuggers as well; see their documentation if you use them. • Use the pdb command-line debugger. For ultimate control, Python comes with a source-code debugger named pdb, available as a module in Python’s standard library. In pdb, you type commands to step line by line, display variables, set and clear breakpoints, continue to a breakpoint or error, and so on. pdb can be launched interactively by importing it, or as a top-level script. Either way, because you can type commands to control the session, it provides a powerful debugging tool. pdb also includes a postmortem function you can run after an exception occurs, to get information from the time of the error. See the Python library manual and Chapter 35 for more details on pdb. • Other options. For more specific debugging requirements, you can find additional tools in the open source domain, including support for multithreaded programs, embedded code, and process attachment. The Winpdb system, for example, is a Which Option Should I Use? | 67 standalone debugger with advanced debugging support and cross-platform GUI and console interfaces. These options will become more important as we start writing larger scripts. Probably the best news on the debugging front, though, is that errors are detected and reported in Python, rather than passing silently or crashing the system altogether. In fact, errors themselves are a well-defined mechanism known as exceptions, which you can catch and process (more on exceptions in Part VII). Making mistakes is never fun, of course, but speaking as someone who recalls when debugging meant getting out a hex calculator and poring over piles of memory dump printouts, Python’s debugging support makes errors much less painful than they might otherwise be. Chapter Summary In this chapter, we’ve looked at common ways to launch Python programs: by running code typed interactively, and by running code stored in files with system command lines, file-icon clicks, module imports, exec calls, and IDE GUIs such as IDLE. We’ve covered a lot of pragmatic startup territory here. This chapter’s goal was to equip you with enough information to enable you to start writing some code, which you’ll do in the next part of the book. There, we will start exploring the Python language itself, beginning with its core data types. First, though, take the usual chapter quiz to exercise what you’ve learned here. Because this is the last chapter in this part of the book, it’s followed with a set of more complete exercises that test your mastery of this entire part’s topics. For help with the latter set of problems, or just for a refresher, be sure to turn to Appendix B after you’ve given the exercises a try. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. 7. 8. How can you start an interactive interpreter session? Where do you type a system command line to launch a script file? Name four or more ways to run the code saved in a script file. Name two pitfalls related to clicking file icons on Windows. Why might you need to reload a module? How do you run a script from within IDLE? Name two pitfalls related to using IDLE. What is a namespace, and how does it relate to module files? 68 | Chapter 3: How You Run Programs Test Your Knowledge: Answers 1. You can start an interactive session on Windows by clicking your Start button, picking the All Programs option, clicking the Python entry, and selecting the “Python (command line)” menu option. You can also achieve the same effect on Windows and other platforms by typing python as a system command line in your system’s console window (a Command Prompt window on Windows). Another alternative is to launch IDLE, as its main Python shell window is an interactive session. If you have not set your system’s PATH variable to find Python, you may need to cd to where Python is installed, or type its full directory path instead of just python (e.g., C:\Python30\python on Windows). 2. You type system command lines in whatever your platform provides as a system console: a Command Prompt window on Windows; an xterm or terminal window on Unix, Linux, and Mac OS X; and so on. 3. Code in a script (really, module) file can be run with system command lines, file icon clicks, imports and reloads, the exec built-in function, and IDE GUI selections such as IDLE’s Run→Run Module menu option. On Unix, they can also be run as executables with the #! trick, and some platforms support more specialized launching techniques (e.g., drag-and-drop). In addition, some text editors have unique ways to run Python code, some Python programs are provided as standalone “frozen binary” executables, and some systems use Python code in embedded mode, where it is run automatically by an enclosing program written in a language like C, C++, or Java. The latter technique is usually done to provide a user customization layer. 4. Scripts that print and then exit cause the output file to disappear immediately, before you can view the output (which is why the input trick comes in handy); error messages generated by your script also appear in an output window that closes before you can examine its contents (which is one reason that system command lines and IDEs such as IDLE are better for most development). 5. Python only imports (loads) a module once per process, by default, so if you’ve changed its source code and want to run the new version without stopping and restarting Python, you’ll have to reload it. You must import a module at least once before you can reload it. Running files of code from a system shell command line, via an icon click, or via an IDE such as IDLE generally makes this a nonissue, as those launch schemes usually run the current version of the source code file each time. 6. Within the text edit window of the file you wish to run, select the window’s Run→Run Module menu option. This runs the window’s source code as a top-level script file and displays its output back in the interactive Python shell window. 7. IDLE can still be hung by some types of programs—especially GUI programs that perform multithreading (an advanced technique beyond this book’s scope). Also, IDLE has some usability features that can burn you once you leave the IDLE GUI: Test Your Knowledge: Answers | 69 a script’s variables are automatically imported to the interactive scope in IDLE, for instance, but not by Python in general. 8. A namespace is just a package of variables (i.e., names). It takes the form of an object with attributes in Python. Each module file is automatically a namespace— that is, a package of variables reflecting the assignments made at the top level of the file. Namespaces help avoid name collisions in Python programs: because each module file is a self-contained namespace, files must explicitly import other files in order to use their names. Test Your Knowledge: Part I Exercises It’s time to start doing a little coding on your own. This first exercise session is fairly simple, but a few of these questions hint at topics to come in later chapters. Be sure to check “Part I, Getting Started” on page 1101 in the solutions appendix (Appendix B) for the answers; the exercises and their solutions sometimes contain supplemental information not discussed in the main text, so you should take a peek at the solutions even if you manage to answer all the questions on your own. 1. Interaction. Using a system command line, IDLE, or another method, start the Python interactive command line (>>> prompt), and type the expression "Hello World!" (including the quotes). The string should be echoed back to you. The purpose of this exercise is to get your environment configured to run Python. In some scenarios, you may need to first run a cd shell command, type the full path to the Python executable, or add its path to your PATH environment variable. If desired, you can set PATH in your .cshrc or .kshrc file to make Python permanently available on Unix systems; on Windows, use a setup.bat, autoexec.bat, or the environment variable GUI. See Appendix A for help with environment variable settings. 2. Programs. With the text editor of your choice, write a simple module file containing the single statement print('Hello module world!') and store it as module1.py. Now, run this file by using any launch option you like: running it in IDLE, clicking on its file icon, passing it to the Python interpreter on the system shell’s command line (e.g., python module1.py), built-in exec calls, imports and reloads, and so on. In fact, experiment by running your file with as many of the launch techniques discussed in this chapter as you can. Which technique seems easiest? (There is no right answer to this, of course.) 3. Modules. Start the Python interactive command line (>>> prompt) and import the module you wrote in exercise 2. Try moving the file to a different directory and importing it again from its original directory (i.e., run Python in the original directory when you import). What happens? (Hint: is there still a module1.pyc byte code file in the original directory?) 70 | Chapter 3: How You Run Programs 4. Scripts. If your platform supports it, add the #! line to the top of your module1.py module file, give the file executable privileges, and run it directly as an executable. What does the first line need to contain? #! usually only has meaning on Unix, Linux, and Unix-like platforms such as Mac OS X; if you’re working on Windows, instead try running your file by listing just its name in a DOS console window without the word “python” before it (this works on recent versions of Windows), or via the Start→Run... dialog box. 5. Errors and debugging. Experiment with typing mathematical expressions and assignments at the Python interactive command line. Along the way, type the expressions 2 ** 500 and 1 / 0, and reference an undefined variable name as we did in this chapter. What happens? You may not know it yet, but when you make a mistake, you’re doing exception processing (a topic we’ll explore in depth in Part VII). As you’ll learn there, you are technically triggering what’s known as the default exception handler—logic that prints a standard error message. If you do not catch an error, the default handler does and prints the standard error message in response. Exceptions are also bound up with the notion of debugging in Python. When you’re first starting out, Python’s default error messages on exceptions will probably provide as much error-handling support as you need—they give the cause of the error, as well as showing the lines in your code that were active when the error occurred. For more about debugging, see the sidebar “Debugging Python Code” on page 67. 6. Breaks and cycles. At the Python command line, type: L = [1, 2] L.append(L) L # Make a 2-item list # Append L as a single item to itself # Print L What happens? In all recent versions of Python, you’ll see a strange output that we’ll describe in the solutions appendix, and which will make more sense when we study references in the next part of the book. If you’re using a Python version older than 1.5.1, a Ctrl-C key combination will probably help on most platforms. Why do you think your version of Python responds the way it does for this code? If you do have a Python older than Release 1.5.1 (a hopefully rare scenario today!), make sure your machine can stop a program with a Ctrl-C key combination of some sort before running this test, or you may be waiting a long time. 7. Documentation. Spend at least 17 minutes browsing the Python library and language manuals before moving on to get a feel for the available tools in the standard library and the structure of the documentation set. It takes at least this long to become familiar with the locations of major topics in the manual set; once you’ve done this, it’s easy to find what you need. You can find this manual via the Python Test Your Knowledge: Part I Exercises | 71 Start button entry on Windows, in the Python Docs option on the Help pull-down menu in IDLE, or online at http://www.python.org/doc. I’ll also have a few more words to say about the manuals and other documentation sources available (including PyDoc and the help function) in Chapter 15. If you still have time, go explore the Python website, as well as its PyPy third-party extension repository. Especially check out the Python.org documentation and search pages; they can be crucial resources. 72 | Chapter 3: How You Run Programs PART II Types and Operations CHAPTER 4 Introducing Python Object Types This chapter begins our tour of the Python language. In an informal sense, in Python, we do things with stuff. “Things” take the form of operations like addition and concatenation, and “stuff” refers to the objects on which we perform those operations. In this part of the book, our focus is on that stuff, and the things our programs can do with it. Somewhat more formally, in Python, data takes the form of objects—either built-in objects that Python provides, or objects we create using Python or external language tools such as C extension libraries. Although we’ll firm up this definition later, objects are essentially just pieces of memory, with values and sets of associated operations. Because objects are the most fundamental notion in Python programming, we’ll start this chapter with a survey of Python’s built-in object types. By way of introduction, however, let’s first establish a clear picture of how this chapter fits into the overall Python picture. From a more concrete perspective, Python programs can be decomposed into modules, statements, expressions, and objects, as follows: 1. 2. 3. 4. Programs are composed of modules. Modules contain statements. Statements contain expressions. Expressions create and process objects. The discussion of modules in Chapter 3 introduced the highest level of this hierarchy. This part’s chapters begin at the bottom, exploring both built-in objects and the expressions you can code to use them. 75 Why Use Built-in Types? If you’ve used lower-level languages such as C or C++, you know that much of your work centers on implementing objects—also known as data structures—to represent the components in your application’s domain. You need to lay out memory structures, manage memory allocation, implement search and access routines, and so on. These chores are about as tedious (and error-prone) as they sound, and they usually distract from your program’s real goals. In typical Python programs, most of this grunt work goes away. Because Python provides powerful object types as an intrinsic part of the language, there’s usually no need to code object implementations before you start solving problems. In fact, unless you have a need for special processing that built-in types don’t provide, you’re almost always better off using a built-in object instead of implementing your own. Here are some reasons why: • Built-in objects make programs easy to write. For simple tasks, built-in types are often all you need to represent the structure of problem domains. Because you get powerful tools such as collections (lists) and search tables (dictionaries) for free, you can use them immediately. You can get a lot of work done with Python’s builtin object types alone. • Built-in objects are components of extensions. For more complex tasks, you may need to provide your own objects using Python classes or C language interfaces. But as you’ll see in later parts of this book, objects implemented manually are often built on top of built-in types such as lists and dictionaries. For instance, a stack data structure may be implemented as a class that manages or customizes a built-in list. • Built-in objects are often more efficient than custom data structures. Python’s built-in types employ already optimized data structure algorithms that are implemented in C for speed. Although you can write similar object types on your own, you’ll usually be hard-pressed to get the level of performance built-in object types provide. • Built-in objects are a standard part of the language. In some ways, Python borrows both from languages that rely on built-in tools (e.g., LISP) and languages that rely on the programmer to provide tool implementations or frameworks of their own (e.g., C++). Although you can implement unique object types in Python, you don’t need to do so just to get started. Moreover, because Python’s built-ins are standard, they’re always the same; proprietary frameworks, on the other hand, tend to differ from site to site. In other words, not only do built-in object types make programming easier, but they’re also more powerful and efficient than most of what can be created from scratch. Regardless of whether you implement new object types, built-in objects form the core of every Python program. 76 | Chapter 4: Introducing Python Object Types Python’s Core Data Types Table 4-1 previews Python’s built-in object types and some of the syntax used to code their literals—that is, the expressions that generate these objects.* Some of these types will probably seem familiar if you’ve used other languages; for instance, numbers and strings represent numeric and textual values, respectively, and files provide an interface for processing files stored on your computer. Table 4-1. Built-in objects preview Object type Example literals/creation Numbers 1234, 3.1415, 3+4j, Decimal, Fraction Strings 'spam', "guido's", b'a\x01c' Lists [1, [2, 'three'], 4] Dictionaries {'food': 'spam', 'taste': 'yum'} Tuples (1, 'spam', 4, 'U') Files myfile = open('eggs', 'r') Sets set('abc'), {'a', 'b', 'c'} Other core types Booleans, types, None Program unit types Functions, modules, classes (Part IV, Part V, Part VI) Implementation-related types Compiled code, stack tracebacks (Part IV, Part VII) Table 4-1 isn’t really complete, because everything we process in Python programs is a kind of object. For instance, when we perform text pattern matching in Python, we create pattern objects, and when we perform network scripting, we use socket objects. These other kinds of objects are generally created by importing and using modules and have behavior all their own. As we’ll see in later parts of the book, program units such as functions, modules, and classes are objects in Python too—they are created with statements and expressions such as def, class, import, and lambda and may be passed around scripts freely, stored within other objects, and so on. Python also provides a set of implementation-related types such as compiled code objects, which are generally of interest to tool builders more than application developers; these are also discussed in later parts of this text. We usually call the other object types in Table 4-1 core data types, though, because they are effectively built into the Python language—that is, there is specific expression syntax for generating most of them. For instance, when you run the following code: >>> 'spam' * In this book, the term literal simply means an expression whose syntax generates an object—sometimes also called a constant. Note that the term “constant” does not imply objects or variables that can never be changed (i.e., this term is unrelated to C++’s const or Python’s “immutable”—a topic explored in the section “Immutability” on page 82). Why Use Built-in Types? | 77 you are, technically speaking, running a literal expression that generates and returns a new string object. There is specific Python language syntax to make this object. Similarly, an expression wrapped in square brackets makes a list, one in curly braces makes a dictionary, and so on. Even though, as we’ll see, there are no type declarations in Python, the syntax of the expressions you run determines the types of objects you create and use. In fact, object-generation expressions like those in Table 4-1 are generally where types originate in the Python language. Just as importantly, once you create an object, you bind its operation set for all time— you can perform only string operations on a string and list operations on a list. As you’ll learn, Python is dynamically typed (it keeps track of types for you automatically instead of requiring declaration code), but it is also strongly typed (you can perform on an object only operations that are valid for its type). Functionally, the object types in Table 4-1 are more general and powerful than what you may be accustomed to. For instance, you’ll find that lists and dictionaries alone are powerful data representation tools that obviate most of the work you do to support collections and searching in lower-level languages. In short, lists provide ordered collections of other objects, while dictionaries store objects by key; both lists and dictionaries may be nested, can grow and shrink on demand, and may contain objects of any type. We’ll study each of the object types in Table 4-1 in detail in upcoming chapters. Before digging into the details, though, let’s begin by taking a quick look at Python’s core objects in action. The rest of this chapter provides a preview of the operations we’ll explore in more depth in the chapters that follow. Don’t expect to find the full story here—the goal of this chapter is just to whet your appetite and introduce some key ideas. Still, the best way to get started is to get started, so let’s jump right into some real code. Numbers If you’ve done any programming or scripting in the past, some of the object types in Table 4-1 will probably seem familiar. Even if you haven’t, numbers are fairly straightforward. Python’s core objects set includes the usual suspects: integers (numbers without a fractional part), floating-point numbers (roughly, numbers with a decimal point in them), and more exotic numeric types (complex numbers with imaginary parts, fixed-precision decimals, rational fractions with numerator and denominator, and fullfeatured sets). Although it offers some fancier options, Python’s basic number types are, well, basic. Numbers in Python support the normal mathematical operations. For instance, the plus sign (+) performs addition, a star (*) is used for multiplication, and two stars (**) are used for exponentiation: 78 | Chapter 4: Introducing Python Object Types >>> 123 + 222 345 >>> 1.5 * 4 6.0 >>> 2 ** 100 1267650600228229401496703205376 # Integer addition # Floating-point multiplication # 2 to the power 100 Notice the last result here: Python 3.0’s integer type automatically provides extra precision for large numbers like this when needed (in 2.6, a separate long integer type handles numbers too large for the normal integer type in similar ways). You can, for instance, compute 2 to the power 1,000,000 as an integer in Python, but you probably shouldn’t try to print the result—with more than 300,000 digits, you may be waiting awhile! >>> len(str(2 ** 1000000)) 301030 # How many digits in a really BIG number? Once you start experimenting with floating-point numbers, you’re likely to stumble across something that may look a bit odd on first glance: >>> 3.1415 * 2 6.2830000000000004 >>> print(3.1415 * 2) 6.283 # repr: as code # str: user-friendly The first result isn’t a bug; it’s a display issue. It turns out that there are two ways to print every object: with full precision (as in the first result shown here), and in a userfriendly form (as in the second). Formally, the first form is known as an object’s ascode repr, and the second is its user-friendly str. The difference can matter when we step up to using classes; for now, if something looks odd, try showing it with a print built-in call statement. Besides expressions, there are a handful of useful numeric modules that ship with Python—modules are just packages of additional tools that we import to use: >>> import math >>> math.pi 3.1415926535897931 >>> math.sqrt(85) 9.2195444572928871 The math module contains more advanced numeric tools as functions, while the random module performs random number generation and random selections (here, from a Python list, introduced later in this chapter): >>> import random >>> random.random() 0.59268735266273953 >>> random.choice([1, 2, 3, 4]) 1 Python also includes more exotic numeric objects—such as complex, fixed-precision, and rational numbers, as well as sets and Booleans—and the third-party open source Numbers | 79 extension domain has even more (e.g., matrixes and vectors). We’ll defer discussion of these types until later in the book. So far, we’ve been using Python much like a simple calculator; to do better justice to its built-in types, let’s move on to explore strings. Strings Strings are used to record textual information as well as arbitrary collections of bytes. They are our first example of what we call a sequence in Python—that is, a positionally ordered collection of other objects. Sequences maintain a left-to-right order among the items they contain: their items are stored and fetched by their relative position. Strictly speaking, strings are sequences of one-character strings; other types of sequences include lists and tuples, covered later. Sequence Operations As sequences, strings support operations that assume a positional ordering among items. For example, if we have a four-character string, we can verify its length with the built-in len function and fetch its components with indexing expressions: >>> >>> 4 >>> 'S' >>> 'p' S = 'Spam' len(S) # Length S[0] # The first item in S, indexing by zero-based position S[1] # The second item from the left In Python, indexes are coded as offsets from the front, and so start from 0: the first item is at index 0, the second is at index 1, and so on. Notice how we assign the string to a variable named S here. We’ll go into detail on how this works later (especially in Chapter 6), but Python variables never need to be declared ahead of time. A variable is created when you assign it a value, may be assigned any type of object, and is replaced with its value when it shows up in an expression. It must also have been previously assigned by the time you use its value. For the purposes of this chapter, it’s enough to know that we need to assign an object to a variable in order to save it for later use. In Python, we can also index backward, from the end—positive indexes count from the left, and negative indexes count back from the right: >>> S[-1] 'm' >>> S[-2] 'a' # The last item from the end in S # The second to last item from the end 80 | Chapter 4: Introducing Python Object Types Formally, a negative index is simply added to the string’s size, so the following two operations are equivalent (though the first is easier to code and less easy to get wrong): >>> S[-1] 'm' >>> S[len(S)-1] 'm' # The last item in S # Negative indexing, the hard way Notice that we can use an arbitrary expression in the square brackets, not just a hardcoded number literal—anywhere that Python expects a value, we can use a literal, a variable, or any expression. Python’s syntax is completely general this way. In addition to simple positional indexing, sequences also support a more general form of indexing known as slicing, which is a way to extract an entire section (slice) in a single step. For example: >>> S 'Spam' >>> S[1:3] 'pa' # A 4-character string # Slice of S from offsets 1 through 2 (not 3) Probably the easiest way to think of slices is that they are a way to extract an entire column from a string in a single step. Their general form, X[I:J], means “give me everything in X from offset I up to but not including offset J.” The result is returned in a new object. The second of the preceding operations, for instance, gives us all the characters in string S from offsets 1 through 2 (that is, 3 – 1) as a new string. The effect is to slice or “parse out” the two characters in the middle. In a slice, the left bound defaults to zero, and the right bound defaults to the length of the sequence being sliced. This leads to some common usage variations: >>> S[1:] 'pam' >>> S 'Spam' >>> S[0:3] 'Spa' >>> S[:3] 'Spa' >>> S[:-1] 'Spa' >>> S[:] 'Spam' # Everything past the first (1:len(S)) # S itself hasn't changed # Everything but the last # Same as S[0:3] # Everything but the last again, but simpler (0:-1) # All of S as a top-level copy (0:len(S)) Note how negative offsets can be used to give bounds for slices, too, and how the last operation effectively copies the entire string. As you’ll learn later, there is no reason to copy a string, but this form can be useful for sequences like lists. Finally, as sequences, strings also support concatenation with the plus sign (joining two strings into a new string) and repetition (making a new string by repeating another): >>> S Spam' >>> S + 'xyz' # Concatenation Strings | 81 'Spamxyz' >>> S # S is unchanged 'Spam' >>> S * 8 # Repetition 'SpamSpamSpamSpamSpamSpamSpamSpam' Notice that the plus sign (+) means different things for different objects: addition for numbers, and concatenation for strings. This is a general property of Python that we’ll call polymorphism later in the book—in sum, the meaning of an operation depends on the objects being operated on. As you’ll see when we study dynamic typing, this polymorphism property accounts for much of the conciseness and flexibility of Python code. Because types aren’t constrained, a Python-coded operation can normally work on many different types of objects automatically, as long as they support a compatible interface (like the + operation here). This turns out to be a huge idea in Python; you’ll learn more about it later on our tour. Immutability Notice that in the prior examples, we were not changing the original string with any of the operations we ran on it. Every string operation is defined to produce a new string as its result, because strings are immutable in Python—they cannot be changed in-place after they are created. For example, you can’t change a string by assigning to one of its positions, but you can always build a new one and assign it to the same name. Because Python cleans up old objects as you go (as you’ll see later), this isn’t as inefficient as it may sound: >>> S 'Spam' >>> S[0] = 'z' # Immutable objects cannot be changed ...error text omitted... TypeError: 'str' object does not support item assignment >>> S = 'z' + S[1:] >>> S 'zpam' # But we can run expressions to make new objects Every object in Python is classified as either immutable (unchangeable) or not. In terms of the core types, numbers, strings, and tuples are immutable; lists and dictionaries are not (they can be changed in-place freely). Among other things, immutability can be used to guarantee that an object remains constant throughout your program. Type-Specific Methods Every string operation we’ve studied so far is really a sequence operation—that is, these operations will work on other sequences in Python as well, including lists and tuples. In addition to generic sequence operations, though, strings also have operations all their own, available as methods—functions attached to the object, which are triggered with a call expression. 82 | Chapter 4: Introducing Python Object Types For example, the string find method is the basic substring search operation (it returns the offset of the passed-in substring, or −1 if it is not present), and the string replace method performs global searches and replacements: >>> S.find('pa') 1 >>> S 'Spam' >>> S.replace('pa', 'XYZ') 'SXYZm' >>> S 'Spam' # Find the offset of a substring # Replace occurrences of a substring with another Again, despite the names of these string methods, we are not changing the original strings here, but creating new strings as the results—because strings are immutable, we have to do it this way. String methods are the first line of text-processing tools in Python. Other methods split a string into substrings on a delimiter (handy as a simple form of parsing), perform case conversions, test the content of the string (digits, letters, and so on), and strip whitespace characters off the ends of the string: >>> line = 'aaa,bbb,ccccc,dd' >>> line.split(',') # Split on a delimiter into a list of substrings ['aaa', 'bbb', 'ccccc', 'dd'] >>> S = 'spam' >>> S.upper() # Upper- and lowercase conversions 'SPAM' >>> S.isalpha() True # Content tests: isalpha, isdigit, etc. >>> line = 'aaa,bbb,ccccc,dd\n' >>> line = line.rstrip() # Remove whitespace characters on the right side >>> line 'aaa,bbb,ccccc,dd' Strings also support an advanced substitution operation known as formatting, available as both an expression (the original) and a string method call (new in 2.6 and 3.0): >>> '%s, eggs, and %s' % ('spam', 'SPAM!') 'spam, eggs, and SPAM!' # Formatting expression (all) >>> '{0}, eggs, and {1}'.format('spam', 'SPAM!') 'spam, eggs, and SPAM!' # Formatting method (2.6, 3.0) One note here: although sequence operations are generic, methods are not—although some types share some method names, string method operations generally work only on strings, and nothing else. As a rule of thumb, Python’s toolset is layered: generic operations that span multiple types show up as built-in functions or expressions (e.g., len(X), X[0]), but type-specific operations are method calls (e.g., aString.upper()). Finding the tools you need among all these categories will become more natural as you use Python more, but the next section gives a few tips you can use right now. Strings | 83 Getting Help The methods introduced in the prior section are a representative, but small, sample of what is available for string objects. In general, this book is not exhaustive in its look at object methods. For more details, you can always call the built-in dir function, which returns a list of all the attributes available for a given object. Because methods are function attributes, they will show up in this list. Assuming S is still the string, here are its attributes on Python 3.0 (Python 2.6 varies slightly): >>> dir(S) ['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '_formatter_field_name_split', '_formatter_parser', 'capitalize', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum','isalpha', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill'] You probably won’t care about the names with underscores in this list until later in the book, when we study operator overloading in classes—they represent the implementation of the string object and are available to support customization. In general, leading and trailing double underscores is the naming pattern Python uses for implementation details. The names without the underscores in this list are the callable methods on string objects. The dir function simply gives the methods’ names. To ask what they do, you can pass them to the help function: >>> help(S.replace) Help on built-in function replace: replace(...) S.replace (old, new[, count]) -> str Return a copy of S with all occurrences of substring old replaced by new. If the optional argument count is given, only the first count occurrences are replaced. help is one of a handful of interfaces to a system of code that ships with Python known as PyDoc—a tool for extracting documentation from objects. Later in the book, you’ll see that PyDoc can also render its reports in HTML format. You can also ask for help on an entire string (e.g., help(S)), but you may get more help than you want to see—i.e., information about every string method. It’s generally better to ask about a specific method. 84 | Chapter 4: Introducing Python Object Types For more details, you can also consult Python’s standard library reference manual or commercially published reference books, but dir and help are the first line of documentation in Python. Other Ways to Code Strings So far, we’ve looked at the string object’s sequence operations and type-specific methods. Python also provides a variety of ways for us to code strings, which we’ll explore in greater depth later. For instance, special characters can be represented as backslash escape sequences: >>> S = 'A\nB\tC' >>> len(S) 5 # \n is end-of-line, \t is tab # Each stands for just one character >>> ord('\n') 10 # \n is a byte with the binary value 10 in ASCII >>> S = 'A\0B\0C' >>> len(S) 5 # \0, a binary zero byte, does not terminate string Python allows strings to be enclosed in single or double quote characters (they mean the same thing). It also allows multiline string literals enclosed in triple quotes (single or double)—when this form is used, all the lines are concatenated together, and endof-line characters are added where line breaks appear. This is a minor syntactic convenience, but it’s useful for embedding things like HTML and XML code in a Python script: >>> msg = """ aaaaaaaaaaaaa bbb'''bbbbbbbbbb""bbbbbbb'bbbb cccccccccccccc""" >>> msg '\naaaaaaaaaaaaa\nbbb\'\'\'bbbbbbbbbb""bbbbbbb\'bbbb\ncccccccccccccc' Python also supports a raw string literal that turns off the backslash escape mechanism (such string literals start with the letter r), as well as Unicode string support that supports internationalization. In 3.0, the basic str string type handles Unicode too (which makes sense, given that ASCII text is a simple kind of Unicode), and a bytes type represents raw byte strings; in 2.6, Unicode is a separate type, and str handles both 8bit strings and binary data. Files are also changed in 3.0 to return and accept str for text and bytes for binary data. We’ll meet all these special string forms in later chapters. Pattern Matching One point worth noting before we move on is that none of the string object’s methods support pattern-based text processing. Text pattern matching is an advanced tool outside this book’s scope, but readers with backgrounds in other scripting languages may be interested to know that to do pattern matching in Python, we import a module called Strings | 85 re. This module has analogous calls for searching, splitting, and replacement, but be- cause we can use patterns to specify substrings, we can be much more general: >>> import re >>> match = re.match('Hello[ \t]*(.*)world', 'Hello >>> match.group(1) 'Python ' Python world') This example searches for a substring that begins with the word “Hello,” followed by zero or more tabs or spaces, followed by arbitrary characters to be saved as a matched group, terminated by the word “world.” If such a substring is found, portions of the substring matched by parts of the pattern enclosed in parentheses are available as groups. The following pattern, for example, picks out three groups separated by slashes: >>> match = re.match('/(.*)/(.*)/(.*)', '/usr/home/lumberjack') >>> match.groups() ('usr', 'home', 'lumberjack') Pattern matching is a fairly advanced text-processing tool by itself, but there is also support in Python for even more advanced language processing, including natural language processing. I’ve already said enough about strings for this tutorial, though, so let’s move on to the next type. Lists The Python list object is the most general sequence provided by the language. Lists are positionally ordered collections of arbitrarily typed objects, and they have no fixed size. They are also mutable—unlike strings, lists can be modified in-place by assignment to offsets as well as a variety of list method calls. Sequence Operations Because they are sequences, lists support all the sequence operations we discussed for strings; the only difference is that the results are usually lists instead of strings. For instance, given a three-item list: >>> L = [123, 'spam', 1.23] >>> len(L) 3 # A list of three different-type objects # Number of items in the list we can index, slice, and so on, just as for strings: >>> L[0] 123 # Indexing by position >>> L[:-1] [123, 'spam'] # Slicing a list returns a new list >>> L + [4, 5, 6] [123, 'spam', 1.23, 4, 5, 6] # Concatenation makes a new list too 86 | Chapter 4: Introducing Python Object Types >>> L [123, 'spam', 1.23] # We're not changing the original list Type-Specific Operations Python’s lists are related to arrays in other languages, but they tend to be more powerful. For one thing, they have no fixed type constraint—the list we just looked at, for example, contains three objects of completely different types (an integer, a string, and a floating-point number). Further, lists have no fixed size. That is, they can grow and shrink on demand, in response to list-specific operations: >>> L.append('NI') >>> L [123, 'spam', 1.23, 'NI'] # Growing: add object at end of list >>> L.pop(2) 1.23 # Shrinking: delete an item in the middle >>> L [123, 'spam', 'NI'] # "del L[2]" deletes from a list too Here, the list append method expands the list’s size and inserts an item at the end; the pop method (or an equivalent del statement) then removes an item at a given offset, causing the list to shrink. Other list methods insert an item at an arbitrary position (insert), remove a given item by value (remove), and so on. Because lists are mutable, most list methods also change the list object in-place, instead of creating a new one: >>> M = ['bb', 'aa', 'cc'] >>> M.sort() >>> M ['aa', 'bb', 'cc'] >>> M.reverse() >>> M ['cc', 'bb', 'aa'] The list sort method here, for example, orders the list in ascending fashion by default, and reverse reverses it—in both cases, the methods modify the list directly. Bounds Checking Although lists have no fixed size, Python still doesn’t allow us to reference items that are not present. Indexing off the end of a list is always a mistake, but so is assigning off the end: >>> L [123, 'spam', 'NI'] >>> L[99] ...error text omitted... IndexError: list index out of range Lists | 87 >>> L[99] = 1 ...error text omitted... IndexError: list assignment index out of range This is intentional, as it’s usually an error to try to assign off the end of a list (and a particularly nasty one in the C language, which doesn’t do as much error checking as Python). Rather than silently growing the list in response, Python reports an error. To grow a list, we call list methods such as append instead. Nesting One nice feature of Python’s core data types is that they support arbitrary nesting—we can nest them in any combination, and as deeply as we like (for example, we can have a list that contains a dictionary, which contains another list, and so on). One immediate application of this feature is to represent matrixes, or “multidimensional arrays” in Python. A list with nested lists will do the job for basic applications: >>> M = [[1, 2, [4, 5, [7, 8, >>> M [[1, 2, 3], [4, 3], 6], 9]] # A 3 × 3 matrix, as nested lists # Code can span lines if bracketed 5, 6], [7, 8, 9]] Here, we’ve coded a list that contains three other lists. The effect is to represent a 3 × 3 matrix of numbers. Such a structure can be accessed in a variety of ways: >>> M[1] [4, 5, 6] # Get row 2 >>> M[1][2] 6 # Get row 2, then get item 3 within the row The first operation here fetches the entire second row, and the second grabs the third item within that row. Stringing together index operations takes us deeper and deeper into our nested-object structure.† Comprehensions In addition to sequence operations and list methods, Python includes a more advanced operation known as a list comprehension expression, which turns out to be a powerful way to process structures like our matrix. Suppose, for instance, that we need to extract the second column of our sample matrix. It’s easy to grab rows by simple indexing † This matrix structure works for small-scale tasks, but for more serious number crunching you will probably want to use one of the numeric extensions to Python, such as the open source NumPy system. Such tools can store and process large matrixes much more efficiently than our nested list structure. NumPy has been said to turn Python into the equivalent of a free and more powerful version of the Matlab system, and organizations such as NASA, Los Alamos, and JPMorgan Chase use this tool for scientific and financial tasks. Search the Web for more details. 88 | Chapter 4: Introducing Python Object Types because the matrix is stored by rows, but it’s almost as easy to get a column with a list comprehension: >>> col2 = [row[1] for row in M] >>> col2 [2, 5, 8] # Collect the items in column 2 >>> M [[1, 2, 3], [4, 5, 6], [7, 8, 9]] # The matrix is unchanged List comprehensions derive from set notation; they are a way to build a new list by running an expression on each item in a sequence, one at a time, from left to right. List comprehensions are coded in square brackets (to tip you off to the fact that they make a list) and are composed of an expression and a looping construct that share a variable name (row, here). The preceding list comprehension means basically what it says: “Give me row[1] for each row in matrix M, in a new list.” The result is a new list containing column 2 of the matrix. List comprehensions can be more complex in practice: >>> [row[1] + 1 for row in M] [3, 6, 9] # Add 1 to each item in column 2 >>> [row[1] for row in M if row[1] % 2 == 0] # Filter out odd items [2, 8] The first operation here, for instance, adds 1 to each item as it is collected, and the second uses an if clause to filter odd numbers out of the result using the % modulus expression (remainder of division). List comprehensions make new lists of results, but they can be used to iterate over any iterable object. Here, for instance, we use list comprehensions to step over a hardcoded list of coordinates and a string: >>> diag = [M[i][i] for i in [0, 1, 2]] >>> diag [1, 5, 9] # Collect a diagonal from matrix >>> doubles = [c * 2 for c in 'spam'] >>> doubles ['ss', 'pp', 'aa', 'mm'] # Repeat characters in a string List comprehensions, and relatives like the map and filter built-in functions, are a bit too involved for me to say more about them here. The main point of this brief introduction is to illustrate that Python includes both simple and advanced tools in its arsenal. List comprehensions are an optional feature, but they tend to be handy in practice and often provide a substantial processing speed advantage. They also work on any type that is a sequence in Python, as well as some types that are not. You’ll hear much more about them later in this book. As a preview, though, you’ll find that in recent Pythons, comprehension syntax in parentheses can also be used to create generators that produce results on demand (the sum built-in, for instance, sums items in a sequence): Lists | 89 >>> G = (sum(row) for row in M) >>> next(G) 6 >>> next(G) 15 # Create a generator of row sums # Run the iteration protocol The map built-in can do similar work, by generating the results of running items through a function. Wrapping it in list forces it to return all its values in Python 3.0: >>> list(map(sum, M)) [6, 15, 24] # Map sum over items in M In Python 3.0, comprehension syntax can also be used to create sets and dictionaries: >>> {sum(row) for row in M} {24, 6, 15} # Create a set of row sums >>> {i : sum(M[i]) for i in range(3)} {0: 6, 1: 15, 2: 24} # Creates key/value table of row sums In fact, lists, sets, and dictionaries can all be built with comprehensions in 3.0: >>> [ord(x) for x in 'spaam'] [115, 112, 97, 97, 109] >>> {ord(x) for x in 'spaam'} {112, 97, 115, 109} >>> {x: ord(x) for x in 'spaam'} {'a': 97, 'p': 112, 's': 115, 'm': 109} # List of character ordinals # Sets remove duplicates # Dictionary keys are unique To understand objects like generators, sets, and dictionaries, though, we must move ahead. Dictionaries Python dictionaries are something completely different (Monty Python reference intended)—they are not sequences at all, but are instead known as mappings. Mappings are also collections of other objects, but they store objects by key instead of by relative position. In fact, mappings don’t maintain any reliable left-to-right order; they simply map keys to associated values. Dictionaries, the only mapping type in Python’s core objects set, are also mutable: they may be changed in-place and can grow and shrink on demand, like lists. Mapping Operations When written as literals, dictionaries are coded in curly braces and consist of a series of “key: value” pairs. Dictionaries are useful anytime we need to associate a set of values with keys—to describe the properties of something, for instance. As an example, consider the following three-item dictionary (with keys “food,” “quantity,” and “color”): >>> D = {'food': 'Spam', 'quantity': 4, 'color': 'pink'} 90 | Chapter 4: Introducing Python Object Types We can index this dictionary by key to fetch and change the keys’ associated values. The dictionary index operation uses the same syntax as that used for sequences, but the item in the square brackets is a key, not a relative position: >>> D['food'] 'Spam' # Fetch value of key 'food' >>> D['quantity'] += 1 # Add 1 to 'quantity' value >>> D {'food': 'Spam', 'color': 'pink', 'quantity': 5} Although the curly-braces literal form does see use, it is perhaps more common to see dictionaries built up in different ways. The following code, for example, starts with an empty dictionary and fills it out one key at a time. Unlike out-of-bounds assignments in lists, which are forbidden, assignments to new dictionary keys create those keys: >>> >>> >>> >>> D = {} D['name'] = 'Bob' D['job'] = 'dev' D['age'] = 40 # Create keys by assignment >>> D {'age': 40, 'job': 'dev', 'name': 'Bob'} >>> print(D['name']) Bob Here, we’re effectively using dictionary keys as field names in a record that describes someone. In other applications, dictionaries can also be used to replace searching operations—indexing a dictionary by key is often the fastest way to code a search in Python. Nesting Revisited In the prior example, we used a dictionary to describe a hypothetical person, with three keys. Suppose, though, that the information is more complex. Perhaps we need to record a first name and a last name, along with multiple job titles. This leads to another application of Python’s object nesting in action. The following dictionary, coded all at once as a literal, captures more structured information: >>> rec = {'name': {'first': 'Bob', 'last': 'Smith'}, 'job': ['dev', 'mgr'], 'age': 40.5} Here, we again have a three-key dictionary at the top (keys “name,” “job,” and “age”), but the values have become more complex: a nested dictionary for the name to support multiple parts, and a nested list for the job to support multiple roles and future expansion. We can access the components of this structure much as we did for our matrix earlier, but this time some of our indexes are dictionary keys, not list offsets: Dictionaries | 91 >>> rec['name'] {'last': 'Smith', 'first': 'Bob'} # 'name' is a nested dictionary >>> rec['name']['last'] 'Smith' # Index the nested dictionary >>> rec['job'] ['dev', 'mgr'] >>> rec['job'][-1] 'mgr' # 'job' is a nested list # Index the nested list >>> rec['job'].append('janitor') # Expand Bob's job description in-place >>> rec {'age': 40.5, 'job': ['dev', 'mgr', 'janitor'], 'name': {'last': 'Smith', 'first': 'Bob'}} Notice how the last operation here expands the nested job list—because the job list is a separate piece of memory from the dictionary that contains it, it can grow and shrink freely (object memory layout will be discussed further later in this book). The real reason for showing you this example is to demonstrate the flexibility of Python’s core data types. As you can see, nesting allows us to build up complex information structures directly and easily. Building a similar structure in a low-level language like C would be tedious and require much more code: we would have to lay out and declare structures and arrays, fill out values, link everything together, and so on. In Python, this is all automatic—running the expression creates the entire nested object structure for us. In fact, this is one of the main benefits of scripting languages like Python. Just as importantly, in a lower-level language we would have to be careful to clean up all of the object’s space when we no longer need it. In Python, when we lose the last reference to the object—by assigning its variable to something else, for example—all of the memory space occupied by that object’s structure is automatically cleaned up for us: >>> rec = 0 # Now the object's space is reclaimed Technically speaking, Python has a feature known as garbage collection that cleans up unused memory as your program runs and frees you from having to manage such details in your code. In Python, the space is reclaimed immediately, as soon as the last reference to an object is removed. We’ll study how this works later in this book; for now, it’s enough to know that you can use objects freely, without worrying about creating their space or cleaning up as you go.‡ ‡ Keep in mind that the rec record we just created really could be a database record, when we employ Python’s object persistence system—an easy way to store native Python objects in files or access-by-key databases. We won’t go into details here, but watch for discussion of Python’s pickle and shelve modules later in this book. 92 | Chapter 4: Introducing Python Object Types Sorting Keys: for Loops As mappings, as we’ve already seen, dictionaries only support accessing items by key. However, they also support type-specific operations with method calls that are useful in a variety of common use cases. As mentioned earlier, because dictionaries are not sequences, they don’t maintain any dependable left-to-right order. This means that if we make a dictionary and print it back, its keys may come back in a different order than that in which we typed them: >>> D = {'a': 1, 'b': 2, 'c': 3} >>> D {'a': 1, 'c': 3, 'b': 2} What do we do, though, if we do need to impose an ordering on a dictionary’s items? One common solution is to grab a list of keys with the dictionary keys method, sort that with the list sort method, and then step through the result with a Python for loop (be sure to press the Enter key twice after coding the for loop below—as explained in Chapter 3, an empty line means “go” at the interactive prompt, and the prompt changes to “...” on some interfaces): >>> Ks = list(D.keys()) >>> Ks ['a', 'c', 'b'] # Unordered keys list # A list in 2.6, "view" in 3.0: use list() >>> Ks.sort() >>> Ks ['a', 'b', 'c'] # Sorted keys list >>> for key in Ks: print(key, '=>', D[key]) # Iterate though sorted keys # <== press Enter twice here a => 1 b => 2 c => 3 This is a three-step process, although, as we’ll see in later chapters, in recent versions of Python it can be done in one step with the newer sorted built-in function. The sorted call returns the result and sorts a variety of object types, in this case sorting dictionary keys automatically: >>> D {'a': 1, 'c': 3, 'b': 2} >>> for key in sorted(D): print(key, '=>', D[key]) a => 1 b => 2 c => 3 Besides showcasing dictionaries, this use case serves to introduce the Python for loop. The for loop is a simple and efficient way to step through all the items in a sequence Dictionaries | 93 and run a block of code for each item in turn. A user-defined loop variable (key, here) is used to reference the current item each time through. The net effect in our example is to print the unordered dictionary’s keys and values, in sorted-key order. The for loop, and its more general cousin the while loop, are the main ways we code repetitive tasks as statements in our scripts. Really, though, the for loop (like its relative the list comprehension, which we met earlier) is a sequence operation. It works on any object that is a sequence and, like the list comprehension, even on some things that are not. Here, for example, it is stepping across the characters in a string, printing the uppercase version of each as it goes: >>> for c in 'spam': print(c.upper()) S P A M Python’s while loop is a more general sort of looping tool, not limited to stepping across sequences: >>> x = 4 >>> while x > 0: print('spam!' * x) x -= 1 spam!spam!spam!spam! spam!spam!spam! spam!spam! spam! We’ll discuss looping statements, syntax, and tools in depth later in the book. Iteration and Optimization If the last section’s for loop looks like the list comprehension expression introduced earlier, it should: both are really general iteration tools. In fact, both will work on any object that follows the iteration protocol—a pervasive idea in Python that essentially means a physically stored sequence in memory, or an object that generates one item at a time in the context of an iteration operation. An object falls into the latter category if it responds to the iter built-in with an object that advances in response to next. The generator comprehension expression we saw earlier is such an object. I’ll have more to say about the iteration protocol later in this book. For now, keep in mind that every Python tool that scans an object from left to right uses the iteration protocol. This is why the sorted call used in the prior section works on the dictionary directly—we don’t have to call the keys method to get a sequence because dictionaries are iterable objects, with a next that returns successive keys. 94 | Chapter 4: Introducing Python Object Types This also means that any list comprehension expression, such as this one, which computes the squares of a list of numbers: >>> squares = [x ** 2 for x in [1, 2, 3, 4, 5]] >>> squares [1, 4, 9, 16, 25] can always be coded as an equivalent for loop that builds the result list manually by appending as it goes: >>> squares = [] >>> for x in [1, 2, 3, 4, 5]: squares.append(x ** 2) # This is what a list comprehension does # Both run the iteration protocol internally >>> squares [1, 4, 9, 16, 25] The list comprehension, though, and related functional programming tools like map and filter, will generally run faster than a for loop today (perhaps even twice as fast)— a property that could matter in your programs for large data sets. Having said that, though, I should point out that performance measures are tricky business in Python because it optimizes so much, and performance can vary from release to release. A major rule of thumb in Python is to code for simplicity and readability first and worry about performance later, after your program is working, and after you’ve proved that there is a genuine performance concern. More often than not, your code will be quick enough as it is. If you do need to tweak code for performance, though, Python includes tools to help you out, including the time and timeit modules and the profile module. You’ll find more on these later in this book, and in the Python manuals. Missing Keys: if Tests One other note about dictionaries before we move on. Although we can assign to a new key to expand a dictionary, fetching a nonexistent key is still a mistake: >>> D {'a': 1, 'c': 3, 'b': 2} >>> D['e'] = 99 >>> D {'a': 1, 'c': 3, 'b': 2, 'e': 99} # Assigning new keys grows dictionaries >>> D['f'] ...error text omitted... KeyError: 'f' # Referencing a nonexistent key is an error This is what we want—it’s usually a programming error to fetch something that isn’t really there. But in some generic programs, we can’t always know what keys will be present when we write our code. How do we handle such cases and avoid errors? One trick is to test ahead of time. The dictionary in membership expression allows us to Dictionaries | 95 query the existence of a key and branch on the result with a Python if statement (as with the for, be sure to press Enter twice to run the if interactively here): >>> 'f' in D False >>> if not 'f' in D: print('missing') missing I’ll have much more to say about the if statement and statement syntax in general later in this book, but the form we’re using here is straightforward: it consists of the word if, followed by an expression that is interpreted as a true or false result, followed by a block of code to run if the test is true. In its full form, the if statement can also have an else clause for a default case, and one or more elif (else if) clauses for other tests. It’s the main selection tool in Python, and it’s the way we code logic in our scripts. Still, there are other ways to create dictionaries and avoid accessing nonexistent keys: the get method (a conditional index with a default); the Python 2.X has_key method (which is no longer available in 3.0); the try statement (a tool we’ll first meet in Chapter 10 that catches and recovers from exceptions altogether); and the if/else expression (essentially, an if statement squeezed onto a single line). Here are a few examples: >>> >>> 0 >>> >>> 0 value = D.get('x', 0) value # Index but with a default value = D['x'] if 'x' in D else 0 value # if/else expression form We’ll save the details on such alternatives until a later chapter. For now, let’s move on to tuples. Tuples The tuple object (pronounced “toople” or “tuhple,” depending on who you ask) is roughly like a list that cannot be changed—tuples are sequences, like lists, but they are immutable, like strings. Syntactically, they are coded in parentheses instead of square brackets, and they support arbitrary types, arbitrary nesting, and the usual sequence operations: >>> T = (1, 2, 3, 4) >>> len(T) 4 # A 4-item tuple # Length >> T + (5, 6) (1, 2, 3, 4, 5, 6) # Concatenation >>> T[0] 1 # Indexing, slicing, and more 96 | Chapter 4: Introducing Python Object Types Tuples also have two type-specific callable methods in Python 3.0, but not nearly as many as lists: >>> T.index(4) 3 >>> T.count(4) 1 # Tuple methods: 4 appears at offset 3 # 4 appears once The primary distinction for tuples is that they cannot be changed once created. That is, they are immutable sequences: >>> T[0] = 2 # Tuples are immutable ...error text omitted... TypeError: 'tuple' object does not support item assignment Like lists and dictionaries, tuples support mixed types and nesting, but they don’t grow and shrink because they are immutable: >>> T = ('spam', 3.0, [11, 22, 33]) >>> T[1] 3.0 >>> T[2][1] 22 >>> T.append(4) AttributeError: 'tuple' object has no attribute 'append' Why Tuples? So, why have a type that is like a list, but supports fewer operations? Frankly, tuples are not generally used as often as lists in practice, but their immutability is the whole point. If you pass a collection of objects around your program as a list, it can be changed anywhere; if you use a tuple, it cannot. That is, tuples provide a sort of integrity constraint that is convenient in programs larger than those we’ll write here. We’ll talk more about tuples later in the book. For now, though, let’s jump ahead to our last major core type: the file. Files File objects are Python code’s main interface to external files on your computer. Files are a core type, but they’re something of an oddball—there is no specific literal syntax for creating them. Rather, to create a file object, you call the built-in open function, passing in an external filename and a processing mode as strings. For example, to create a text output file, you would pass in its name and the 'w' processing mode string to write data: >>> >>> 6 >>> 6 >>> f = open('data.txt', 'w') f.write('Hello\n') # Make a new file in output mode # Write strings of bytes to it f.write('world\n') # Returns number of bytes written in Python 3.0 f.close() # Close to flush output buffers to disk Files | 97 This creates a file in the current directory and writes text to it (the filename can be a full directory path if you need to access a file elsewhere on your computer). To read back what you just wrote, reopen the file in 'r' processing mode, for reading text input—this is the default if you omit the mode in the call. Then read the file’s content into a string, and display it. A file’s contents are always a string in your script, regardless of the type of data the file contains: >>> f = open('data.txt') >>> text = f.read() >>> text 'Hello\nworld\n' # 'r' is the default processing mode # Read entire file into a string >>> print(text) Hello world # print interprets control characters >>> text.split() ['Hello', 'world'] # File content is always a string Other file object methods support additional features we don’t have time to cover here. For instance, file objects provide more ways of reading and writing (read accepts an optional byte size, readline reads one line at a time, and so on), as well as other tools (seek moves to a new file position). As we’ll see later, though, the best way to read a file today is to not read it at all—files provide an iterator that automatically reads line by line in for loops and other contexts. We’ll meet the full set of file methods later in this book, but if you want a quick preview now, run a dir call on any open file and a help on any of the method names that come back: >>> dir(f) [ ...many names omitted... 'buffer', 'close', 'closed', 'encoding', 'errors', 'fileno', 'flush', 'isatty', 'line_buffering', 'mode', 'name', 'newlines', 'read', 'readable', 'readline', 'readlines', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write', 'writelines'] >>>help(f.seek) ...try it and see... Later in the book, we’ll also see that files in Python 3.0 draw a sharp distinction between text and binary data. Text files represent content as strings and perform Unicode encoding and decoding automatically, while binary files represent content as a special bytes string type and allow you to access file content unaltered: >>> data = open('data.bin', 'rb').read() >>> data b'\x00\x00\x00\x07spam\x00\x08' >>> data[4:8] b'spam' 98 | Chapter 4: Introducing Python Object Types # Open binary file # bytes string holds binary data Although you won’t generally need to care about this distinction if you deal only with ASCII text, Python 3.0’s strings and files are an asset if you deal with internationalized applications or byte-oriented data. Other File-Like Tools The open function is the workhorse for most file processing you will do in Python. For more advanced tasks, though, Python comes with additional file-like tools: pipes, FIFOs, sockets, keyed-access files, persistent object shelves, descriptor-based files, relational and object-oriented database interfaces, and more. Descriptor files, for instance, support file locking and other low-level tools, and sockets provide an interface for networking and interprocess communication. We won’t cover many of these topics in this book, but you’ll find them useful once you start programming Python in earnest. Other Core Types Beyond the core types we’ve seen so far, there are others that may or may not qualify for membership in the set, depending on how broadly it is defined. Sets, for example, are a recent addition to the language that are neither mappings nor sequences; rather, they are unordered collections of unique and immutable objects. Sets are created by calling the built-in set function or using new set literals and expressions in 3.0, and they support the usual mathematical set operations (the choice of new {...} syntax for set literals in 3.0 makes sense, since sets are much like the keys of a valueless dictionary): >>> X = set('spam') # Make a set out of a sequence in 2.6 and 3.0 >>> Y = {'h', 'a', 'm'} # Make a set with new 3.0 set literals >>> X, Y ({'a', 'p', 's', 'm'}, {'a', 'h', 'm'}) >>> X & Y {'a', 'm'} # Intersection >>> X | Y {'a', 'p', 's', 'h', 'm'} # Union >>> X – Y {'p', 's'} # Difference >>> {x ** 2 for x in [1, 2, 3, 4]} {16, 1, 4, 9} # Set comprehensions in 3.0 In addition, Python recently grew a few new numeric types: decimal numbers (fixedprecision floating-point numbers) and fraction numbers (rational numbers with both a numerator and a denominator). Both can be used to work around the limitations and inherent inaccuracies of floating-point math: >>> 1 / 3 0.33333333333333331 >>> (2/3) + (1/2) # Floating-point (use .0 in Python 2.6) Other Core Types | 99 1.1666666666666665 >>> import decimal >>> d = decimal.Decimal('3.141') >>> d + 1 Decimal('4.141') # Decimals: fixed precision >>> decimal.getcontext().prec = 2 >>> decimal.Decimal('1.00') / decimal.Decimal('3.00') Decimal('0.33') >>> from fractions import Fraction >>> f = Fraction(2, 3) >>> f + 1 Fraction(5, 3) >>> f + Fraction(1, 2) Fraction(7, 6) # Fractions: numerator+denominator Python also comes with Booleans (with predefined True and False objects that are essentially just the integers 1 and 0 with custom display logic), and it has long supported a special placeholder object called None commonly used to initialize names and objects: >>> 1 > 2, 1 < 2 (False, True) >>> bool('spam') True # Booleans >>> X = None # None placeholder >>> print(X) None >>> L = [None] * 100 # Initialize a list of 100 Nones >>> L [None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, None, ...a list of 100 Nones...] How to Break Your Code’s Flexibility I’ll have more to say about all of Python’s object types later, but one merits special treatment here. The type object, returned by the type built-in function, is an object that gives the type of another object; its result differs slightly in 3.0, because types have merged with classes completely (something we’ll explore in the context of “new-style” classes in Part VI). Assuming L is still the list of the prior section: # In Python 2.6: >>> type(L) <type 'list'> >>> type(type(L)) <type 'type'> # Types: type of L is list type object # Even types are objects # In Python 3.0: >>> type(L) <class 'list'> 100 | Chapter 4: Introducing Python Object Types # 3.0: types are classes, and vice versa >>> type(type(L)) <class 'type'> # See Chapter 31 for more on class types Besides allowing you to explore your objects interactively, the practical application of this is that it allows code to check the types of the objects it processes. In fact, there are at least three ways to do so in a Python script: >>> if type(L) == type([]): print('yes') # Type testing, if you must... yes >>> if type(L) == list: print('yes') # Using the type name yes >>> if isinstance(L, list): print('yes') # Object-oriented tests yes Now that I’ve shown you all these ways to do type testing, however, I am required by law to tell you that doing so is almost always the wrong thing to do in a Python program (and often a sign of an ex-C programmer first starting to use Python!). The reason why won’t become completely clear until later in the book, when we start writing larger code units such as functions, but it’s a (perhaps the) core Python concept. By checking for specific types in your code, you effectively break its flexibility—you limit it to working on just one type. Without such tests, your code may be able to work on a whole range of types. This is related to the idea of polymorphism mentioned earlier, and it stems from Python’s lack of type declarations. As you’ll learn, in Python, we code to object interfaces (operations supported), not to types. Not caring about specific types means that code is automatically applicable to many of them—any object with a compatible interface will work, regardless of its specific type. Although type checking is supported— and even required, in some rare cases—you’ll see that it’s not usually the “Pythonic” way of thinking. In fact, you’ll find that polymorphism is probably the key idea behind using Python well. User-Defined Classes We’ll study object-oriented programming in Python—an optional but powerful feature of the language that cuts development time by supporting programming by customization—in depth later in this book. In abstract terms, though, classes define new types of objects that extend the core set, so they merit a passing glance here. Say, for example, that you wish to have a type of object that models employees. Although there is no such specific core type in Python, the following user-defined class might fit the bill: >>> class Worker: def __init__(self, name, pay): self.name = name # Initialize when created # self is the new object Other Core Types | 101 self.pay = pay def lastName(self): return self.name.split()[-1] def giveRaise(self, percent): self.pay *= (1.0 + percent) # Split string on blanks # Update pay in-place This class defines a new kind of object that will have name and pay attributes (sometimes called state information), as well as two bits of behavior coded as functions (normally called methods). Calling the class like a function generates instances of our new type, and the class’s methods automatically receive the instance being processed by a given method call (in the self argument): >>> bob = Worker('Bob Smith', 50000) >>> sue = Worker('Sue Jones', 60000) >>> bob.lastName() 'Smith' >>> sue.lastName() 'Jones' >>> sue.giveRaise(.10) >>> sue.pay 66000.0 # Make two instances # Each has name and pay attrs # Call method: bob is self # sue is the self subject # Updates sue's pay The implied “self” object is why we call this an object-oriented model: there is always an implied subject in functions within a class. In a sense, though, the class-based type simply builds on and uses core types—a user-defined Worker object here, for example, is just a collection of a string and a number (name and pay, respectively), plus functions for processing those two built-in objects. The larger story of classes is that their inheritance mechanism supports software hierarchies that lend themselves to customization by extension. We extend software by writing new classes, not by changing what already works. You should also know that classes are an optional feature of Python, and simpler built-in types such as lists and dictionaries are often better tools than user-coded classes. This is all well beyond the bounds of our introductory object-type tutorial, though, so consider this just a preview; for full disclosure on user-defined types coded with classes, you’ll have to read on to Part VI. And Everything Else As mentioned earlier, everything you can process in a Python script is a type of object, so our object type tour is necessarily incomplete. However, even though everything in Python is an “object,” only those types of objects we’ve met so far are considered part of Python’s core type set. Other types in Python either are objects related to program execution (like functions, modules, classes, and compiled code), which we will study later, or are implemented by imported module functions, not language syntax. The latter of these also tend to have application-specific roles—text patterns, database interfaces, network connections, and so on. 102 | Chapter 4: Introducing Python Object Types Moreover, keep in mind that the objects we’ve met here are objects, but not necessarily object-oriented—a concept that usually requires inheritance and the Python class statement, which we’ll meet again later in this book. Still, Python’s core objects are the workhorses of almost every Python script you’re likely to meet, and they usually are the basis of larger noncore types. Chapter Summary And that’s a wrap for our concise data type tour. This chapter has offered a brief introduction to Python’s core object types and the sorts of operations we can apply to them. We’ve studied generic operations that work on many object types (sequence operations such as indexing and slicing, for example), as well as type-specific operations available as method calls (for instance, string splits and list appends). We’ve also defined some key terms, such as immutability, sequences, and polymorphism. Along the way, we’ve seen that Python’s core object types are more flexible and powerful than what is available in lower-level languages such as C. For instance, Python’s lists and dictionaries obviate most of the work you do to support collections and searching in lower-level languages. Lists are ordered collections of other objects, and dictionaries are collections of other objects that are indexed by key instead of by position. Both dictionaries and lists may be nested, can grow and shrink on demand, and may contain objects of any type. Moreover, their space is automatically cleaned up as you go. I’ve skipped most of the details here in order to provide a quick tour, so you shouldn’t expect all of this chapter to have made sense yet. In the next few chapters, we’ll start to dig deeper, filling in details of Python’s core object types that were omitted here so you can gain a more complete understanding. We’ll start off in the next chapter with an in-depth look at Python numbers. First, though, another quiz to review. Test Your Knowledge: Quiz We’ll explore the concepts introduced in this chapter in more detail in upcoming chapters, so we’ll just cover the big ideas here: 1. Name four of Python’s core data types. 2. Why are they called “core” data types? 3. What does “immutable” mean, and which three of Python’s core types are considered immutable? 4. What does “sequence” mean, and which three types fall into that category? Test Your Knowledge: Quiz | 103 5. What does “mapping” mean, and which core type is a mapping? 6. What is “polymorphism,” and why should you care? Test Your Knowledge: Answers 1. Numbers, strings, lists, dictionaries, tuples, files, and sets are generally considered to be the core object (data) types. Types, None, and Booleans are sometimes classified this way as well. There are multiple number types (integer, floating point, complex, fraction, and decimal) and multiple string types (simple strings and Unicode strings in Python 2.X, and text strings and byte strings in Python 3.X). 2. They are known as “core” types because they are part of the Python language itself and are always available; to create other objects, you generally must call functions in imported modules. Most of the core types have specific syntax for generating the objects: 'spam', for example, is an expression that makes a string and determines the set of operations that can be applied to it. Because of this, core types are hardwired into Python’s syntax. In contrast, you must call the built-in open function to create a file object. 3. An “immutable” object is an object that cannot be changed after it is created. Numbers, strings, and tuples in Python fall into this category. While you cannot change an immutable object in-place, you can always make a new one by running an expression. 4. A “sequence” is a positionally ordered collection of objects. Strings, lists, and tuples are all sequences in Python. They share common sequence operations, such as indexing, concatenation, and slicing, but also have type-specific method calls. 5. The term “mapping” denotes an object that maps keys to associated values. Python’s dictionary is the only mapping type in the core type set. Mappings do not maintain any left-to-right positional ordering; they support access to data stored by key, plus type-specific method calls. 6. “Polymorphism” means that the meaning of an operation (like a +) depends on the objects being operated on. This turns out to be a key idea (perhaps the key idea) behind using Python well—not constraining code to specific types makes that code automatically applicable to many types. 104 | Chapter 4: Introducing Python Object Types CHAPTER 5 Numeric Types This chapter begins our in-depth tour of the Python language. In Python, data takes the form of objects—either built-in objects that Python provides, or objects we create using Python tools and other languages such as C. In fact, objects are the basis of every Python program you will ever write. Because they are the most fundamental notion in Python programming, objects are also our first focus in this book. In the preceding chapter, we took a quick pass over Python’s core object types. Although essential terms were introduced in that chapter, we avoided covering too many specifics in the interest of space. Here, we’ll begin a more careful second look at data type concepts, to fill in details we glossed over earlier. Let’s get started by exploring our first data type category: Python’s numeric types. Numeric Type Basics Most of Python’s number types are fairly typical and will probably seem familiar if you’ve used almost any other programming language in the past. They can be used to keep track of your bank balance, the distance to Mars, the number of visitors to your website, and just about any other numeric quantity. In Python, numbers are not really a single object type, but a category of similar types. Python supports the usual numeric types (integers and floating points), as well as literals for creating numbers and expressions for processing them. In addition, Python provides more advanced numeric programming support and objects for more advanced work. A complete inventory of Python’s numeric toolbox includes: • Integers and floating-point numbers • Complex numbers • Fixed-precision decimal numbers 105 • • • • • Rational fraction numbers Sets Booleans Unlimited integer precision A variety of numeric built-ins and modules This chapter starts with basic numbers and fundamentals, then moves on to explore the other tools in this list. Before we jump into code, though, the next few sections get us started with a brief overview of how we write and process numbers in our scripts. Numeric Literals Among its basic types, Python provides integers (positive and negative whole numbers) and floating-point numbers (numbers with a fractional part, sometimes called “floats” for economy). Python also allows us to write integers using hexadecimal, octal, and binary literals; offers a complex number type; and allows integers to have unlimited precision (they can grow to have as many digits as your memory space allows). Table 5-1 shows what Python’s numeric types look like when written out in a program, as literals. Table 5-1. Basic numeric literals Literal Interpretation 1234, −24, 0, 99999999999999 Integers (unlimited size) 1.23, 1., 3.14e-10, 4E210, 4.0e+210 Floating-point numbers 0177, 0x9ff, 0b101010 Octal, hex, and binary literals in 2.6 0o177, 0x9ff, 0b101010 Octal, hex, and binary literals in 3.0 3+4j, 3.0+4.0j, 3J Complex number literals In general, Python’s numeric type literals are straightforward to write, but a few coding concepts are worth highlighting here: Integer and floating-point literals Integers are written as strings of decimal digits. Floating-point numbers have a decimal point and/or an optional signed exponent introduced by an e or E and followed by an optional sign. If you write a number with a decimal point or exponent, Python makes it a floating-point object and uses floating-point (not integer) math when the object is used in an expression. Floating-point numbers are implemented as C “doubles,” and therefore get as much precision as the C compiler used to build the Python interpreter gives to doubles. 106 | Chapter 5: Numeric Types Integers in Python 2.6: normal and long In Python 2.6 there are two integer types, normal (32 bits) and long (unlimited precision), and an integer may end in an l or L to force it to become a long integer. Because integers are automatically converted to long integers when their values overflow 32 bits, you never need to type the letter L yourself—Python automatically converts up to long integer when extra precision is needed. Integers in Python 3.0: a single type In Python 3.0, the normal and long integer types have been merged—there is only integer, which automatically supports the unlimited precision of Python 2.6’s separate long integer type. Because of this, integers can no longer be coded with a trailing l or L, and integers never print with this character either. Apart from this, most programs are unaffected by this change, unless they do type testing that checks for 2.6 long integers. Hexadecimal, octal, and binary literals Integers may be coded in decimal (base 10), hexadecimal (base 16), octal (base 8), or binary (base 2). Hexadecimals start with a leading 0x or 0X, followed by a string of hexadecimal digits (0–9 and A–F). Hex digits may be coded in lower- or uppercase. Octal literals start with a leading 0o or 0O (zero and lower- or uppercase letter “o”), followed by a string of digits (0–7). In 2.6 and earlier, octal literals can also be coded with just a leading 0, but not in 3.0 (this original octal form is too easily confused with decimal, and is replaced by the new 0o format). Binary literals, new in 2.6 and 3.0, begin with a leading 0b or 0B, followed by binary digits (0–1). Note that all of these literals produce integer objects in program code; they are just alternative syntaxes for specifying values. The built-in calls hex(I), oct(I), and bin(I) convert an integer to its representation string in these three bases, and int(str, base) converts a runtime string to an integer per a given base. Complex numbers Python complex literals are written as realpart+imaginarypart, where the imaginarypart is terminated with a j or J. The realpart is technically optional, so the imaginarypart may appear on its own. Internally, complex numbers are implemented as pairs of floating-point numbers, but all numeric operations perform complex math when applied to complex numbers. Complex numbers may also be created with the complex(real, imag) built-in call. Coding other numeric types As we’ll see later in this chapter, there are additional, more advanced number types not included in Table 5-1. Some of these are created by calling functions in imported modules (e.g., decimals and fractions), and others have literal syntax all their own (e.g., sets). Numeric Type Basics | 107 Built-in Numeric Tools Besides the built-in number literals shown in Table 5-1, Python provides a set of tools for processing number objects: Expression operators +, -, *, /, >>, **, &, etc. Built-in mathematical functions pow, abs, round, int, hex, bin, etc. Utility modules random, math, etc. We’ll meet all of these as we go along. Although numbers are primarily processed with expressions, built-ins, and modules, they also have a handful of type-specific methods today, which we’ll meet in this chapter as well. Floating-point numbers, for example, have an as_integer_ratio method that is useful for the fraction number type, and an is_integer method to test if the number is an integer. Integers have various attributes, including a new bit_length method in the upcoming Python 3.1 release that gives the number of bits necessary to represent the object’s value. Moreover, as part collection and part number, sets also support both methods and expressions. Since expressions are the most essential tool for most number types, though, let’s turn to them next. Python Expression Operators Perhaps the most fundamental tool that processes numbers is the expression: a combination of numbers (or other objects) and operators that computes a value when executed by Python. In Python, expressions are written using the usual mathematical notation and operator symbols. For instance, to add two numbers X and Y you would say X + Y, which tells Python to apply the + operator to the values named by X and Y. The result of the expression is the sum of X and Y, another number object. Table 5-2 lists all the operator expressions available in Python. Many are self-explanatory; for instance, the usual mathematical operators (+, −, *, /, and so on) are supported. A few will be familiar if you’ve used other languages in the past: % computes a division remainder, << performs a bitwise left-shift, & computes a bitwise AND result, and so on. Others are more Python-specific, and not all are numeric in nature: for example, the is operator tests object identity (i.e., address in memory, a strict form of equality), and lambda creates unnamed functions. 108 | Chapter 5: Numeric Types Table 5-2. Python expression operators and precedence Operators Description yield x Generator function send protocol lambda args: expression Anonymous function generation x if y else z Ternary selection (x is evaluated only if y is true) x or y Logical OR (y is evaluated only if x is false) x and y Logical AND (y is evaluated only if x is true) not x Logical negation x in y, x not in y Membership (iterables, sets) x is y, x is not y Object identity tests x < y, x <= y, x > y, x >= y Magnitude comparison, set subset and superset; x == y, x != y Value equality operators x | y Bitwise OR, set union x ^ y Bitwise XOR, set symmetric difference x & y Bitwise AND, set intersection x << y, x >> y Shift x left or right by y bits x + y Addition, concatenation; x – y Subtraction, set difference x * y Multiplication, repetition; x % y Remainder, format; x / y, x // y Division: true and floor −x, +x Negation, identity ˜x Bitwise NOT (inversion) x ** y Power (exponentiation) x[i] Indexing (sequence, mapping, others) x[i:j:k] Slicing x(...) Call (function, method, class, other callable) x.attr Attribute reference (...) Tuple, expression, generator expression [...] List, list comprehension {...} Dictionary, set, set and dictionary comprehensions Numeric Type Basics | 109 Since this book addresses both Python 2.6 and 3.0, here are some notes about version differences and recent additions related to the operators in Table 5-2: • In Python 2.6, value inequality can be written as either X != Y or X <> Y. In Python 3.0, the latter of these options is removed because it is redundant. In either version, best practice is to use X != Y for all value inequality tests. • In Python 2.6, a backquotes expression `X` works the same as repr(X) and converts objects to display strings. Due to its obscurity, this expression is removed in Python 3.0; use the more readable str and repr built-in functions, described in “Numeric Display Formats” on page 115. • The X // Y floor division expression always truncates fractional remainders in both Python 2.6 and 3.0. The X / Y expression performs true division in 3.0 (retaining remainders) and classic division in 2.6 (truncating for integers). See “Division: Classic, Floor, and True” on page 117. • The syntax [...] is used for both list literals and list comprehension expressions. The latter of these performs an implied loop and collects expression results in a new list. See Chapters 4, 14, and 20 for examples. • The syntax (...) is used for tuples and expressions, as well as generator expressions—a form of list comprehension that produces results on demand, instead of building a result list. See Chapters 4 and 20 for examples. The parentheses may sometimes be omitted in all three constructs. • The syntax {...} is used for dictionary literals, and in Python 3.0 for set literals and both dictionary and set comprehensions. See the set coverage in this chapter and Chapters 4, 8, 14, and 20 for examples. • The yield and ternary if/else selection expressions are available in Python 2.5 and later. The former returns send(...) arguments in generators; the latter is shorthand for a multiline if statement. yield requires parentheses if not alone on the right side of an assignment statement. • Comparison operators may be chained: X < Y < Z produces the same result as X < Y and Y < X. See “Comparisons: Normal and Chained” on page 116 for details. • In recent Pythons, the slice expression X[I:J:K] is equivalent to indexing with a slice object: X[slice(I, J, K)]. • In Python 2.X, magnitude comparisons of mixed types—converting numbers to a common type, and ordering other mixed types according to the type name—are allowed. In Python 3.0, nonnumeric mixed-type magnitude comparisons are not allowed and raise exceptions; this includes sorts by proxy. • Magnitude comparisons for dictionaries are also no longer supported in Python 3.0 (though equality tests are); comparing sorted(dict.items()) is one possible replacement. We’ll see most of the operators in Table 5-2 in action later; first, though, we need to take a quick look at the ways these operators may be combined in expressions. 110 | Chapter 5: Numeric Types Mixed operators follow operator precedence As in most languages, in Python, more complex expressions are coded by stringing together the operator expressions in Table 5-2. For instance, the sum of two multiplications might be written as a mix of variables and operators: A * B + C * D So, how does Python know which operation to perform first? The answer to this question lies in operator precedence. When you write an expression with more than one operator, Python groups its parts according to what are called precedence rules, and this grouping determines the order in which the expression’s parts are computed. Table 5-2 is ordered by operator precedence: • Operators lower in the table have higher precedence, and so bind more tightly in mixed expressions. • Operators in the same row in Table 5-2 generally group from left to right when combined (except for exponentiation, which groups right to left, and comparisons, which chain left to right). For example, if you write X + Y * Z, Python evaluates the multiplication first (Y * Z), then adds that result to X because * has higher precedence (is lower in the table) than +. Similarly, in this section’s original example, both multiplications (A * B and C * D) will happen before their results are added. Parentheses group subexpressions You can forget about precedence completely if you’re careful to group parts of expressions with parentheses. When you enclose subexpressions in parentheses, you override Python’s precedence rules; Python always evaluates expressions in parentheses first before using their results in the enclosing expressions. For instance, instead of coding X + Y * Z, you could write one of the following to force Python to evaluate the expression in the desired order: (X + Y) * Z X + (Y * Z) In the first case, + is applied to X and Y first, because this subexpression is wrapped in parentheses. In the second case, the * is performed first (just as if there were no parentheses at all). Generally speaking, adding parentheses in large expressions is a good idea—it not only forces the evaluation order you want, but also aids readability. Mixed types are converted up Besides mixing operators in expressions, you can also mix numeric types. For instance, you can add an integer to a floating-point number: 40 + 3.14 Numeric Type Basics | 111 But this leads to another question: what type is the result—integer or floating-point? The answer is simple, especially if you’ve used almost any other language before: in mixed-type numeric expressions, Python first converts operands up to the type of the most complicated operand, and then performs the math on same-type operands. This behavior is similar to type conversions in the C language. Python ranks the complexity of numeric types like so: integers are simpler than floatingpoint numbers, which are simpler than complex numbers. So, when an integer is mixed with a floating point, as in the preceding example, the integer is converted up to a floating-point value first, and floating-point math yields the floating-point result. Similarly, any mixed-type expression where one operand is a complex number results in the other operand being converted up to a complex number, and the expression yields a complex result. (In Python 2.6, normal integers are also converted to long integers whenever their values are too large to fit in a normal integer; in 3.0, integers subsume longs entirely.) You can force the issue by calling built-in functions to convert types manually: >>> int(3.1415) 3 >>> float(3) 3.0 # Truncates float to integer # Converts integer to float However, you won’t usually need to do this: because Python automatically converts up to the more complex type within an expression, the results are normally what you want. Also, keep in mind that all these mixed-type conversions apply only when mixing numeric types (e.g., an integer and a floating-point) in an expression, including those using numeric and comparison operators. In general, Python does not convert across any other type boundaries automatically. Adding a string to an integer, for example, results in an error, unless you manually convert one or the other; watch for an example when we meet strings in Chapter 7. In Python 2.6, nonnumeric mixed types can be compared, but no conversions are performed (mixed types compare according to a fixed but arbitrary rule). In 3.0, nonnumeric mixed-type comparisons are not allowed and raise exceptions. Preview: Operator overloading and polymorphism Although we’re focusing on built-in numbers right now, all Python operators may be overloaded (i.e., implemented) by Python classes and C extension types to work on objects you create. For instance, you’ll see later that objects coded with classes may be added or concatenated with + expressions, indexed with [i] expressions, and so on. Furthermore, Python itself automatically overloads some operators, such that they perform different actions depending on the type of built-in objects being processed. 112 | Chapter 5: Numeric Types For example, the + operator performs addition when applied to numbers but performs concatenation when applied to sequence objects such as strings and lists. In fact, + can mean anything at all when applied to objects you define with classes. As we saw in the prior chapter, this property is usually called polymorphism—a term indicating that the meaning of an operation depends on the type of the objects being operated on. We’ll revisit this concept when we explore functions in Chapter 16, because it becomes a much more obvious feature in that context. Numbers in Action On to the code! Probably the best way to understand numeric objects and expressions is to see them in action, so let’s start up the interactive command line and try some basic but illustrative operations (see Chapter 3 for pointers if you need help starting an interactive session). Variables and Basic Expressions First of all, let’s exercise some basic math. In the following interaction, we first assign two variables (a and b) to integers so we can use them later in a larger expression. Variables are simply names—created by you or Python—that are used to keep track of information in your program. We’ll say more about this in the next chapter, but in Python: • • • • Variables are created when they are first assigned values. Variables are replaced with their values when used in expressions. Variables must be assigned before they can be used in expressions. Variables refer to objects and are never declared ahead of time. In other words, these assignments cause the variables a and b to spring into existence automatically: % python >>> a = 3 >>> b = 4 # Name created I’ve also used a comment here. Recall that in Python code, text after a # mark and continuing to the end of the line is considered to be a comment and is ignored. Comments are a way to write human-readable documentation for your code. Because code you type interactively is temporary, you won’t normally write comments in this context, but I’ve added them to some of this book’s examples to help explain the code.* In the next part of the book, we’ll meet a related feature—documentation strings—that attaches the text of your comments to objects. * If you’re working along, you don’t need to type any of the comment text from the # through to the end of the line; comments are simply ignored by Python and not required parts of the statements we’re running. Numbers in Action | 113 Now, let’s use our new integer objects in some expressions. At this point, the values of a and b are still 3 and 4, respectively. Variables like these are replaced with their values whenever they’re used inside an expression, and the expression results are echoed back immediately when working interactively: >>> a + 1, a (4, 2) >>> b * 3, b (12, 2.0) >>> a % 2, b (1, 16) >>> 2 + 4.0, (6.0, 16.0) – 1 # Addition (3 + 1), subtraction (3 - 1) / 2 # Multiplication (4 * 3), division (4 / 2) ** 2 # Modulus (remainder), power (4 ** 2) 2.0 ** b # Mixed-type conversions Technically, the results being echoed back here are tuples of two values because the lines typed at the prompt contain two expressions separated by commas; that’s why the results are displayed in parentheses (more on tuples later). Note that the expressions work because the variables a and b within them have been assigned values. If you use a different variable that has never been assigned, Python reports an error rather than filling in some default value: >>> c * 2 Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'c' is not defined You don’t need to predeclare variables in Python, but they must have been assigned at least once before you can use them. In practice, this means you have to initialize counters to zero before you can add to them, initialize lists to an empty list before you can append to them, and so on. Here are two slightly larger expressions to illustrate operator grouping and more about conversions: >>> b / 2 + a 5.0 >>> print(b / (2.0 + a)) 0.8 # Same as ((4 / 2) + 3) # Same as (4 / (2.0 + 3)) In the first expression, there are no parentheses, so Python automatically groups the components according to its precedence rules—because / is lower in Table 5-2 than +, it binds more tightly and so is evaluated first. The result is as if the expression had been organized with parentheses as shown in the comment to the right of the code. Also, notice that all the numbers are integers in the first expression. Because of that, Python 2.6 performs integer division and addition and will give a result of 5, whereas Python 3.0 performs true division with remainders and gives the result shown. If you want integer division in 3.0, code this as b // 2 + a (more on division in a moment). In the second expression, parentheses are added around the + part to force Python to evaluate it first (i.e., before the /). We also made one of the operands floating-point by adding a decimal point: 2.0. Because of the mixed types, Python converts the integer 114 | Chapter 5: Numeric Types referenced by a to a floating-point value (3.0) before performing the +. If all the numbers in this expression were integers, integer division (4 / 5) would yield the truncated integer 0 in Python 2.6 but the floating-point 0.8 in Python 3.0 (again, stay tuned for division details). Numeric Display Formats Notice that we used a print operation in the last of the preceding examples. Without the print, you’ll see something that may look a bit odd at first glance: >>> b / (2.0 + a) 0.80000000000000004 # Auto echo output: more digits >>> print(b / (2.0 + a)) 0.8 # print rounds off digits The full story behind this odd result has to do with the limitations of floating-point hardware and its inability to exactly represent some values in a limited number of bits. Because computer architecture is well beyond this book’s scope, though, we’ll finesse this by saying that all of the digits in the first output are really there in your computer’s floating-point hardware—it’s just that you’re not accustomed to seeing them. In fact, this is really just a display issue—the interactive prompt’s automatic result echo shows more digits than the print statement. If you don’t want to see all the digits, use print; as the sidebar “str and repr Display Formats” on page 116 will explain, you’ll get a user-friendly display. Note, however, that not all values have so many digits to display: >>> 1 / 2.0 0.5 and that there are more ways to display the bits of a number inside your computer than using print and automatic echoes: >>> num = 1 / 3.0 >>> num 0.33333333333333331 >>> print(num) 0.333333333333 >>> '%e' % num '3.333333e-001' >>> '%4.2f' % num '0.33' >>> '{0:4.2f}'.format(num) '0.33' # Echoes # print rounds # String formatting expression # Alternative floating-point format # String formatting method (Python 2.6 and 3.0) The last three of these expressions employ string formatting, a tool that allows for format flexibility, which we will explore in the upcoming chapter on strings (Chapter 7). Its results are strings that are typically printed to displays or reports. Numbers in Action | 115 str and repr Display Formats Technically, the difference between default interactive echoes and print corresponds to the difference between the built-in repr and str functions: >>> num = 1 / 3 >>> repr(num) '0.33333333333333331' >>> str(num) '0.333333333333' # Used by echoes: as-code form # Used by print: user-friendly form Both of these convert arbitrary objects to their string representations: repr (and the default interactive echo) produces results that look as though they were code; str (and the print operation) converts to a typically more user-friendly format if available. Some objects have both—a str for general use, and a repr with extra details. This notion will resurface when we study both strings and operator overloading in classes, and you’ll find more on these built-ins in general later in the book. Besides providing print strings for arbitrary objects, the str built-in is also the name of the string data type and may be called with an encoding name to decode a Unicode string from a byte string. We’ll study the latter advanced role in Chapter 36 of this book. Comparisons: Normal and Chained So far, we’ve been dealing with standard numeric operations (addition and multiplication), but numbers can also be compared. Normal comparisons work for numbers exactly as you’d expect—they compare the relative magnitudes of their operands and return a Boolean result (which we would normally test in a larger statement): >>> 1 < True >>> 2.0 True >>> 2.0 True >>> 2.0 False 2 # Less than >= 1 # Greater than or equal: mixed-type 1 converted to 1.0 == 2.0 # Equal value != 2.0 # Not equal value Notice again how mixed types are allowed in numeric expressions (only); in the second test here, Python compares values in terms of the more complex type, float. Interestingly, Python also allows us to chain multiple comparisons together to perform range tests. Chained comparisons are a sort of shorthand for larger Boolean expressions. In short, Python lets us string together magnitude comparison tests to code chained comparisons such as range tests. The expression (A < B < C), for instance, tests whether B is between A and C; it is equivalent to the Boolean test (A < B and B < C) but is easier on the eyes (and the keyboard). For example, assume the following assignments: 116 | Chapter 5: Numeric Types >>> X = 2 >>> Y = 4 >>> Z = 6 The following two expressions have identical effects, but the first is shorter to type, and it may run slightly faster since Python needs to evaluate Y only once: >>> X < Y < Z True >>> X < Y and Y < Z True # Chained comparisons: range tests The same equivalence holds for false results, and arbitrary chain lengths are allowed: >>> X < Y > Z False >>> X < Y and Y > Z False >>> 1 < 2 < 3.0 < 4 True >>> 1 > 2 > 3.0 > 4 False You can use other comparisons in chained tests, but the resulting expressions can become nonintuitive unless you evaluate them the way Python does. The following, for instance, is false just because 1 is not equal to 2: >>> 1 == 2 < 3 False # Same as: 1 == 2 and 2 < 3 # Not same as: False < 3 (which means 0 < 3, which is true) Python does not compare the 1 == 2 False result to 3—this would technically mean the same as 0 < 3, which would be True (as we’ll see later in this chapter, True and False are just customized 1 and 0). Division: Classic, Floor, and True You’ve seen how division works in the previous sections, so you should know that it behaves slightly differently in Python 3.0 and 2.6. In fact, there are actually three flavors of division, and two different division operators, one of which changes in 3.0: X / Y Classic and true division. In Python 2.6 and earlier, this operator performs classic division, truncating results for integers and keeping remainders for floating-point numbers. In Python 3.0, it performs true division, always keeping remainders regardless of types. X // Y Floor division. Added in Python 2.2 and available in both Python 2.6 and 3.0, this operator always truncates fractional remainders down to their floor, regardless of types. Numbers in Action | 117 True division was added to address the fact that the results of the original classic division model are dependent on operand types, and so can be difficult to anticipate in a dynamically typed language like Python. Classic division was removed in 3.0 because of this constraint—the / and // operators implement true and floor division in 3.0. In sum: • In 3.0, the / now always performs true division, returning a float result that includes any remainder, regardless of operand types. The // performs floor division, which truncates the remainder and returns an integer for integer operands or a float if any operand is a float. • In 2.6, the / does classic division, performing truncating integer division if both operands are integers and float division (keeping remainders) otherwise. The // does floor division and works as it does in 3.0, performing truncating division for integers and floor division for floats. Here are the two operators at work in 3.0 and 2.6: C:\misc> C:\Python30\python >>> >>> 10 / 4 # Differs in 3.0: keeps remainder 2.5 >>> 10 // 4 # Same in 3.0: truncates remainder 2 >>> 10 / 4.0 # Same in 3.0: keeps remainder 2.5 >>> 10 // 4.0 # Same in 3.0: truncates to floor 2.0 C:\misc> C:\Python26\python >>> >>> 10 / 4 2 >>> 10 // 4 2 >>> 10 / 4.0 2.5 >>> 10 // 4.0 2.0 Notice that the data type of the result for // is still dependent on the operand types in 3.0: if either is a float, the result is a float; otherwise, it is an integer. Although this may seem similar to the type-dependent behavior of / in 2.X that motivated its change in 3.0, the type of the return value is much less critical than differences in the return value itself. Moreover, because // was provided in part as a backward-compatibility tool for programs that rely on truncating integer division (and this is more common than you might expect), it must return integers for integers. 118 | Chapter 5: Numeric Types Supporting either Python Although / behavior differs in 2.6 and 3.0, you can still support both versions in your code. If your programs depend on truncating integer division, use // in both 2.6 and 3.0. If your programs require floating-point results with remainders for integers, use float to guarantee that one operand is a float around a / when run in 2.6: X = Y // Z # Always truncates, always an int result for ints in 2.6 and 3.0 X = Y / float(Z) # Guarantees float division with remainder in either 2.6 or 3.0 Alternatively, you can enable 3.0 / division in 2.6 with a __future__ import, rather than forcing it with float conversions: C:\misc> C:\Python26\python >>> from __future__ import division >>> 10 / 4 2.5 >>> 10 // 4 2 # Enable 3.0 "/" behavior Floor versus truncation One subtlety: the // operator is generally referred to as truncating division, but it’s more accurate to refer to it as floor division—it truncates the result down to its floor, which means the closest whole number below the true result. The net effect is to round down, not strictly truncate, and this matters for negatives. You can see the difference for yourself with the Python math module (modules must be imported before you can use their contents; more on this later): >>> >>> 2 >>> -3 >>> 2 >>> -2 import math math.floor(2.5) math.floor(-2.5) math.trunc(2.5) math.trunc(-2.5) When running division operators, you only really truncate for positive results, since truncation is the same as floor; for negatives, it’s a floor result (really, they are both floor, but floor is the same as truncation for positives). Here’s the case for 3.0: C:\misc> c:\python30\python >>> 5 / 2, 5 / −2 (2.5, −2.5) >>> 5 // 2, 5 // −2 (2, −3) # Truncates to floor: rounds to first lower integer # 2.5 becomes 2, −2.5 becomes −3 >>> 5 / 2.0, 5 / −2.0 (2.5, −2.5) Numbers in Action | 119 >>> 5 // 2.0, 5 // −2.0 (2.0, −3.0) # Ditto for floats, though result is float too The 2.6 case is similar, but / results differ again: C:\misc> c:\python26\python >>> 5 / 2, 5 / −2 (2, −3) >>> 5 // 2, 5 // −2 (2, −3) # Differs in 3.0 # This and the rest are the same in 2.6 and 3.0 >>> 5 / 2.0, 5 / −2.0 (2.5, −2.5) >>> 5 // 2.0, 5 // −2.0 (2.0, −3.0) If you really want truncation regardless of sign, you can always run a float division result through math.trunc, regardless of Python version (also see the round built-in for related functionality): C:\misc> c:\python30\python >>> import math >>> 5 / −2 −2.5 >>> 5 // −2 -3 >>> math.trunc(5 / −2) −2 C:\misc> c:\python26\python >>> import math >>> 5 / float(−2) −2.5 >>> 5 / −2, 5 // −2 (−3, −3) >>> math.trunc(5 / float(−2)) −2 # Keep remainder # Floor below result # Truncate instead of floor # Remainder in 2.6 # Floor in 2.6 # Truncate in 2.6 Why does truncation matter? If you are using 3.0, here is the short story on division operators for reference: >>> (5 / 2), (5 / 2.0), (5 / −2.0), (5 / −2) (2.5, 2.5, −2.5, −2.5) # 3.0 true division >>> (5 // 2), (5 // 2.0), (5 // −2.0), (5 // −2) (2, 2.0, −3.0, −3) # 3.0 floor division >>> (9 / 3), (9.0 / 3), (9 // 3), (9 // 3.0) (3.0, 3.0, 3, 3.0) # Both For 2.6 readers, division works as follows: >>> (5 / 2), (5 / 2.0), (5 / −2.0), (5 / −2) (2, 2.5, −2.5, −3) 120 | Chapter 5: Numeric Types # 2.6 classic division >>> (5 // 2), (5 // 2.0), (5 // −2.0), (5 // −2) (2, 2.0, −3.0, −3) # 2.6 floor division (same) >>> (9 / 3), (9.0 / 3), (9 // 3), (9 // 3.0) (3, 3.0, 3, 3.0) # Both Although results have yet to come in, it’s possible that the nontruncating behavior of / in 3.0 may break a significant number of programs. Perhaps because of a C language legacy, many programmers rely on division truncation for integers and will have to learn to use // in such contexts instead. Watch for a simple prime number while loop example in Chapter 13, and a corresponding exercise at the end of Part IV that illustrates the sort of code that may be impacted by this / change. Also stay tuned for more on the special from command used in this section; it’s discussed further in Chapter 24. Integer Precision Division may differ slightly across Python releases, but it’s still fairly standard. Here’s something a bit more exotic. As mentioned earlier, Python 3.0 integers support unlimited size: >>> 999999999999999999999999999999 + 1 1000000000000000000000000000000 Python 2.6 has a separate type for long integers, but it automatically converts any number too large to store in a normal integer to this type. Hence, you don’t need to code any special syntax to use longs, and the only way you can tell that you’re using 2.6 longs is that they print with a trailing “L”: >>> 999999999999999999999999999999 + 1 1000000000000000000000000000000L Unlimited-precision integers are a convenient built-in tool. For instance, you can use them to count the U.S. national debt in pennies in Python directly (if you are so inclined, and have enough memory on your computer for this year’s budget!). They are also why we were able to raise 2 to such large powers in the examples in Chapter 3. Here are the 3.0 and 2.6 cases: >>> 2 ** 200 1606938044258990275541962092341162602522202993782792835301376 >>> 2 ** 200 1606938044258990275541962092341162602522202993782792835301376L Because Python must do extra work to support their extended precision, integer math is usually substantially slower than normal when numbers grow large. However, if you need the precision, the fact that it’s built in for you to use will likely outweigh its performance penalty. Numbers in Action | 121 Complex Numbers Although less widely used than the types we’ve been exploring thus far, complex numbers are a distinct core object type in Python. If you know what they are, you know why they are useful; if not, consider this section optional reading. Complex numbers are represented as two floating-point numbers—the real and imaginary parts—and are coded by adding a j or J suffix to the imaginary part. We can also write complex numbers with a nonzero real part by adding the two parts with a +. For example, the complex number with a real part of 2 and an imaginary part of −3 is written 2 + −3j. Here are some examples of complex math at work: >>> 1j * 1J (-1+0j) >>> 2 + 1j * 3 (2+3j) >>> (2 + 1j) * 3 (6+3j) Complex numbers also allow us to extract their parts as attributes, support all the usual mathematical expressions, and may be processed with tools in the standard cmath module (the complex version of the standard math module). Complex numbers typically find roles in engineering-oriented programs. Because they are advanced tools, check Python’s language reference manual for additional details. Hexadecimal, Octal, and Binary Notation As described earlier in this chapter, Python integers can be coded in hexadecimal, octal, and binary notation, in addition to the normal base 10 decimal coding. The coding rules were laid out at the start of this chapter; let’s look at some live examples here. Keep in mind that these literals are simply an alternative syntax for specifying the value of an integer object. For example, the following literals coded in Python 3.0 or 2.6 produce normal integers with the specified values in all three bases: >>> (1, >>> (1, >>> (1, 0o1, 0o20, 0o377 16, 255) 0x01, 0x10, 0xFF 16, 255) 0b1, 0b10000, 0b11111111 16, 255) # Octal literals # Hex literals # Binary literals Here, the octal value 0o377, the hex value 0xFF, and the binary value 0b11111111 are all decimal 255. Python prints in decimal (base 10) by default but provides built-in functions that allow you to convert integers to other bases’ digit strings: >>> oct(64), hex(64), bin(64) ('0100', '0x40', '0b1000000') 122 | Chapter 5: Numeric Types The oct function converts decimal to octal, hex to hexadecimal, and bin to binary. To go the other way, the built-in int function converts a string of digits to an integer, and an optional second argument lets you specify the numeric base: >>> int('64'), int('100', 8), int('40', 16), int('1000000', 2) (64, 64, 64, 64) # Literals okay too >>> int('0x40', 16), int('0b1000000', 2) (64, 64) The eval function, which you’ll meet later in this book, treats strings as though they were Python code. Therefore, it has a similar effect (but usually runs more slowly—it actually compiles and runs the string as a piece of a program, and it assumes you can trust the source of the string being run; a clever user might be able to submit a string that deletes files on your machine!): >>> eval('64'), eval('0o100'), eval('0x40'), eval('0b1000000') (64, 64, 64, 64) Finally, you can also convert integers to octal and hexadecimal strings with string formatting method calls and expressions: >>> '{0:o}, {1:x}, {2:b}'.format(64, 64, 64) '100, 40, 1000000' >>> '%o, %x, %X' % (64, 255, 255) '100, ff, FF' String formatting is covered in more detail in Chapter 7. Two notes before moving on. First, Python 2.6 users should remember that you can code octals with simply a leading zero, the original octal format in Python: >>> (1, >>> (1, 0o1, 0o20, 0o377 16, 255) 01, 020, 0377 16, 255) # New octal format in 2.6 (same as 3.0) # Old octal literals in 2.6 (and earlier) In 3.0, the syntax in the second of these examples generates an error. Even though it’s not an error in 2.6, be careful not to begin a string of digits with a leading zero unless you really mean to code an octal value. Python 2.6 will treat it as base 8, which may not work as you’d expect—010 is always decimal 8 in 2.6, not decimal 10 (despite what you may or may not think!). This, along with symmetry with the hex and binary forms, is why the octal format was changed in 3.0—you must use 0o010 in 3.0, and probably should in 2.6. Secondly, note that these literals can produce arbitrarily long integers. The following, for instance, creates an integer with hex notation and then displays it first in decimal and then in octal and binary with converters: >>> X = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFF >>> X 5192296858534827628530496329220095L >>> oct(X) Numbers in Action | 123 '017777777777777777777777777777777777777L' >>> bin(X) '0b1111111111111111111111111111111111111111111111111111111111 ...and so on... Speaking of binary digits, the next section shows tools for processing individual bits. Bitwise Operations Besides the normal numeric operations (addition, subtraction, and so on), Python supports most of the numeric expressions available in the C language. This includes operators that treat integers as strings of binary bits. For instance, here it is at work performing bitwise shift and Boolean operations: >>> >>> 4 >>> 3 >>> 1 x = 1 x << 2 # 0001 # Shift left 2 bits: 0100 x | 2 # Bitwise OR: 0011 x & 1 # Bitwise AND: 0001 In the first expression, a binary 1 (in base 2, 0001) is shifted left two slots to create a binary 4 (0100). The last two operations perform a binary OR (0001|0010 = 0011) and a binary AND (0001&0001 = 0001). Such bit-masking operations allow us to encode multiple flags and other values within a single integer. This is one area where the binary and hexadecimal number support in Python 2.6 and 3.0 become especially useful—they allow us to code and inspect numbers by bit-strings: >>> X = 0b0001 >>> X << 2 4 >>> bin(X << 2) '0b100' # Binary literals # Shift left >>> bin(X | 0b010) '0b11' >>> bin(X & 0b1) '0b1' # Bitwise OR # Binary digits string # Bitwise AND >>> X = 0xFF # Hex literals >>> bin(X) '0b11111111' >>> X ^ 0b10101010 # Bitwise XOR 85 >>> bin(X ^ 0b10101010) '0b1010101' >>> int('1010101', 2) 85 >>> hex(85) '0x55' 124 | Chapter 5: Numeric Types # String to int per base # Hex digit string We won’t go into much more detail on “bit-twiddling” here. It’s supported if you need it, and it comes in handy if your Python code must deal with things like network packets or packed binary data produced by a C program. Be aware, though, that bitwise operations are often not as important in a high-level language such as Python as they are in a low-level language such as C. As a rule of thumb, if you find yourself wanting to flip bits in Python, you should think about which language you’re really coding. In general, there are often better ways to encode information in Python than bit strings. In the upcoming Python 3.1 release, the integer bit_length method also allows you to query the number of bits required to represent a number’s value in binary. The same effect can often be achieved by subtracting 2 from the length of the bin string using the len built-in function we met in Chapter 4, though it may be less efficient: >>> X = 99 >>> bin(X), X.bit_length() ('0b1100011', 7) >>> bin(256), (256).bit_length() ('0b100000000', 9) >>> len(bin(256)) - 2 9 Other Built-in Numeric Tools In addition to its core object types, Python also provides both built-in functions and standard library modules for numeric processing. The pow and abs built-in functions, for instance, compute powers and absolute values, respectively. Here are some examples of the built-in math module (which contains most of the tools in the C language’s math library) and a few built-in functions at work: >>> import math >>> math.pi, math.e (3.1415926535897931, 2.7182818284590451) # Common constants >>> math.sin(2 * math.pi / 180) 0.034899496702500969 # Sine, tangent, cosine >>> math.sqrt(144), math.sqrt(2) (12.0, 1.4142135623730951) # Square root >>> pow(2, 4), 2 ** 4 (16, 16) # Exponentiation (power) >>> abs(-42.0), sum((1, 2, 3, 4)) (42.0, 10) # Absolute value, summation >>> min(3, 1, 2, 4), max(3, 1, 2, 4) (1, 4) # Minimum, maximum The sum function shown here works on a sequence of numbers, and min and max accept either a sequence or individual arguments. There are a variety of ways to drop the Numbers in Action | 125 decimal digits of floating-point numbers. We met truncation and floor earlier; we can also round, both numerically and for display purposes: >>> math.floor(2.567), math.floor(-2.567) (2, −3) # Floor (next-lower integer) >>> math.trunc(2.567), math.trunc(−2.567) (2, −2) # Truncate (drop decimal digits) >>> int(2.567), int(−2.567) (2, −2) # Truncate (integer conversion) >>> round(2.567), round(2.467), round(2.567, 2) (3, 2, 2.5699999999999998) # Round (Python 3.0 version) >>> '%.1f' % 2.567, '{0:.2f}'.format(2.567) ('2.6', '2.57') # Round for display (Chapter 7) As we saw earlier, the last of these produces strings that we would usually print and supports a variety of formatting options. As also described earlier, the second to last test here will output (3, 2, 2.57) if we wrap it in a print call to request a more userfriendly display. The last two lines still differ, though—round rounds a floating-point number but still yields a floating-point number in memory, whereas string formatting produces a string and doesn’t yield a modified number: >>> (1 / 3), round(1 / 3, 2), ('%.2f' % (1 / 3)) (0.33333333333333331, 0.33000000000000002, '0.33') Interestingly, there are three ways to compute square roots in Python: using a module function, an expression, or a built-in function (if you’re interested in performance, we will revisit these in an exercise and its solution at the end of Part IV, to see which runs quicker): >>> import math >>> math.sqrt(144) 12.0 >>> 144 ** .5 12.0 >>> pow(144, .5) 12.0 >>> math.sqrt(1234567890) 35136.418286444619 >>> 1234567890 ** .5 35136.418286444619 >>> pow(1234567890, .5) 35136.418286444619 # Module # Expression # Built-in # Larger numbers Notice that standard library modules such as math must be imported, but built-in functions such as abs and round are always available without imports. In other words, modules are external components, but built-in functions live in an implied namespace that Python automatically searches to find names used in your program. This namespace corresponds to the module called builtins in Python 3.0 (__builtin__ in 2.6). There 126 | Chapter 5: Numeric Types is much more about name resolution in the function and module parts of this book; for now, when you hear “module,” think “import.” The standard library random module must be imported as well. This module provides tools for picking a random floating-point number between 0 and 1, selecting a random integer between two numbers, choosing an item at random from a sequence, and more: >>> import random >>> random.random() 0.44694718823781876 >>> random.random() 0.28970426439292829 >>> random.randint(1, 10) 5 >>> random.randint(1, 10) 4 >>> random.choice(['Life of Brian', 'Holy Grail', 'Meaning of Life']) 'Life of Brian' >>> random.choice(['Life of Brian', 'Holy Grail', 'Meaning of Life']) 'Holy Grail' The random module can be useful for shuffling cards in games, picking images at random in a slideshow GUI, performing statistical simulations, and much more. For more details, see Python’s library manual. Other Numeric Types So far in this chapter, we’ve been using Python’s core numeric types—integer, floating point, and complex. These will suffice for most of the number crunching that most programmers will ever need to do. Python comes with a handful of more exotic numeric types, though, that merit a quick look here. Decimal Type Python 2.4 introduced a new core numeric type: the decimal object, formally known as Decimal. Syntactically, decimals are created by calling a function within an imported module, rather than running a literal expression. Functionally, decimals are like floating-point numbers, but they have a fixed number of decimal points. Hence, decimals are fixed-precision floating-point values. For example, with decimals, we can have a floating-point value that always retains just two decimal digits. Furthermore, we can specify how to round or truncate the extra decimal digits beyond the object’s cutoff. Although it generally incurs a small performance penalty compared to the normal floating-point type, the decimal type is well suited to representing fixed-precision quantities like sums of money and can help you achieve better numeric accuracy. Other Numeric Types | 127 The basics The last point merits elaboration. As you may or may not already know, floating-point math is less than exact, because of the limited space used to store values. For example, the following should yield zero, but it does not. The result is close to zero, but there are not enough bits to be precise here: >>> 0.1 + 0.1 + 0.1 - 0.3 5.5511151231257827e-17 Printing the result to produce the user-friendly display format doesn’t completely help either, because the hardware related to floating-point math is inherently limited in terms of accuracy: >>> print(0.1 + 0.1 + 0.1 - 0.3) 5.55111512313e-17 However, with decimals, the result can be dead-on: >>> from decimal import Decimal >>> Decimal('0.1') + Decimal('0.1') + Decimal('0.1') - Decimal('0.3') Decimal('0.0') As shown here, we can make decimal objects by calling the Decimal constructor function in the decimal module and passing in strings that have the desired number of decimal digits for the resulting object (we can use the str function to convert floating-point values to strings if needed). When decimals of different precision are mixed in expressions, Python converts up to the largest number of decimal digits automatically: >>> Decimal('0.1') + Decimal('0.10') + Decimal('0.10') - Decimal('0.30') Decimal('0.00') In Python 3.1 (to be released after this book’s publication), it’s also possible to create a decimal object from a floating-point object, with a call of the form decimal.Decimal.from_float(1.25). The conversion is exact but can sometimes yield a large number of digits. Setting precision globally Other tools in the decimal module can be used to set the precision of all decimal numbers, set up error handling, and more. For instance, a context object in this module allows for specifying precision (number of decimal digits) and rounding modes (down, ceiling, etc.). The precision is applied globally for all decimals created in the calling thread: >>> import decimal >>> decimal.Decimal(1) / decimal.Decimal(7) Decimal('0.1428571428571428571428571429') >>> decimal.getcontext().prec = 4 >>> decimal.Decimal(1) / decimal.Decimal(7) Decimal('0.1429') 128 | Chapter 5: Numeric Types This is especially useful for monetary applications, where cents are represented as two decimal digits. Decimals are essentially an alternative to manual rounding and string formatting in this context: >>> 1999 + 1.33 2000.3299999999999 >>> >>> decimal.getcontext().prec = 2 >>> pay = decimal.Decimal(str(1999 + 1.33)) >>> pay Decimal('2000.33') Decimal context manager In Python 2.6 and 3.0 (and later), it’s also possible to reset precision temporarily by using the with context manager statement. The precision is reset to its original value on statement exit: C:\misc> C:\Python30\python >>> import decimal >>> decimal.Decimal('1.00') / decimal.Decimal('3.00') Decimal('0.3333333333333333333333333333') >>> >>> with decimal.localcontext() as ctx: ... ctx.prec = 2 ... decimal.Decimal('1.00') / decimal.Decimal('3.00') ... Decimal('0.33') >>> >>> decimal.Decimal('1.00') / decimal.Decimal('3.00') Decimal('0.3333333333333333333333333333') Though useful, this statement requires much more background knowledge than you’ve obtained at this point; watch for coverage of the with statement in Chapter 33. Because use of the decimal type is still relatively rare in practice, I’ll defer to Python’s standard library manuals and interactive help for more details. And because decimals address some of the same floating-point accuracy issues as the fraction type, let’s move on to the next section to see how the two compare. Fraction Type Python 2.6 and 3.0 debut a new numeric type, Fraction, which implements a rational number object. It essentially keeps both a numerator and a denominator explicitly, so as to avoid some of the inaccuracies and limitations of floating-point math. The basics Fraction is a sort of cousin to the existing Decimal fixed-precision type described in the prior section, as both can be used to control numerical accuracy by fixing decimal digits and specifying rounding or truncation policies. It’s also used in similar ways—like Other Numeric Types | 129 Decimal, Fraction resides in a module; import its constructor and pass in a numerator and a denominator to make one. The following interaction shows how: >>> from fractions import Fraction >>> x = Fraction(1, 3) >>> y = Fraction(4, 6) # Numerator, denominator # Simplified to 2, 3 by gcd >>> x Fraction(1, 3) >>> y Fraction(2, 3) >>> print(y) 2/3 Once created, Fractions can be used in mathematical expressions as usual: >>> x + y Fraction(1, 1) >>> x – y Fraction(-1, 3) >>> x * y Fraction(2, 9) # Results are exact: numerator, denominator Fraction objects can also be created from floating-point number strings, much like decimals: >>> Fraction('.25') Fraction(1, 4) >>> Fraction('1.25') Fraction(5, 4) >>> >>> Fraction('.25') + Fraction('1.25') Fraction(3, 2) Numeric accuracy Notice that this is different from floating-point-type math, which is constrained by the underlying limitations of floating-point hardware. To compare, here are the same operations run with floating-point objects, and notes on their limited accuracy: >>> a = 1 / 3.0 >>> b = 4 / 6.0 >>> a 0.33333333333333331 >>> b 0.66666666666666663 # Only as accurate as floating-point hardware # Can lose precision over calculations >>> a + b 1.0 >>> a - b -0.33333333333333331 >>> a * b 0.22222222222222221 This floating-point limitation is especially apparent for values that cannot be represented accurately given their limited number of bits in memory. Both Fraction and 130 | Chapter 5: Numeric Types Decimal provide ways to get exact results, albeit at the cost of some speed. For instance, in the following example (repeated from the prior section), floating-point numbers do not accurately give the zero answer expected, but both of the other types do: # This should be zero (close, but not exact) >>> 0.1 + 0.1 + 0.1 - 0.3 5.5511151231257827e-17 >>> from fractions import Fraction >>> Fraction(1, 10) + Fraction(1, 10) + Fraction(1, 10) - Fraction(3, 10) Fraction(0, 1) >>> from decimal import Decimal >>> Decimal('0.1') + Decimal('0.1') + Decimal('0.1') - Decimal('0.3') Decimal('0.0') Moreover, fractions and decimals both allow more intuitive and accurate results than floating points sometimes can, in different ways (by using rational representation and by limiting precision): >>> 1 / 3 0.33333333333333331 # Use 3.0 in Python 2.6 for true "/" >>> Fraction(1, 3) Fraction(1, 3) # Numeric accuracy >>> import decimal >>> decimal.getcontext().prec = 2 >>> decimal.Decimal(1) / decimal.Decimal(3) Decimal('0.33') In fact, fractions both retain accuracy and automatically simplify results. Continuing the preceding interaction: >>> (1 / 3) + (6 / 12) 0.83333333333333326 # Use ".0" in Python 2.6 for true "/" >>> Fraction(6, 12) Fraction(1, 2) # Automatically simplified >>> Fraction(1, 3) + Fraction(6, 12) Fraction(5, 6) >>> decimal.Decimal(str(1/3)) + decimal.Decimal(str(6/12)) Decimal('0.83') >>> 1000.0 / 1234567890 8.1000000737100011e-07 >>> Fraction(1000, 1234567890) Fraction(100, 123456789) Conversions and mixed types To support fraction conversions, floating-point objects now have a method that yields their numerator and denominator ratio, fractions have a from_float method, and Other Numeric Types | 131 float accepts a Fraction as an argument. Trace through the following interaction to see how this pans out (the * in the second test is special syntax that expands a tuple into individual arguments; more on this when we study function argument passing in Chapter 18): >>> (2.5).as_integer_ratio() (5, 2) >>> f = 2.5 >>> z = Fraction(*f.as_integer_ratio()) >>> z Fraction(5, 2) # float object method # Convert float -> fraction: two args # Same as Fraction(5, 2) >>> x Fraction(1, 3) >>> x + z Fraction(17, 6) # x from prior interaction >>> float(x) 0.33333333333333331 >>> float(z) 2.5 >>> float(x + z) 2.8333333333333335 >>> 17 / 6 2.8333333333333335 # Convert fraction -> float >>> Fraction.from_float(1.75) Fraction(7, 4) >>> Fraction(*(1.75).as_integer_ratio()) Fraction(7, 4) # Convert float -> fraction: other way # 5/2 + 1/3 = 15/6 + 2/6 Finally, some type mixing is allowed in expressions, though Fraction must sometimes be manually propagated to retain accuracy. Study the following interaction to see how this works: >>> x Fraction(1, 3) >>> x + 2 Fraction(7, 3) >>> x + 2.0 2.3333333333333335 >>> x + (1./3) 0.66666666666666663 >>> x + (4./3) 1.6666666666666665 >>> x + Fraction(4, 3) Fraction(5, 3) # Fraction + int -> Fraction # Fraction + float -> float # Fraction + float -> float # Fraction + Fraction -> Fraction Caveat: although you can convert from floating-point to fraction, in some cases there is an unavoidable precision loss when you do so, because the number is inaccurate in its original floating-point form. When needed, you can simplify such results by limiting the maximum denominator value: 132 | Chapter 5: Numeric Types >>> 4.0 / 3 1.3333333333333333 >>> (4.0 / 3).as_integer_ratio() (6004799503160661, 4503599627370496) # Precision loss from float >>> x Fraction(1, 3) >>> a = x + Fraction(*(4.0 / 3).as_integer_ratio()) >>> a Fraction(22517998136852479, 13510798882111488) >>> 22517998136852479 / 13510798882111488. 1.6666666666666667 # 5 / 3 (or close to it!) >>> a.limit_denominator(10) Fraction(5, 3) # Simplify to closest fraction For more details on the Fraction type, experiment further on your own and consult the Python 2.6 and 3.0 library manuals and other documentation. Sets Python 2.4 also introduced a new collection type, the set—an unordered collection of unique and immutable objects that supports operations corresponding to mathematical set theory. By definition, an item appears only once in a set, no matter how many times it is added. As such, sets have a variety of applications, especially in numeric and database-focused work. Because sets are collections of other objects, they share some behavior with objects such as lists and dictionaries that are outside the scope of this chapter. For example, sets are iterable, can grow and shrink on demand, and may contain a variety of object types. As we’ll see, a set acts much like the keys of a valueless dictionary, but it supports extra operations. However, because sets are unordered and do not map keys to values, they are neither sequence nor mapping types; they are a type category unto themselves. Moreover, because sets are fundamentally mathematical in nature (and for many readers, may seem more academic and be used much less often than more pervasive objects like dictionaries), we’ll explore the basic utility of Python’s set objects here. Set basics in Python 2.6 There are a few ways to make sets today, depending on whether you are using Python 2.6 or 3.0. Since this book covers both, let’s begin with the 2.6 case, which also is available (and sometimes still required) in 3.0; we’ll refine this for 3.0 extensions in a moment. To make a set object, pass in a sequence or other iterable object to the builtin set function: >>> x = set('abcde') >>> y = set('bdxyz') Other Numeric Types | 133 You get back a set object, which contains all the items in the object passed in (notice that sets do not have a positional ordering, and so are not sequences): >>> x set(['a', 'c', 'b', 'e', 'd']) # 2.6 display format Sets made this way support the common mathematical set operations with expression operators. Note that we can’t perform these expressions on plain sequences—we must create sets from them in order to apply these tools: >>> 'e' in x True # Membership >>> x – y set(['a', 'c', 'e']) # Difference >>> x | y set(['a', 'c', 'b', 'e', 'd', 'y', 'x', 'z']) # Union >>> x & y set(['b', 'd']) # Intersection >>> x ^ y set(['a', 'c', 'e', 'y', 'x', 'z']) # Symmetric difference (XOR) >>> x > y, x < y (False, False) # Superset, subset In addition to expressions, the set object provides methods that correspond to these operations and more, and that support set changes—the set add method inserts one item, update is an in-place union, and remove deletes an item by value (run a dir call on any set instance or the set type name to see all the available methods). Assuming x and y are still as they were in the prior interaction: >>> z = x.intersection(y) >>> z set(['b', 'd']) >>> z.add('SPAM') >>> z set(['b', 'd', 'SPAM']) >>> z.update(set(['X', 'Y'])) >>> z set(['Y', 'X', 'b', 'd', 'SPAM']) >>> z.remove('b') >>> z set(['Y', 'X', 'd', 'SPAM']) # Same as x & y # Insert one item # Merge: in-place union # Delete one item As iterable containers, sets can also be used in operations such as len, for loops, and list comprehensions. Because they are unordered, though, they don’t support sequence operations like indexing and slicing: >>> for item in set('abc'): print(item * 3) ... aaa 134 | Chapter 5: Numeric Types ccc bbb Finally, although the set expressions shown earlier generally require two sets, their method-based counterparts can often work with any iterable type as well: >>> S = set([1, 2, 3]) >>> S | set([3, 4]) # Expressions require both to be sets set([1, 2, 3, 4]) >>> S | [3, 4] TypeError: unsupported operand type(s) for |: 'set' and 'list' >>> S.union([3, 4]) # But their methods allow any iterable set([1, 2, 3, 4]) >>> S.intersection((1, 3, 5)) set([1, 3]) >>> S.issubset(range(-5, 5)) True For more details on set operations, see Python’s library reference manual or a reference book. Although set operations can be coded manually in Python with other types, like lists and dictionaries (and often were in the past), Python’s built-in sets use efficient algorithms and implementation techniques to provide quick and standard operation. Set literals in Python 3.0 If you think sets are “cool,” they recently became noticeably cooler. In Python 3.0 we can still use the set built-in to make set objects, but 3.0 also adds a new set literal form, using the curly braces formerly reserved for dictionaries. In 3.0, the following are equivalent: set([1, 2, 3, 4]) {1, 2, 3, 4} # Built-in call # 3.0 set literals This syntax makes sense, given that sets are essentially like valueless dictionaries— because they are unordered, unique, and immutable, a set’s items behave much like a dictionary’s keys. This operational similarity is even more striking given that dictionary key lists in 3.0 are view objects, which support set-like behavior such as intersections and unions (see Chapter 8 for more on dictionary view objects). In fact, regardless of how a set is made, 3.0 displays it using the new literal format. The set built-in is still required in 3.0 to create empty sets and to build sets from existing iterable objects (short of using set comprehensions, discussed later in this chapter), but the new literal is convenient for initializing sets of known structure: C:\Misc> c:\python30\python >>> set([1, 2, 3, 4]) {1, 2, 3, 4} >>> set('spam') {'a', 'p', 's', 'm'} >>> {1, 2, 3, 4} # Built-in: same as in 2.6 # Add all items in an iterable # Set literals: new in 3.0 Other Numeric Types | 135 {1, 2, 3, 4} >>> S = {'s', 'p', 'a', 'm'} >>> S.add('alot') >>> S {'a', 'p', 's', 'm', 'alot'} All the set processing operations discussed in the prior section work the same in 3.0, but the result sets print differently: >>> S1 = {1, 2, 3, 4} >>> S1 & {1, 3} {1, 3} >>> {1, 5, 3, 6} | S1 {1, 2, 3, 4, 5, 6} >>> S1 - {1, 3, 4} {2} >>> S1 > {1, 3} True # Intersection # Union # Difference # Superset Note that {} is still a dictionary in Python. Empty sets must be created with the set built-in, and print the same way: >>> S1 - {1, 2, 3, 4} set() >>> type({}) <class 'dict'> # Empty sets print differently >>> S = set() >>> S.add(1.23) >>> S {1.23} # Initialize an empty set # Because {} is an empty dictionary As in Python 2.6, sets created with 3.0 literals support the same methods, some of which allow general iterable operands that expressions do not: >>> {1, 2, 3} | {3, 4} {1, 2, 3, 4} >>> {1, 2, 3} | [3, 4] TypeError: unsupported operand type(s) for |: 'set' and 'list' >>> {1, >>> {1, >>> {1, {1, 2, 3}.union([3, 4]) 2, 3, 4} {1, 2, 3}.union({3, 4}) 2, 3, 4} {1, 2, 3}.union(set([3, 4])) 2, 3, 4} >>> {1, 2, 3}.intersection((1, 3, 5)) {1, 3} >>> {1, 2, 3}.issubset(range(-5, 5)) True Immutable constraints and frozen sets Sets are powerful and flexible objects, but they do have one constraint in both 3.0 and 2.6 that you should keep in mind—largely because of their implementation, sets can 136 | Chapter 5: Numeric Types only contain immutable (a.k.a “hashable”) object types. Hence, lists and dictionaries cannot be embedded in sets, but tuples can if you need to store compound values. Tuples compare by their full values when used in set operations: >>> S {1.23} >>> S.add([1, 2, 3]) TypeError: unhashable type: 'list' >>> S.add({'a':1}) TypeError: unhashable type: 'dict' >>> S.add((1, 2, 3)) >>> S {1.23, (1, 2, 3)} >>> S | {(4, 5, 6), (1, 2, 3)} {1.23, (4, 5, 6), (1, 2, 3)} >>> (1, 2, 3) in S True >>> (1, 4, 3) in S False # Only mutable objects work in a set # No list or dict, but tuple okay # Union: same as S.union(...) # Membership: by complete values Tuples in a set, for instance, might be used to represent dates, records, IP addresses, and so on (more on tuples later in this part of the book). Sets themselves are mutable too, and so cannot be nested in other sets directly; if you need to store a set inside another set, the frozenset built-in call works just like set but creates an immutable set that cannot change and thus can be embedded in other sets. Set comprehensions in Python 3.0 In addition to literals, 3.0 introduces a set comprehension construct; it is similar in form to the list comprehension we previewed in Chapter 4, but is coded in curly braces instead of square brackets and run to make a set instead of a list. Set comprehensions run a loop and collect the result of an expression on each iteration; a loop variable gives access to the current iteration value for use in the collection expression. The result is a new set created by running the code, with all the normal set behavior: >>> {x ** 2 for x in [1, 2, 3, 4]} {16, 1, 4, 9} # 3.0 set comprehension In this expression, the loop is coded on the right, and the collection expression is coded on the left (x ** 2). As for list comprehensions, we get back pretty much what this expression says: “Give me a new set containing X squared, for every X in a list.” Comprehensions can also iterate across other kinds of objects, such as strings (the first of the following examples illustrates the comprehension-based way to make a set from an existing iterable): >>> {x for x in 'spam'} {'a', 'p', 's', 'm'} # Same as: set('spam') >>> {c * 4 for c in 'spam'} {'ssss', 'aaaa', 'pppp', 'mmmm'} >>> {c * 4 for c in 'spamham'} # Set of collected expression results Other Numeric Types | 137 {'ssss', 'aaaa', 'hhhh', 'pppp', 'mmmm'} >>> S = {c * 4 for c in 'spam'} >>> S | {'mmmm', 'xxxx'} {'ssss', 'aaaa', 'pppp', 'mmmm', 'xxxx'} >>> S & {'mmmm', 'xxxx'} {'mmmm'} Because the rest of the comprehensions story relies upon underlying concepts we’re not yet prepared to address, we’ll postpone further details until later in this book. In Chapter 8, we’ll meet a first cousin in 3.0, the dictionary comprehension, and I’ll have much more to say about all comprehensions (list, set, dictionary, and generator) later, especially in Chapters14 and 20. As we’ll learn later, all comprehensions, including sets, support additional syntax not shown here, including nested loops and if tests, which can be difficult to understand until you’ve had a chance to study larger statements. Why sets? Set operations have a variety of common uses, some more practical than mathematical. For example, because items are stored only once in a set, sets can be used to filter duplicates out of other collections. Simply convert the collection to a set, and then convert it back again (because sets are iterable, they work in the list call here): >>> >>> {1, >>> >>> [1, L = [1, 2, 1, 3, 2, 4, 5] set(L) 2, 3, 4, 5} L = list(set(L)) L 2, 3, 4, 5] # Remove duplicates Sets can also be used to keep track of where you’ve already been when traversing a graph or other cyclic structure. For example, the transitive module reloader and inheritance tree lister examples we’ll study in Chapters 24 and 30, respectively, must keep track of items visited to avoid loops. Although recording states visited as keys in a dictionary is efficient, sets offer an alternative that’s essentially equivalent (and may be more or less intuitive, depending on who you ask). Finally, sets are also convenient when dealing with large data sets (database query results, for example)—the intersection of two sets contains objects in common to both categories, and the union contains all items in either set. To illustrate, here’s a somewhat more realistic example of set operations at work, applied to lists of people in a hypothetical company, using 3.0 set literals (use set in 2.6): >>> engineers = {'bob', 'sue', 'ann', 'vic'} >>> managers = {'tom', 'sue'} >>> 'bob' in engineers True # Is bob an engineer? >>> engineers & managers # Who is both engineer and manager? 138 | Chapter 5: Numeric Types {'sue'} >>> engineers | managers {'vic', 'sue', 'tom', 'bob', 'ann'} # All people in either category >>> engineers – managers {'vic', 'bob', 'ann'} # Engineers who are not managers >>> managers – engineers {'tom'} # Managers who are not engineers >>> engineers > managers False # Are all managers engineers? (superset) >>> {'bob', 'sue'} < engineers True # Are both engineers? (subset) >>> (managers | engineers) > managers True # All people is a superset of managers >>> managers ^ engineers {'vic', 'bob', 'ann', 'tom'} # Who is in one but not both? >>> (managers | engineers) - (managers ^ engineers) {'sue'} # Intersection! You can find more details on set operations in the Python library manual and some mathematical and relational database theory texts. Also stay tuned for Chapter 8’s revival of some of the set operations we’ve seen here, in the context of dictionary view objects in Python 3.0. Booleans Some argue that the Python Boolean type, bool, is numeric in nature because its two values, True and False, are just customized versions of the integers 1 and 0 that print themselves differently. Although that’s all most programmers need to know, let’s explore this type in a bit more detail. More formally, Python today has an explicit Boolean data type called bool, with the values True and False available as new preassigned built-in names. Internally, the names True and False are instances of bool, which is in turn just a subclass (in the objectoriented sense) of the built-in integer type int. True and False behave exactly like the integers 1 and 0, except that they have customized printing logic—they print themselves as the words True and False, instead of the digits 1 and 0. bool accomplishes this by redefining str and repr string formats for its two objects. Because of this customization, the output of Boolean expressions typed at the interactive prompt prints as the words True and False instead of the older and less obvious 1 and 0. In addition, Booleans make truth values more explicit. For instance, an infinite loop can now be coded as while True: instead of the less intuitive while 1:. Similarly, Other Numeric Types | 139 flags can be initialized more clearly with flag = False. We’ll discuss these statements further in Part III. Again, though, for all other practical purposes, you can treat True and False as though they are predefined variables set to integer 1 and 0. Most programmers used to preassign True and False to 1 and 0 anyway; the bool type simply makes this standard. Its implementation can lead to curious results, though. Because True is just the integer 1 with a custom display format, True + 4 yields 5 in Python: >>> type(True) <class 'bool'> >>> isinstance(True, int) True >>> True == 1 True >>> True is 1 False >>> True or False True >>> True + 4 5 # Same value # But different object: see the next chapter # Same as: 1 or 0 # (Hmmm) Since you probably won’t come across an expression like the last of these in real Python code, you can safely ignore its deeper metaphysical implications.... We’ll revisit Booleans in Chapter 9 (to define Python’s notion of truth) and again in Chapter 12 (to see how Boolean operators like and and or work). Numeric Extensions Finally, although Python core numeric types offer plenty of power for most applications, there is a large library of third-party open source extensions available to address more focused needs. Because numeric programming is a popular domain for Python, you’ll find a wealth of advanced tools. For example, if you need to do serious number crunching, an optional extension for Python called NumPy (Numeric Python) provides advanced numeric programming tools, such as a matrix data type, vector processing, and sophisticated computation libraries. Hardcore scientific programming groups at places like Los Alamos and NASA use Python with NumPy to implement the sorts of tasks they previously coded in C++, FORTRAN, or Matlab. The combination of Python and NumPy is often compared to a free, more flexible version of Matlab—you get NumPy’s performance, plus the Python language and its libraries. Because it’s so advanced, we won’t talk further about NumPy in this book. You can find additional support for advanced numeric programming in Python, including graphics and plotting tools, statistics libraries, and the popular SciPy package at Python’s PyPI site, or by searching the Web. Also note that NumPy is currently an optional extension; it doesn’t come with Python and must be installed separately. 140 | Chapter 5: Numeric Types Chapter Summary This chapter has taken a tour of Python’s numeric object types and the operations we can apply to them. Along the way, we met the standard integer and floating-point types, as well as some more exotic and less commonly used types such as complex numbers, fractions, and sets. We also explored Python’s expression syntax, type conversions, bitwise operations, and various literal forms for coding numbers in scripts. Later in this part of the book, I’ll fill in some details about the next object type, the string. In the next chapter, however, we’ll take some time to explore the mechanics of variable assignment in more detail than we have here. This turns out to be perhaps the most fundamental idea in Python, so make sure you check out the next chapter before moving on. First, though, it’s time to take the usual chapter quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. 7. 8. 9. What is the value of the expression 2 * (3 + 4) in Python? What is the value of the expression 2 * 3 + 4 in Python? What is the value of the expression 2 + 3 * 4 in Python? What tools can you use to find a number’s square root, as well as its square? What is the type of the result of the expression 1 + 2.0 + 3? How can you truncate and round a floating-point number? How can you convert an integer to a floating-point number? How would you display an integer in octal, hexadecimal, or binary notation? How might you convert an octal, hexadecimal, or binary string to a plain integer? Test Your Knowledge: Answers 1. The value will be 14, the result of 2 * 7, because the parentheses force the addition to happen before the multiplication. 2. The value will be 10, the result of 6 + 4. Python’s operator precedence rules are applied in the absence of parentheses, and multiplication has higher precedence than (i.e., happens before) addition, per Table 5-2. 3. This expression yields 14, the result of 2 + 12, for the same precedence reasons as in the prior question. 4. Functions for obtaining the square root, as well as pi, tangents, and more, are available in the imported math module. To find a number’s square root, import math and call math.sqrt(N). To get a number’s square, use either the exponent Test Your Knowledge: Answers | 141 5. 6. 7. 8. 9. expression X ** 2 or the built-in function pow(X, 2). Either of these last two can also compute the square root when given a power of 0.5 (e.g., X ** .5). The result will be a floating-point number: the integers are converted up to floating point, the most complex type in the expression, and floating-point math is used to evaluate it. The int(N) and math.trunc(N) functions truncate, and the round(N, digits) function rounds. We can also compute the floor with math.floor(N) and round for display with string formatting operations. The float(I) function converts an integer to a floating point; mixing an integer with a floating point within an expression will result in a conversion as well. In some sense, Python 3.0 / division converts too—it always returns a floating-point result that includes the remainder, even if both operands are integers. The oct(I) and hex(I) built-in functions return the octal and hexadecimal string forms for an integer. The bin(I) call also returns a number’s binary digits string in Python 2.6 and 3.0. The % string formatting expression and format string method also provide targets for some such conversions. The int(S, base) function can be used to convert from octal and hexadecimal strings to normal integers (pass in 8, 16, or 2 for the base). The eval(S) function can be used for this purpose too, but it’s more expensive to run and can have security issues. Note that integers are always stored in binary in computer memory; these are just display string format conversions. 142 | Chapter 5: Numeric Types CHAPTER 6 The Dynamic Typing Interlude In the prior chapter, we began exploring Python’s core object types in depth with a look at Python numbers. We’ll resume our object type tour in the next chapter, but before we move on, it’s important that you get a handle on what may be the most fundamental idea in Python programming and is certainly the basis of much of both the conciseness and flexibility of the Python language—dynamic typing, and the polymorphism it yields. As you’ll see here and later in this book, in Python, we do not declare the specific types of the objects our scripts use. In fact, programs should not even care about specific types; in exchange, they are naturally applicable in more contexts than we can sometimes even plan ahead for. Because dynamic typing is the root of this flexibility, let’s take a brief look at the model here. The Case of the Missing Declaration Statements If you have a background in compiled or statically typed languages like C, C++, or Java, you might find yourself a bit perplexed at this point in the book. So far, we’ve been using variables without declaring their existence or their types, and it somehow works. When we type a = 3 in an interactive session or program file, for instance, how does Python know that a should stand for an integer? For that matter, how does Python know what a is at all? Once you start asking such questions, you’ve crossed over into the domain of Python’s dynamic typing model. In Python, types are determined automatically at runtime, not in response to declarations in your code. This means that you never declare variables ahead of time (a concept that is perhaps simpler to grasp if you keep in mind that it all boils down to variables, objects, and the links between them). 143 Variables, Objects, and References As you’ve seen in many of the examples used so far in this book, when you run an assignment statement such as a = 3 in Python, it works even if you’ve never told Python to use the name a as a variable, or that a should stand for an integer-type object. In the Python language, this all pans out in a very natural way, as follows: Variable creation A variable (i.e., name), like a, is created when your code first assigns it a value. Future assignments change the value of the already created name. Technically, Python detects some names before your code runs, but you can think of it as though initial assignments make variables. Variable types A variable never has any type information or constraints associated with it. The notion of type lives with objects, not names. Variables are generic in nature; they always simply refer to a particular object at a particular point in time. Variable use When a variable appears in an expression, it is immediately replaced with the object that it currently refers to, whatever that may be. Further, all variables must be explicitly assigned before they can be used; referencing unassigned variables results in errors. In sum, variables are created when assigned, can reference any type of object, and must be assigned before they are referenced. This means that you never need to declare names used by your script, but you must initialize names before you can update them; counters, for example, must be initialized to zero before you can add to them. This dynamic typing model is strikingly different from the typing model of traditional languages. When you are first starting out, the model is usually easier to understand if you keep clear the distinction between names and objects. For example, when we say this: >>> a = 3 at least conceptually, Python will perform three distinct steps to carry out the request. These steps reflect the operation of all assignments in the Python language: 1. Create an object to represent the value 3. 2. Create the variable a, if it does not yet exist. 3. Link the variable a to the new object 3. The net result will be a structure inside Python that resembles Figure 6-1. As sketched, variables and objects are stored in different parts of memory and are associated by links (the link is shown as a pointer in the figure). Variables always link to objects and never to other variables, but larger objects may link to other objects (for instance, a list object has links to the objects it contains). 144 | Chapter 6: The Dynamic Typing Interlude Figure 6-1. Names and objects after running the assignment a = 3. Variable a becomes a reference to the object 3. Internally, the variable is really a pointer to the object’s memory space created by running the literal expression 3. These links from variables to objects are called references in Python—that is, a reference is a kind of association, implemented as a pointer in memory.* Whenever the variables are later used (i.e., referenced), Python automatically follows the variable-to-object links. This is all simpler than the terminology may imply. In concrete terms: • Variables are entries in a system table, with spaces for links to objects. • Objects are pieces of allocated memory, with enough space to represent the values for which they stand. • References are automatically followed pointers from variables to objects. At least conceptually, each time you generate a new value in your script by running an expression, Python creates a new object (i.e., a chunk of memory) to represent that value. Internally, as an optimization, Python caches and reuses certain kinds of unchangeable objects, such as small integers and strings (each 0 is not really a new piece of memory—more on this caching behavior later). But, from a logical perspective, it works as though each expression’s result value is a distinct object and each object is a distinct piece of memory. Technically speaking, objects have more structure than just enough space to represent their values. Each object also has two standard header fields: a type designator used to mark the type of the object, and a reference counter used to determine when it’s OK to reclaim the object. To understand how these two header fields factor into the model, we need to move on. Types Live with Objects, Not Variables To see how object types come into play, watch what happens if we assign a variable multiple times: * Readers with a background in C may find Python references similar to C pointers (memory addresses). In fact, references are implemented as pointers, and they often serve the same roles, especially with objects that can be changed in-place (more on this later). However, because references are always automatically dereferenced when used, you can never actually do anything useful with a reference itself; this is a feature that eliminates a vast category of C bugs. You can think of Python references as C “void*” pointers, which are automatically followed whenever used. The Case of the Missing Declaration Statements | 145 >>> a = 3 >>> a = 'spam' >>> a = 1.23 # It's an integer # Now it's a string # Now it's a floating point This isn’t typical Python code, but it does work—a starts out as an integer, then becomes a string, and finally becomes a floating-point number. This example tends to look especially odd to ex-C programmers, as it appears as though the type of a changes from integer to string when we say a = 'spam'. However, that’s not really what’s happening. In Python, things work more simply. Names have no types; as stated earlier, types live with objects, not names. In the preceding listing, we’ve simply changed a to reference different objects. Because variables have no type, we haven’t actually changed the type of the variable a; we’ve simply made the variable reference a different type of object. In fact, again, all we can ever say about a variable in Python is that it references a particular object at a particular point in time. Objects, on the other hand, know what type they are—each object contains a header field that tags the object with its type. The integer object 3, for example, will contain the value 3, plus a designator that tells Python that the object is an integer (strictly speaking, a pointer to an object called int, the name of the integer type). The type designator of the 'spam' string object points to the string type (called str) instead. Because objects know their types, variables don’t have to. To recap, types are associated with objects in Python, not with variables. In typical code, a given variable usually will reference just one kind of object. Because this isn’t a requirement, though, you’ll find that Python code tends to be much more flexible than you may be accustomed to—if you use Python well, your code might work on many types automatically. I mentioned that objects have two header fields, a type designator and a reference counter. To understand the latter of these, we need to move on and take a brief look at what happens at the end of an object’s life. Objects Are Garbage-Collected In the prior section’s listings, we assigned the variable a to different types of objects in each assignment. But when we reassign a variable, what happens to the value it was previously referencing? For example, after the following statements, what happens to the object 3? >>> a = 3 >>> a = 'spam' The answer is that in Python, whenever a name is assigned to a new object, the space held by the prior object is reclaimed (if it is not referenced by any other name or object). This automatic reclamation of objects’ space is known as garbage collection. To illustrate, consider the following example, which sets the name x to a different object on each assignment: 146 | Chapter 6: The Dynamic Typing Interlude >>> >>> >>> >>> x x x x = = = = 42 'shrubbery' 3.1415 [1, 2, 3] # Reclaim 42 now (unless referenced elsewhere) # Reclaim 'shrubbery' now # Reclaim 3.1415 now First, notice that x is set to a different type of object each time. Again, though this is not really the case, the effect is as though the type of x is changing over time. Remember, in Python types live with objects, not names. Because names are just generic references to objects, this sort of code works naturally. Second, notice that references to objects are discarded along the way. Each time x is assigned to a new object, Python reclaims the prior object’s space. For instance, when it is assigned the string 'shrubbery', the object 42 is immediately reclaimed (assuming it is not referenced anywhere else)—that is, the object’s space is automatically thrown back into the free space pool, to be reused for a future object. Internally, Python accomplishes this feat by keeping a counter in every object that keeps track of the number of references currently pointing to that object. As soon as (and exactly when) this counter drops to zero, the object’s memory space is automatically reclaimed. In the preceding listing, we’re assuming that each time x is assigned to a new object, the prior object’s reference counter drops to zero, causing it to be reclaimed. The most immediately tangible benefit of garbage collection is that it means you can use objects liberally without ever needing to free up space in your script. Python will clean up unused space for you as your program runs. In practice, this eliminates a substantial amount of bookkeeping code required in lower-level languages such as C and C++. Technically speaking, Python’s garbage collection is based mainly upon reference counters, as described here; however, it also has a component that detects and reclaims objects with cyclic references in time. This component can be disabled if you’re sure that your code doesn’t create cycles, but it is enabled by default. Because references are implemented as pointers, it’s possible for an object to reference itself, or reference another object that does. For example, exercise 3 at the end of Part I and its solution in Appendix B show how to create a cycle by embedding a reference to a list within itself. The same phenomenon can occur for assignments to attributes of objects created from user-defined classes. Though relatively rare, because the reference counts for such objects never drop to zero, they must be treated specially. For more details on Python’s cycle detector, see the documentation for the gc module in Python’s library manual. Also note that this description of Python’s garbage collector applies to the standard CPython only; Jython and IronPython may use different schemes, though the net effect in all is similar—unused space is reclaimed for you automatically. The Case of the Missing Declaration Statements | 147 Shared References So far, we’ve seen what happens as a single variable is assigned references to objects. Now let’s introduce another variable into our interaction and watch what happens to its names and objects: >>> a = 3 >>> b = a Typing these two statements generates the scene captured in Figure 6-2. The second line causes Python to create the variable b; the variable a is being used and not assigned here, so it is replaced with the object it references (3), and b is made to reference that object. The net effect is that the variables a and b wind up referencing the same object (that is, pointing to the same chunk of memory). This scenario, with multiple names referencing the same object, is called a shared reference in Python. Figure 6-2. Names and objects after next running the assignment b = a. Variable b becomes a reference to the object 3. Internally, the variable is really a pointer to the object’s memory space created by running the literal expression 3. Next, suppose we extend the session with one more statement: >>> a = 3 >>> b = a >>> a = 'spam' As with all Python assignments, this statement simply makes a new object to represent the string value 'spam' and sets a to reference this new object. It does not, however, change the value of b; b still references the original object, the integer 3. The resulting reference structure is shown in Figure 6-3. The same sort of thing would happen if we changed b to 'spam' instead—the assignment would change only b, not a. This behavior also occurs if there are no type differences at all. For example, consider these three statements: >>> a = 3 >>> b = a >>> a = a + 2 148 | Chapter 6: The Dynamic Typing Interlude Figure 6-3. Names and objects after finally running the assignment a = ‘spam’. Variable a references the new object (i.e., piece of memory) created by running the literal expression ‘spam’, but variable b still refers to the original object 3. Because this assignment is not an in-place change to the object 3, it changes only variable a, not b. In this sequence, the same events transpire. Python makes the variable a reference the object 3 and makes b reference the same object as a, as in Figure 6-2; as before, the last assignment then sets a to a completely different object (in this case, the integer 5, which is the result of the + expression). It does not change b as a side effect. In fact, there is no way to ever overwrite the value of the object 3—as introduced in Chapter 4, integers are immutable and thus can never be changed in-place. One way to think of this is that, unlike in some languages, in Python variables are always pointers to objects, not labels of changeable memory areas: setting a variable to a new value does not alter the original object, but rather causes the variable to reference an entirely different object. The net effect is that assignment to a variable can impact only the single variable being assigned. When mutable objects and in-place changes enter the equation, though, the picture changes somewhat; to see how, let’s move on. Shared References and In-Place Changes As you’ll see later in this part’s chapters, there are objects and operations that perform in-place object changes. For instance, an assignment to an offset in a list actually changes the list object itself in-place, rather than generating a brand new list object. For objects that support such in-place changes, you need to be more aware of shared references, since a change from one name may impact others. To further illustrate, let’s take another look at the list objects introduced in Chapter 4. Recall that lists, which do support in-place assignments to positions, are simply collections of other objects, coded in square brackets: >>> L1 = [2, 3, 4] >>> L2 = L1 Shared References | 149 L1 here is a list containing the objects 2, 3, and 4. Items inside a list are accessed by their positions, so L1[0] refers to object 2, the first item in the list L1. Of course, lists are also objects in their own right, just like integers and strings. After running the two prior assignments, L1 and L2 reference the same object, just like a and b in the prior example (see Figure 6-2). Now say that, as before, we extend this interaction to say the following: >>> L1 = 24 This assignment simply sets L1 is to a different object; L2 still references the original list. If we change this statement’s syntax slightly, however, it has a radically different effect: >>> L1 = [2, 3, 4] >>> L2 = L1 >>> L1[0] = 24 # A mutable object # Make a reference to the same object # An in-place change >>> L1 [24, 3, 4] >>> L2 [24, 3, 4] # L1 is different # But so is L2! Really, we haven’t changed L1 itself here; we’ve changed a component of the object that L1 references. This sort of change overwrites part of the list object in-place. Because the list object is shared by (referenced from) other variables, though, an in-place change like this doesn’t only affect L1—that is, you must be aware that when you make such changes, they can impact other parts of your program. In this example, the effect shows up in L2 as well because it references the same object as L1. Again, we haven’t actually changed L2, either, but its value will appear different because it has been overwritten. This behavior is usually what you want, but you should be aware of how it works, so that it’s expected. It’s also just the default: if you don’t want such behavior, you can request that Python copy objects instead of making references. There are a variety of ways to copy a list, including using the built-in list function and the standard library copy module. Perhaps the most common way is to slice from start to finish (see Chapters 4 and 7 for more on slicing): >>> L1 = [2, 3, 4] >>> L2 = L1[:] >>> L1[0] = 24 >>> L1 [24, 3, 4] >>> L2 [2, 3, 4] # Make a copy of L1 # L2 is not changed Here, the change made through L1 is not reflected in L2 because L2 references a copy of the object L1 references; that is, the two variables point to different pieces of memory. 150 | Chapter 6: The Dynamic Typing Interlude Note that this slicing technique won’t work on the other major mutable core types, dictionaries and sets, because they are not sequences—to copy a dictionary or set, instead use their X.copy() method call. Also, note that the standard library copy module has a call for copying any object type generically, as well as a call for copying nested object structures (a dictionary with nested lists, for example): import copy X = copy.copy(Y) X = copy.deepcopy(Y) # Make top-level "shallow" copy of any object Y # Make deep copy of any object Y: copy all nested parts We’ll explore lists and dictionaries in more depth, and revisit the concept of shared references and copies, in Chapters 8 and 9. For now, keep in mind that objects that can be changed in-place (that is, mutable objects) are always open to these kinds of effects. In Python, this includes lists, dictionaries, and some objects defined with class statements. If this is not the desired behavior, you can simply copy your objects as needed. Shared References and Equality In the interest of full disclosure, I should point out that the garbage-collection behavior described earlier in this chapter may be more conceptual than literal for certain types. Consider these statements: >>> x = 42 >>> x = 'shrubbery' # Reclaim 42 now? Because Python caches and reuses small integers and small strings, as mentioned earlier, the object 42 here is probably not literally reclaimed; instead, it will likely remain in a system table to be reused the next time you generate a 42 in your code. Most kinds of objects, though, are reclaimed immediately when they are no longer referenced; for those that are not, the caching mechanism is irrelevant to your code. For instance, because of Python’s reference model, there are two different ways to check for equality in a Python program. Let’s create a shared reference to demonstrate: >>> L >>> M >>> L True >>> L True = [1, 2, 3] = L == M # M and L reference the same object # Same value is M # Same object The first technique here, the == operator, tests whether the two referenced objects have the same values; this is the method almost always used for equality checks in Python. The second method, the is operator, instead tests for object identity—it returns True only if both names point to the exact same object, so it is a much stronger form of equality testing. Shared References | 151 Really, is simply compares the pointers that implement references, and it serves as a way to detect shared references in your code if needed. It returns False if the names point to equivalent but different objects, as is the case when we run two different literal expressions: >>> L >>> M >>> L True >>> L False = [1, 2, 3] = [1, 2, 3] == M # M and L reference different objects # Same values is M # Different objects Now, watch what happens when we perform the same operations on small numbers: >>> X >>> Y >>> X True >>> X True = 42 = 42 == Y # Should be two different objects is Y # Same object anyhow: caching at work! In this interaction, X and Y should be == (same value), but not is (same object) because we ran two different literal expressions. Because small integers and strings are cached and reused, though, is tells us they reference the same single object. In fact, if you really want to look under the hood, you can always ask Python how many references there are to an object: the getrefcount function in the standard sys module returns the object’s reference count. When I ask about the integer object 1 in the IDLE GUI, for instance, it reports 837 reuses of this same object (most of which are in IDLE’s system code, not mine): >>> import sys >>> sys.getrefcount(1) 837 # 837 pointers to this shared piece of memory This object caching and reuse is irrelevant to your code (unless you run the is check!). Because you cannot change numbers or strings in-place, it doesn’t matter how many references there are to the same object. Still, this behavior reflects one of the many ways Python optimizes its model for execution speed. Dynamic Typing Is Everywhere Of course, you don’t really need to draw name/object diagrams with circles and arrows to use Python. When you’re starting out, though, it sometimes helps you understand unusual cases if you can trace their reference structures. If a mutable object changes out from under you when passed around your program, for example, chances are you are witnessing some of this chapter’s subject matter firsthand. Moreover, even if dynamic typing seems a little abstract at this point, you probably will care about it eventually. Because everything seems to work by assignment and references in Python, a basic understanding of this model is useful in many different 152 | Chapter 6: The Dynamic Typing Interlude contexts. As you’ll see, it works the same in assignment statements, function arguments, for loop variables, module imports, class attributes, and more. The good news is that there is just one assignment model in Python; once you get a handle on dynamic typing, you’ll find that it works the same everywhere in the language. At the most practical level, dynamic typing means there is less code for you to write. Just as importantly, though, dynamic typing is also the root of Python’s polymorphism, a concept we introduced in Chapter 4 and will revisit again later in this book. Because we do not constrain types in Python code, it is highly flexible. As you’ll see, when used well, dynamic typing and the polymorphism it provides produce code that automatically adapts to new requirements as your systems evolve. Chapter Summary This chapter took a deeper look at Python’s dynamic typing model—that is, the way that Python keeps track of object types for us automatically, rather than requiring us to code declaration statements in our scripts. Along the way, we learned how variables and objects are associated by references in Python; we also explored the idea of garbage collection, learned how shared references to objects can affect multiple variables, and saw how references impact the notion of equality in Python. Because there is just one assignment model in Python, and because assignment pops up everywhere in the language, it’s important that you have a handle on the model before moving on. The following quiz should help you review some of this chapter’s ideas. After that, we’ll resume our object tour in the next chapter, with strings. Test Your Knowledge: Quiz 1. Consider the following three statements. Do they change the value printed for A? A = "spam" B = A B = "shrubbery" 2. Consider these three statements. Do they change the printed value of A? A = ["spam"] B = A B[0] = "shrubbery" 3. How about these—is A changed now? A = ["spam"] B = A[:] B[0] = "shrubbery" Test Your Knowledge: Quiz | 153 Test Your Knowledge: Answers 1. No: A still prints as "spam". When B is assigned to the string "shrubbery", all that happens is that the variable B is reset to point to the new string object. A and B initially share (i.e., reference/point to) the same single string object "spam", but two names are never linked together in Python. Thus, setting B to a different object has no effect on A. The same would be true if the last statement here was B = B + 'shrubbery', by the way—the concatenation would make a new object for its result, which would then be assigned to B only. We can never overwrite a string (or number, or tuple) in-place, because strings are immutable. 2. Yes: A now prints as ["shrubbery"]. Technically, we haven’t really changed either A or B; instead, we’ve changed part of the object they both reference (point to) by overwriting that object in-place through the variable B. Because A references the same object as B, the update is reflected in A as well. 3. No: A still prints as ["spam"]. The in-place assignment through B has no effect this time because the slice expression made a copy of the list object before it was assigned to B. After the second assignment statement, there are two different list objects that have the same value (in Python, we say they are ==, but not is). The third statement changes the value of the list object pointed to by B, but not that pointed to by A. 154 | Chapter 6: The Dynamic Typing Interlude CHAPTER 7 Strings The next major type on our built-in object tour is the Python string—an ordered collection of characters used to store and represent text-based information. We looked briefly at strings in Chapter 4. Here, we will revisit them in more depth, filling in some of the details we skipped then. From a functional perspective, strings can be used to represent just about anything that can be encoded as text: symbols and words (e.g., your name), contents of text files loaded into memory, Internet addresses, Python programs, and so on. They can also be used to hold the absolute binary values of bytes, and multibyte Unicode text used in internationalized programs. You may have used strings in other languages, too. Python’s strings serve the same role as character arrays in languages such as C, but they are a somewhat higher-level tool than arrays. Unlike in C, in Python, strings come with a powerful set of processing tools. Also unlike languages such as C, Python has no distinct type for individual characters; instead, you just use one-character strings. Strictly speaking, Python strings are categorized as immutable sequences, meaning that the characters they contain have a left-to-right positional order and that they cannot be changed in-place. In fact, strings are the first representative of the larger class of objects called sequences that we will study here. Pay special attention to the sequence operations introduced in this chapter, because they will work the same on other sequence types we’ll explore later, such as lists and tuples. Table 7-1 previews common string literals and operations we will discuss in this chapter. Empty strings are written as a pair of quotation marks (single or double) with nothing in between, and there are a variety of ways to code strings. For processing, strings support expression operations such as concatenation (combining strings), slicing (extracting sections), indexing (fetching by offset), and so on. Besides expressions, Python also provides a set of string methods that implement common string-specific tasks, as well as modules for more advanced text-processing tasks such as pattern matching. We’ll explore all of these later in the chapter. 155 Table 7-1. Common string literals and operations Operation Interpretation S = '' Empty string S = "spam's" Double quotes, same as single S = 's\np\ta\x00m' Escape sequences S = """...""" Triple-quoted block strings S = r'\temp\spam' Raw strings S = b'spam' Byte strings in 3.0 (Chapter 36) S = u'spam' Unicode strings in 2.6 only (Chapter 36) S1 + S2 Concatenate, repeat S * 3 S[i] Index, slice, length S[i:j] len(S) "a %s parrot" % kind String formatting expression "a {0} parrot".format(kind) String formatting method in 2.6 and 3.0 S.find('pa') String method calls: search, S.rstrip() remove whitespace, S.replace('pa', 'xx') replacement, S.split(',') split on delimiter, S.isdigit() content test, S.lower() case conversion, S.endswith('spam') end test, 'spam'.join(strlist) delimiter join, S.encode('latin-1') Unicode encoding, etc. for x in S: print(x) Iteration, membership 'spam' in S [c * 2 for c in S] map(ord, S) Beyond the core set of string tools in Table 7-1, Python also supports more advanced pattern-based string processing with the standard library’s re (regular expression) module, introduced in Chapter 4, and even higher-level text processing tools such as XML parsers, discussed briefly in Chapter 36. This book’s scope, though, is focused on the fundamentals represented by Table 7-1. 156 | Chapter 7: Strings To cover the basics, this chapter begins with an overview of string literal forms and string expressions, then moves on to look at more advanced tools such as string methods and formatting. Python comes with many string tools, and we won’t look at them all here; the complete story is chronicled in the Python library manual. Our goal here is to explore enough commonly used tools to give you a representative sample; methods we won’t see in action here, for example, are largely analogous to those we will. Content note: Technically speaking, this chapter tells only part of the string story in Python—the part most programmers need to know. It presents the basic str string type, which handles ASCII text and works the same regardless of which version of Python you use. That is, this chapter intentionally limits its scope to the string processing essentials that are used in most Python scripts. From a more formal perspective, ASCII is a simple form of Unicode text. Python addresses the distinction between text and binary data by including distinct object types: • In Python 3.0 there are three string types: str is used for Unicode text (ASCII or otherwise), bytes is used for binary data (including encoded text), and bytearray is a mutable variant of bytes. • In Python 2.6, unicode strings represent wide Unicode text, and str strings handle both 8-bit text and binary data. The bytearray type is also available as a back-port in 2.6, but not earlier, and it’s not as closely bound to binary data as it is in 3.0. Because most programmers don’t need to dig into the details of Unicode encodings or binary data formats, though, I’ve moved all such details to the Advanced Topics part of this book, in Chapter 36. If you do need to deal with more advanced string concepts such as alternative character sets or packed binary data and files, see Chapter 36 after reading the material here. For now, we’ll focus on the basic string type and its operations. As you’ll find, the basics we’ll study here also apply directly to the more advanced string types in Python’s toolset. String Literals By and large, strings are fairly easy to use in Python. Perhaps the most complicated thing about them is that there are so many ways to write them in your code: • • • • • Single quotes: 'spa"m' Double quotes: "spa'm" Triple quotes: '''... spam ...''', """... spam ...""" Escape sequences: "s\tp\na\0m" Raw strings: r"C:\new\test.spm" String Literals | 157 • Byte strings in 3.0 (see Chapter 36): b'sp\x01am' • Unicode strings in 2.6 only (see Chapter 36): u'eggs\u0020spam' The single- and double-quoted forms are by far the most common; the others serve specialized roles, and we’re postponing discussion of the last two advanced forms until Chapter 36. Let’s take a quick look at all the other options in turn. Single- and Double-Quoted Strings Are the Same Around Python strings, single and double quote characters are interchangeable. That is, string literals can be written enclosed in either two single or two double quotes— the two forms work the same and return the same type of object. For example, the following two strings are identical, once coded: >>> 'shrubbery', "shrubbery" ('shrubbery', 'shrubbery') The reason for supporting both is that it allows you to embed a quote character of the other variety inside a string without escaping it with a backslash. You may embed a single quote character in a string enclosed in double quote characters, and vice versa: >>> 'knight"s', "knight's" ('knight"s', "knight's") Incidentally, Python automatically concatenates adjacent string literals in any expression, although it is almost as simple to add a + operator between them to invoke concatenation explicitly (as we’ll see in Chapter 12, wrapping this form in parentheses also allows it to span multiple lines): >>> title = "Meaning " 'of' " Life" >>> title 'Meaning of Life' # Implicit concatenation Notice that adding commas between these strings would result in a tuple, not a string. Also notice in all of these outputs that Python prefers to print strings in single quotes, unless they embed one. You can also embed quotes by escaping them with backslashes: >>> 'knight\'s', "knight\"s" ("knight's", 'knight"s') To understand why, you need to know how escapes work in general. Escape Sequences Represent Special Bytes The last example embedded a quote inside a string by preceding it with a backslash. This is representative of a general pattern in strings: backslashes are used to introduce special byte codings known as escape sequences. Escape sequences let us embed byte codes in strings that cannot easily be typed on a keyboard. The character \, and one or more characters following it in the string literal, are replaced with a single character in the resulting string object, which has the binary 158 | Chapter 7: Strings value specified by the escape sequence. For example, here is a five-character string that embeds a newline and a tab: >>> s = 'a\nb\tc' The two characters \n stand for a single character—the byte containing the binary value of the newline character in your character set (usually, ASCII code 10). Similarly, the sequence \t is replaced with the tab character. The way this string looks when printed depends on how you print it. The interactive echo shows the special characters as escapes, but print interprets them instead: >>> s 'a\nb\tc' >>> print(s) a b c To be completely sure how many bytes are in this string, use the built-in len function— it returns the actual number of bytes in a string, regardless of how it is displayed: >>> len(s) 5 This string is five bytes long: it contains an ASCII a byte, a newline byte, an ASCII b byte, and so on. Note that the original backslash characters are not really stored with the string in memory; they are used to tell Python to store special byte values in the string. For coding such special bytes, Python recognizes a full set of escape code sequences, listed in Table 7-2. Table 7-2. String backslash characters Escape Meaning \newline Ignored (continuation line) \\ Backslash (stores one \) \' Single quote (stores ') \" Double quote (stores ") \a Bell \b Backspace \f Formfeed \n Newline (linefeed) \r Carriage return \t Horizontal tab \v Vertical tab \xhh Character with hex value hh (at most 2 digits) \ooo Character with octal value ooo (up to 3 digits) \0 Null: binary 0 character (doesn’t end string) String Literals | 159 a Escape Meaning \N{ id } Unicode database ID \uhhhh Unicode 16-bit hex \Uhhhhhhhh Unicode 32-bit hexa \other Not an escape (keeps both \ and other) The \Uhhhh... escape sequence takes exactly eight hexadecimal digits (h); both \u and \U can be used only in Unicode string literals. Some escape sequences allow you to embed absolute binary values into the bytes of a string. For instance, here’s a five-character string that embeds two binary zero bytes (coded as octal escapes of one digit): >>> s = 'a\0b\0c' >>> s 'a\x00b\x00c' >>> len(s) 5 In Python, the zero (null) byte does not terminate a string the way it typically does in C. Instead, Python keeps both the string’s length and text in memory. In fact, no character terminates a string in Python. Here’s a string that is all absolute binary escape codes—a binary 1 and 2 (coded in octal), followed by a binary 3 (coded in hexadecimal): >>> s = '\001\002\x03' >>> s '\x01\x02\x03' >>> len(s) 3 Notice that Python displays nonprintable characters in hex, regardless of how they were specified. You can freely combine absolute value escapes and the more symbolic escape types in Table 7-2. The following string contains the characters “spam”, a tab and newline, and an absolute zero value byte coded in hex: >>> S = "s\tp\na\x00m" >>> S 's\tp\na\x00m' >>> len(S) 7 >>> print(S) s p a m This becomes more important to know when you process binary data files in Python. Because their contents are represented as strings in your scripts, it’s OK to process binary files that contain any sorts of binary byte values (more on files in Chapter 9).* * If you need to care about binary data files, the chief distinction is that you open them in binary mode (using open mode flags with a b, such as 'rb', 'wb', and so on). In Python 3.0, binary file content is a bytes string, with an interface similar to that of normal strings; in 2.6, such content is a normal str string. See also the standard struct module introduced in Chapter 9, which can parse binary data loaded from a file, and the extended coverage of binary files and byte strings in Chapter 36. 160 | Chapter 7: Strings Finally, as the last entry in Table 7-2 implies, if Python does not recognize the character after a \ as being a valid escape code, it simply keeps the backslash in the resulting string: >>> x = "C:\py\code" >>> x 'C:\\py\\code' >>> len(x) 10 # Keeps \ literally Unless you’re able to commit all of Table 7-2 to memory, though, you probably shouldn’t rely on this behavior.† To code literal backslashes explicitly such that they are retained in your strings, double them up (\\ is an escape for one \) or use raw strings; the next section shows how. Raw Strings Suppress Escapes As we’ve seen, escape sequences are handy for embedding special byte codes within strings. Sometimes, though, the special treatment of backslashes for introducing escapes can lead to trouble. It’s surprisingly common, for instance, to see Python newcomers in classes trying to open a file with a filename argument that looks something like this: myfile = open('C:\new\text.dat', 'w') thinking that they will open a file called text.dat in the directory C:\new. The problem here is that \n is taken to stand for a newline character, and \t is replaced with a tab. In effect, the call tries to open a file named C:(newline)ew(tab)ext.dat, with usually less than stellar results. This is just the sort of thing that raw strings are useful for. If the letter r (uppercase or lowercase) appears just before the opening quote of a string, it turns off the escape mechanism. The result is that Python retains your backslashes literally, exactly as you type them. Therefore, to fix the filename problem, just remember to add the letter r on Windows: myfile = open(r'C:\new\text.dat', 'w') Alternatively, because two backslashes are really an escape sequence for one backslash, you can keep your backslashes by simply doubling them up: myfile = open('C:\\new\\text.dat', 'w') In fact, Python itself sometimes uses this doubling scheme when it prints strings with embedded backslashes: >>> path = r'C:\new\text.dat' >>> path 'C:\\new\\text.dat' >>> print(path) # Show as Python code # User-friendly format † In classes, I’ve met people who have indeed committed most or all of this table to memory; I’d probably think that was really sick, but for the fact that I’m a member of the set, too. String Literals | 161 C:\new\text.dat >>> len(path) 15 # String length As with numeric representation, the default format at the interactive prompt prints results as if they were code, and therefore escapes backslashes in the output. The print statement provides a more user-friendly format that shows that there is actually only one backslash in each spot. To verify this is the case, you can check the result of the built-in len function, which returns the number of bytes in the string, independent of display formats. If you count the characters in the print(path) output, you’ll see that there really is just 1 character per backslash, for a total of 15. Besides directory paths on Windows, raw strings are also commonly used for regular expressions (text pattern matching, supported with the re module introduced in Chapter 4). Also note that Python scripts can usually use forward slashes in directory paths on Windows and Unix because Python tries to interpret paths portably (i.e., 'C:/new/ text.dat' works when opening files, too). Raw strings are useful if you code paths using native Windows backslashes, though. Despite its role, even a raw string cannot end in a single backslash, because the backslash escapes the following quote character—you still must escape the surrounding quote character to embed it in the string. That is, r"...\" is not a valid string literal—a raw string cannot end in an odd number of backslashes. If you need to end a raw string with a single backslash, you can use two and slice off the second (r'1\nb\tc\\'[:-1]), tack one on manually (r'1\nb\tc' + '\\'), or skip the raw string syntax and just double up the backslashes in a normal string ('1\\nb\\tc\\'). All three of these forms create the same eightcharacter string containing three backslashes. Triple Quotes Code Multiline Block Strings So far, you’ve seen single quotes, double quotes, escapes, and raw strings in action. Python also has a triple-quoted string literal format, sometimes called a block string, that is a syntactic convenience for coding multiline text data. This form begins with three quotes (of either the single or double variety), is followed by any number of lines of text, and is closed with the same triple-quote sequence that opened it. Single and double quotes embedded in the string’s text may be, but do not have to be, escaped— the string does not end until Python sees three unescaped quotes of the same kind used to start the literal. For example: >>> mantra = """Always look ... on the bright ... side of life.""" >>> >>> mantra 'Always look\n on the bright\nside of life.' 162 | Chapter 7: Strings This string spans three lines (in some interfaces, the interactive prompt changes to ... on continuation lines; IDLE simply drops down one line). Python collects all the triple-quoted text into a single multiline string, with embedded newline characters (\n) at the places where your code has line breaks. Notice that, as in the literal, the second line in the result has a leading space, but the third does not—what you type is truly what you get. To see the string with the newlines interpreted, print it instead of echoing: >>> print(mantra) Always look on the bright side of life. Triple-quoted strings are useful any time you need multiline text in your program; for example, to embed multiline error messages or HTML or XML code in your source code files. You can embed such blocks directly in your scripts without resorting to external text files or explicit concatenation and newline characters. Triple-quoted strings are also commonly used for documentation strings, which are string literals that are taken as comments when they appear at specific points in your file (more on these later in the book). These don’t have to be triple-quoted blocks, but they usually are to allow for multiline comments. Finally, triple-quoted strings are also sometimes used as a “horribly hackish” way to temporarily disable lines of code during development (OK, it’s not really too horrible, and it’s actually a fairly common practice). If you wish to turn off a few lines of code and run your script again, simply put three quotes above and below them, like this: X = 1 """ import os print(os.getcwd()) """ Y = 2 # Disable this code temporarily I said this was hackish because Python really does make a string out of the lines of code disabled this way, but this is probably not significant in terms of performance. For large sections of code, it’s also easier than manually adding hash marks before each line and later removing them. This is especially true if you are using a text editor that does not have support for editing Python code specifically. In Python, practicality often beats aesthetics. Strings in Action Once you’ve created a string with the literal expressions we just met, you will almost certainly want to do things with it. This section and the next two demonstrate string expressions, methods, and formatting—the first line of text-processing tools in the Python language. Strings in Action | 163 Basic Operations Let’s begin by interacting with the Python interpreter to illustrate the basic string operations listed earlier in Table 7-1. Strings can be concatenated using the + operator and repeated using the * operator: % python >>> len('abc') 3 >>> 'abc' + 'def' 'abcdef' >>> 'Ni!' * 4 'Ni!Ni!Ni!Ni!' # Length: number of items # Concatenation: a new string # Repetition: like "Ni!" + "Ni!" + ... Formally, adding two string objects creates a new string object, with the contents of its operands joined. Repetition is like adding a string to itself a number of times. In both cases, Python lets you create arbitrarily sized strings; there’s no need to predeclare anything in Python, including the sizes of data structures.‡ The len built-in function returns the length of a string (or any other object with a length). Repetition may seem a bit obscure at first, but it comes in handy in a surprising number of contexts. For example, to print a line of 80 dashes, you can count up to 80, or let Python count for you: >>> print('------- ...more... ---') >>> print('-' * 80) # 80 dashes, the hard way # 80 dashes, the easy way Notice that operator overloading is at work here already: we’re using the same + and * operators that perform addition and multiplication when using numbers. Python does the correct operation because it knows the types of the objects being added and multiplied. But be careful: the rules aren’t quite as liberal as you might expect. For instance, Python doesn’t allow you to mix numbers and strings in + expressions: 'abc'+9 raises an error instead of automatically converting 9 to a string. As shown in the last row in Table 7-1, you can also iterate over strings in loops using for statements and test membership for both characters and substrings with the in expression operator, which is essentially a search. For substrings, in is much like the str.find() method covered later in this chapter, but it returns a Boolean result instead of the substring’s position: >>> myjob = "hacker" >>> for c in myjob: print(c, end=' ') ... # Step through items ‡ Unlike with C character arrays, you don’t need to allocate or manage storage arrays when using Python strings; you can simply create string objects as needed and let Python manage the underlying memory space. As discussed in Chapter 6, Python reclaims unused objects’ memory space automatically, using a referencecount garbage-collection strategy. Each object keeps track of the number of names, data structures, etc., that reference it; when the count reaches zero, Python frees the object’s space. This scheme means Python doesn’t have to stop and scan all the memory to find unused space to free (an additional garbage component also collects cyclic objects). 164 | Chapter 7: Strings h a c k e r >>> "k" in myjob True >>> "z" in myjob False >>> 'spam' in 'abcspamdef' True # Found # Not found # Substring search, no position returned The for loop assigns a variable to successive items in a sequence (here, a string) and executes one or more statements for each item. In effect, the variable c becomes a cursor stepping across the string here. We will discuss iteration tools like these and others listed in Table 7-1 in more detail later in this book (especially in Chapters 14 and 20). Indexing and Slicing Because strings are defined as ordered collections of characters, we can access their components by position. In Python, characters in a string are fetched by indexing— providing the numeric offset of the desired component in square brackets after the string. You get back the one-character string at the specified position. As in the C language, Python offsets start at 0 and end at one less than the length of the string. Unlike C, however, Python also lets you fetch items from sequences such as strings using negative offsets. Technically, a negative offset is added to the length of a string to derive a positive offset. You can also think of negative offsets as counting backward from the end. The following interaction demonstrates: >>> S = 'spam' >>> S[0], S[−2] ('s', 'a') >>> S[1:3], S[1:], S[:−1] ('pa', 'pam', 'spa') # Indexing from front or end # Slicing: extract a section The first line defines a four-character string and assigns it the name S. The next line indexes it in two ways: S[0] fetches the item at offset 0 from the left (the one-character string 's'), and S[−2] gets the item at offset 2 back from the end (or equivalently, at offset (4 + (–2)) from the front). Offsets and slices map to cells as shown in Figure 7-1.§ The last line in the preceding example demonstrates slicing, a generalized form of indexing that returns an entire section, not a single item. Probably the best way to think of slicing is that it is a type of parsing (analyzing structure), especially when applied to strings—it allows us to extract an entire section (substring) in a single step. Slices can be used to extract columns of data, chop off leading and trailing text, and more. In fact, we’ll explore slicing in the context of text parsing later in this chapter. The basics of slicing are straightforward. When you index a sequence object such as a string on a pair of offsets separated by a colon, Python returns a new object containing § More mathematically minded readers (and students in my classes) sometimes detect a small asymmetry here: the leftmost item is at offset 0, but the rightmost is at offset –1. Alas, there is no such thing as a distinct –0 value in Python. Strings in Action | 165 Figure 7-1. Offsets and slices: positive offsets start from the left end (offset 0 is the first item), and negatives count back from the right end (offset −1 is the last item). Either kind of offset can be used to give positions in indexing and slicing operations. the contiguous section identified by the offset pair. The left offset is taken to be the lower bound (inclusive), and the right is the upper bound (noninclusive). That is, Python fetches all items from the lower bound up to but not including the upper bound, and returns a new object containing the fetched items. If omitted, the left and right bounds default to 0 and the length of the object you are slicing, respectively. For instance, in the example we just saw, S[1:3] extracts the items at offsets 1 and 2: it grabs the second and third items, and stops before the fourth item at offset 3. Next, S[1:] gets all items beyond the first—the upper bound, which is not specified, defaults to the length of the string. Finally, S[:−1] fetches all but the last item—the lower bound defaults to 0, and −1 refers to the last item, noninclusive. This may seem confusing at first glance, but indexing and slicing are simple and powerful tools to use, once you get the knack. Remember, if you’re unsure about the effects of a slice, try it out interactively. In the next chapter, you’ll see that it’s even possible to change an entire section of another object in one step by assigning to a slice (though not for immutables like strings). Here’s a summary of the details for reference: • Indexing (S[i]) fetches components at offsets: — The first item is at offset 0. — Negative indexes mean to count backward from the end or right. — S[0] fetches the first item. — S[−2] fetches the second item from the end (like S[len(S)−2]). • Slicing (S[i:j]) extracts contiguous sections of sequences: — The upper bound is noninclusive. — Slice boundaries default to 0 and the sequence length, if omitted. — S[1:3] fetches items at offsets 1 up to but not including 3. — S[1:] fetches items at offset 1 through the end (the sequence length). 166 | Chapter 7: Strings — S[:3] fetches items at offset 0 up to but not including 3. — S[:−1] fetches items at offset 0 up to but not including the last item. — S[:] fetches items at offsets 0 through the end—this effectively performs a toplevel copy of S. The last item listed here turns out to be a very common trick: it makes a full top-level copy of a sequence object—an object with the same value, but a distinct piece of memory (you’ll find more on copies in Chapter 9). This isn’t very useful for immutable objects like strings, but it comes in handy for objects that may be changed in-place, such as lists. In the next chapter, you’ll see that the syntax used to index by offset (square brackets) is used to index dictionaries by key as well; the operations look the same but have different interpretations. Extended slicing: the third limit and slice objects In Python 2.3 and later, slice expressions have support for an optional third index, used as a step (sometimes called a stride). The step is added to the index of each item extracted. The full-blown form of a slice is now X[I:J:K], which means “extract all the items in X, from offset I through J−1, by K.” The third limit, K, defaults to 1, which is why normally all items in a slice are extracted from left to right. If you specify an explicit value, however, you can use the third limit to skip items or to reverse their order. For instance, X[1:10:2] will fetch every other item in X from offsets 1–9; that is, it will collect the items at offsets 1, 3, 5, 7, and 9. As usual, the first and second limits default to 0 and the length of the sequence, respectively, so X[::2] gets every other item from the beginning to the end of the sequence: >>> S = 'abcdefghijklmnop' >>> S[1:10:2] 'bdfhj' >>> S[::2] 'acegikmo' You can also use a negative stride. For example, the slicing expression "hello"[::−1] returns the new string "olleh"—the first two bounds default to 0 and the length of the sequence, as before, and a stride of −1 indicates that the slice should go from right to left instead of the usual left to right. The effect, therefore, is to reverse the sequence: >>> S = 'hello' >>> S[::−1] 'olleh' With a negative stride, the meanings of the first two bounds are essentially reversed. That is, the slice S[5:1:−1] fetches the items from 2 to 5, in reverse order (the result contains items from offsets 5, 4, 3, and 2): Strings in Action | 167 >>> S = 'abcedfg' >>> S[5:1:−1] 'fdec' Skipping and reversing like this are the most common use cases for three-limit slices, but see Python’s standard library manual for more details (or run a few experiments interactively). We’ll revisit three-limit slices again later in this book, in conjunction with the for loop statement. Later in the book, we’ll also learn that slicing is equivalent to indexing with a slice object, a finding of importance to class writers seeking to support both operations: >>> 'spam'[1:3] 'pa' >>> 'spam'[slice(1, 3)] 'pa' >>> 'spam'[::-1] 'maps' >>> 'spam'[slice(None, None, −1)] 'maps' # Slicing syntax # Slice objects Why You Will Care: Slices Throughout this book, I will include common use case sidebars (such as this one) to give you a peek at how some of the language features being introduced are typically used in real programs. Because you won’t be able to make much sense of real use cases until you’ve seen more of the Python picture, these sidebars necessarily contain many references to topics not introduced yet; at most, you should consider them previews of ways that you may find these abstract language concepts useful for common programming tasks. For instance, you’ll see later that the argument words listed on a system command line used to launch a Python program are made available in the argv attribute of the builtin sys module: # File echo.py import sys print(sys.argv) % python echo.py −a −b −c ['echo.py', '−a', '−b', '−c'] Usually, you’re only interested in inspecting the arguments that follow the program name. This leads to a very typical application of slices: a single slice expression can be used to return all but the first item of a list. Here, sys.argv[1:] returns the desired list, ['−a', '−b', '−c']. You can then process this list without having to accommodate the program name at the front. Slices are also often used to clean up lines read from input files. If you know that a line will have an end-of-line character at the end (a \n newline marker), you can get rid of it with a single expression such as line[:−1], which extracts all but the last character in the line (the lower limit defaults to 0). In both cases, slices do the job of logic that must be explicit in a lower-level language. 168 | Chapter 7: Strings Note that calling the line.rstrip method is often preferred for stripping newline characters because this call leaves the line intact if it has no newline character at the end— a common case for files created with some text-editing tools. Slicing works if you’re sure the line is properly terminated. String Conversion Tools One of Python’s design mottos is that it refuses the temptation to guess. As a prime example, you cannot add a number and a string together in Python, even if the string looks like a number (i.e., is all digits): >>> "42" + 1 TypeError: cannot concatenate 'str' and 'int' objects This is by design: because + can mean both addition and concatenation, the choice of conversion would be ambiguous. So, Python treats this as an error. In Python, magic is generally omitted if it will make your life more complex. What to do, then, if your script obtains a number as a text string from a file or user interface? The trick is that you need to employ conversion tools before you can treat a string like a number, or vice versa. For instance: >>> int("42"), str(42) (42, '42') >>> repr(42) '42' # Convert from/to string # Convert to as-code string The int function converts a string to a number, and the str function converts a number to its string representation (essentially, what it looks like when printed). The repr function (and the older backquotes expression, removed in Python 3.0) also converts an object to its string representation, but returns the object as a string of code that can be rerun to recreate the object. For strings, the result has quotes around it if displayed with a print statement: >>> print(str('spam'), repr('spam')) ('spam', "'spam'") See the sidebar “str and repr Display Formats” on page 116 for more on this topic. Of these, int and str are the generally prescribed conversion techniques. Now, although you can’t mix strings and number types around operators such as +, you can manually convert operands before that operation if needed: >>> S = "42" >>> I = 1 >>> S + I TypeError: cannot concatenate 'str' and 'int' objects >>> int(S) + I 43 # Force addition Strings in Action | 169 >>> S + str(I) '421' # Force concatenation Similar built-in functions handle floating-point number conversions to and from strings: >>> str(3.1415), float("1.5") ('3.1415', 1.5) >>> text = "1.234E-10" >>> float(text) 1.2340000000000001e-010 Later, we’ll further study the built-in eval function; it runs a string containing Python expression code and so can convert a string to any kind of object. The functions int and float convert only to numbers, but this restriction means they are usually faster (and more secure, because they do not accept arbitrary expression code). As we saw briefly in Chapter 5, the string formatting expression also provides a way to convert numbers to strings. We’ll discuss formatting further later in this chapter. Character code conversions On the subject of conversions, it is also possible to convert a single character to its underlying ASCII integer code by passing it to the built-in ord function—this returns the actual binary value of the corresponding byte in memory. The chr function performs the inverse operation, taking an ASCII integer code and converting it to the corresponding character: >>> ord('s') 115 >>> chr(115) 's' You can use a loop to apply these functions to all characters in a string. These tools can also be used to perform a sort of string-based math. To advance to the next character, for example, convert and do the math in integer: >>> >>> >>> '6' >>> >>> '7' S = '5' S = chr(ord(S) + 1) S S = chr(ord(S) + 1) S At least for single-character strings, this provides an alternative to using the built-in int function to convert from string to integer: >>> int('5') 5 >>> ord('5') - ord('0') 5 170 | Chapter 7: Strings Such conversions can be used in conjunction with looping statements, introduced in Chapter 4 and covered in depth in the next part of this book, to convert a string of binary digits to their corresponding integer values. Each time through the loop, multiply the current value by 2 and add the next digit’s integer value: >>> >>> >>> ... ... ... >>> 13 B = '1101' # Convert binary digits to integer with ord I = 0 while B != '': I = I * 2 + (ord(B[0]) - ord('0')) B = B[1:] I A left-shift operation (I << 1) would have the same effect as multiplying by 2 here. We’ll leave this change as a suggested exercise, though, both because we haven’t studied loops in detail yet and because the int and bin built-ins we met in Chapter 5 handle binary conversion tasks for us in Python 2.6 and 3.0: >>> int('1101', 2) 13 >>> bin(13) '0b1101' # Convert binary to integer: built-in # Convert integer to binary Given enough time, Python tends to automate most common tasks! Changing Strings Remember the term “immutable sequence”? The immutable part means that you can’t change a string in-place (e.g., by assigning to an index): >>> S = 'spam' >>> S[0] = "x" Raises an error! So, how do you modify text information in Python? To change a string, you need to build and assign a new string using tools such as concatenation and slicing, and then, if desired, assign the result back to the string’s original name: >>> S = S + 'SPAM!' # To change a string, make a new one >>> S 'spamSPAM!' >>> S = S[:4] + 'Burger' + S[−1] >>> S 'spamBurger!' The first example adds a substring at the end of S, by concatenation (really, it makes a new string and assigns it back to S, but you can think of this as “changing” the original string). The second example replaces four characters with six by slicing, indexing, and concatenating. As you’ll see in the next section, you can achieve similar effects with string method calls like replace: Strings in Action | 171 >>> S = 'splot' >>> S = S.replace('pl', 'pamal') >>> S 'spamalot' Like every operation that yields a new string value, string methods generate new string objects. If you want to retain those objects, you can assign them to variable names. Generating a new string object for each string change is not as inefficient as it may sound—remember, as discussed in the preceding chapter, Python automatically garbage collects (reclaims the space of) old unused string objects as you go, so newer objects reuse the space held by prior values. Python is usually more efficient than you might expect. Finally, it’s also possible to build up new text values with string formatting expressions. Both of the following substitute objects into a string, in a sense converting the objects to strings and changing the original string according to a format specification: >>> 'That is %d %s bird!' % (1, 'dead') That is 1 dead bird! >>> 'That is {0} {1} bird!'.format(1, 'dead') 'That is 1 dead bird!' # Format expression # Format method in 2.6 and 3.0 Despite the substitution metaphor, though, the result of formatting is a new string object, not a modified one. We’ll study formatting later in this chapter; as we’ll find, formatting turns out to be more general and useful than this example implies. Because the second of the preceding calls is provided as a method, though, let’s get a handle on string method calls before we explore formatting further. As we’ll see in Chapter 36, Python 3.0 and 2.6 introduce a new string type known as bytearray, which is mutable and so may be changed in place. bytearray objects aren’t really strings; they’re sequences of small, 8-bit integers. However, they support most of the same operations as normal strings and print as ASCII characters when displayed. As such, they provide another option for large amounts of text that must be changed frequently. In Chapter 36 we’ll also see that ord and chr handle Unicode characters, too, which might not be stored in single bytes. String Methods In addition to expression operators, strings provide a set of methods that implement more sophisticated text-processing tasks. Methods are simply functions that are associated with particular objects. Technically, they are attributes attached to objects that happen to reference callable functions. In Python, expressions and built-in functions may work across a range of types, but methods are generally specific to object types— string methods, for example, work only on string objects. The method sets of some types intersect in Python 3.0 (e.g., many types have a count method), but they are still more type-specific than other tools. 172 | Chapter 7: Strings In finer-grained detail, functions are packages of code, and method calls combine two operations at once (an attribute fetch and a call): Attribute fetches An expression of the form object.attribute means “fetch the value of attribute in object.” Call expressions An expression of the form function(arguments) means “invoke the code of function, passing zero or more comma-separated argument objects to it, and return function’s result value.” Putting these two together allows us to call a method of an object. The method call expression object.method(arguments) is evaluated from left to right—Python will first fetch the method of the object and then call it, passing in the arguments. If the method computes a result, it will come back as the result of the entire method-call expression. As you’ll see throughout this part of the book, most objects have callable methods, and all are accessed using this same method-call syntax. To call an object method, as you’ll see in the following sections, you have to go through an existing object. Table 7-3 summarizes the methods and call patterns for built-in string objects in Python 3.0; these change frequently, so be sure to check Python’s standard library manual for the most up-to-date list, or run a help call on any string interactively. Python 2.6’s string methods vary slightly; it includes a decode, for example, because of its different handling of Unicode data (something we’ll discuss in Chapter 36). In this table, S is a string object, and optional arguments are enclosed in square brackets. String methods in this table implement higher-level operations such as splitting and joining, case conversions, content tests, and substring searches and replacements. Table 7-3. String method calls in Python 3.0 S.capitalize() S.ljust(width [, fill]) S.center(width [, fill]) S.lower() S.count(sub [, start [, end]]) S.lstrip([chars]) S.encode([encoding [,errors]]) S.maketrans(x[, y[, z]]) S.endswith(suffix [, start [, end]]) S.partition(sep) S.expandtabs([tabsize]) S.replace(old, new [, count]) S.find(sub [, start [, end]]) S.rfind(sub [,start [,end]]) S.format(fmtstr, *args, **kwargs) S.rindex(sub [, start [, end]]) S.index(sub [, start [, end]]) S.rjust(width [, fill]) S.isalnum() S.rpartition(sep) S.isalpha() S.rsplit([sep[, maxsplit]]) S.isdecimal() S.rstrip([chars]) S.isdigit() S.split([sep [,maxsplit]]) String Methods | 173 S.isidentifier() S.splitlines([keepends]) S.islower() S.startswith(prefix [, start [, end]]) S.isnumeric() S.strip([chars]) S.isprintable() S.swapcase() S.isspace() S.title() S.istitle() S.translate(map) S.isupper() S.upper() S.join(iterable) S.zfill(width) As you can see, there are quite a few string methods, and we don’t have space to cover them all; see Python’s library manual or reference texts for all the fine points. To help you get started, though, let’s work through some code that demonstrates some of the most commonly used methods in action, and illustrates Python text-processing basics along the way. String Method Examples: Changing Strings As we’ve seen, because strings are immutable, they cannot be changed in-place directly. To make a new text value from an existing string, you construct a new string with operations such as slicing and concatenation. For example, to replace two characters in the middle of a string, you can use code like this: >>> S = 'spammy' >>> S = S[:3] + 'xx' + S[5:] >>> S 'spaxxy' But, if you’re really just out to replace a substring, you can use the string replace method instead: >>> S = 'spammy' >>> S = S.replace('mm', 'xx') >>> S 'spaxxy' The replace method is more general than this code implies. It takes as arguments the original substring (of any length) and the string (of any length) to replace it with, and performs a global search and replace: >>> 'aa$bb$cc$dd'.replace('$', 'SPAM') 'aaSPAMbbSPAMccSPAMdd' In such a role, replace can be used as a tool to implement template replacements (e.g., in form letters). Notice that this time we simply printed the result, instead of assigning it to a name—you need to assign results to names only if you want to retain them for later use. 174 | Chapter 7: Strings If you need to replace one fixed-size string that can occur at any offset, you can do a replacement again, or search for the substring with the string find method and then slice: >>> S = 'xxxxSPAMxxxxSPAMxxxx' >>> where = S.find('SPAM') # Search for position >>> where # Occurs at offset 4 4 >>> S = S[:where] + 'EGGS' + S[(where+4):] >>> S 'xxxxEGGSxxxxSPAMxxxx' The find method returns the offset where the substring appears (by default, searching from the front), or −1 if it is not found. As we saw earlier, it’s a substring search operation just like the in expression, but find returns the position of a located substring. Another option is to use replace with a third argument to limit it to a single substitution: >>> S = 'xxxxSPAMxxxxSPAMxxxx' >>> S.replace('SPAM', 'EGGS') 'xxxxEGGSxxxxEGGSxxxx' >>> S.replace('SPAM', 'EGGS', 1) 'xxxxEGGSxxxxSPAMxxxx' # Replace all # Replace one Notice that replace returns a new string object each time. Because strings are immutable, methods never really change the subject strings in-place, even if they are called “replace”! The fact that concatenation operations and the replace method generate new string objects each time they are run is actually a potential downside of using them to change strings. If you have to apply many changes to a very large string, you might be able to improve your script’s performance by converting the string to an object that does support in-place changes: >>> S = 'spammy' >>> L = list(S) >>> L ['s', 'p', 'a', 'm', 'm', 'y'] The built-in list function (or an object construction call) builds a new list out of the items in any sequence—in this case, “exploding” the characters of a string into a list. Once the string is in this form, you can make multiple changes to it without generating a new copy for each change: >>> L[3] = 'x' >>> L[4] = 'x' >>> L ['s', 'p', 'a', 'x', 'x', 'y'] # Works for lists, not strings If, after your changes, you need to convert back to a string (e.g., to write to a file), use the string join method to “implode” the list back into a string: String Methods | 175 >>> S = ''.join(L) >>> S 'spaxxy' The join method may look a bit backward at first sight. Because it is a method of strings (not of lists), it is called through the desired delimiter. join puts the strings in a list (or other iterable) together, with the delimiter between list items; in this case, it uses an empty string delimiter to convert from a list back to a string. More generally, any string delimiter and iterable of strings will do: >>> 'SPAM'.join(['eggs', 'sausage', 'ham', 'toast']) 'eggsSPAMsausageSPAMhamSPAMtoast' In fact, joining substrings all at once this way often runs much faster than concatenating them individually. Be sure to also see the earlier note about the mutable bytearray string in Python 3.0 and 2.6, described fully in Chapter 36; because it may be changed in place, it offers an alternative to this list/join combination for some kinds of text that must be changed often. String Method Examples: Parsing Text Another common role for string methods is as a simple form of text parsing—that is, analyzing structure and extracting substrings. To extract substrings at fixed offsets, we can employ slicing techniques: >>> line = 'aaa bbb ccc' >>> col1 = line[0:3] >>> col3 = line[8:] >>> col1 'aaa' >>> col3 'ccc' Here, the columns of data appear at fixed offsets and so may be sliced out of the original string. This technique passes for parsing, as long as the components of your data have fixed positions. If instead some sort of delimiter separates the data, you can pull out its components by splitting. This will work even if the data may show up at arbitrary positions within the string: >>> line = 'aaa bbb ccc' >>> cols = line.split() >>> cols ['aaa', 'bbb', 'ccc'] The string split method chops up a string into a list of substrings, around a delimiter string. We didn’t pass a delimiter in the prior example, so it defaults to whitespace— the string is split at groups of one or more spaces, tabs, and newlines, and we get back a list of the resulting substrings. In other applications, more tangible delimiters may separate the data. This example splits (and hence parses) the string at commas, a separator common in data returned by some database tools: 176 | Chapter 7: Strings >>> line = 'bob,hacker,40' >>> line.split(',') ['bob', 'hacker', '40'] Delimiters can be longer than a single character, too: >>> line = "i'mSPAMaSPAMlumberjack" >>> line.split("SPAM") ["i'm", 'a', 'lumberjack'] Although there are limits to the parsing potential of slicing and splitting, both run very fast and can handle basic text-extraction chores. Other Common String Methods in Action Other string methods have more focused roles—for example, to strip off whitespace at the end of a line of text, perform case conversions, test content, and test for a substring at the end or front: >>> line = "The knights who say Ni!\n" >>> line.rstrip() 'The knights who say Ni!' >>> line.upper() 'THE KNIGHTS WHO SAY NI!\n' >>> line.isalpha() False >>> line.endswith('Ni!\n') True >>> line.startswith('The') True Alternative techniques can also sometimes be used to achieve the same results as string methods—the in membership operator can be used to test for the presence of a substring, for instance, and length and slicing operations can be used to mimic endswith: >>> line 'The knights who say Ni!\n' >>> line.find('Ni') != −1 True >>> 'Ni' in line True >>> sub = 'Ni!\n' >>> line.endswith(sub) True >>> line[-len(sub):] == sub True # Search via method call or expression # End test via method call or slice See also the format string formatting method described later in this chapter; it provides more advanced substitution tools that combine many operations in a single step. Again, because there are so many methods available for strings, we won’t look at every one here. You’ll see some additional string examples later in this book, but for more String Methods | 177 details you can also turn to the Python library manual and other documentation sources, or simply experiment interactively on your own. You can also check the help(S.method) results for a method of any string object S for more hints. Note that none of the string methods accepts patterns—for pattern-based text processing, you must use the Python re standard library module, an advanced tool that was introduced in Chapter 4 but is mostly outside the scope of this text (one further example appears at the end of Chapter 36). Because of this limitation, though, string methods may sometimes run more quickly than the re module’s tools. The Original string Module (Gone in 3.0) The history of Python’s string methods is somewhat convoluted. For roughly the first decade of its existence, Python provided a standard library module called string that contained functions that largely mirrored the current set of string object methods. In response to user requests, in Python 2.0 these functions were made available as methods of string objects. Because so many people had written so much code that relied on the original string module, however, it was retained for backward compatibility. Today, you should use only string methods, not the original string module. In fact, the original module-call forms of today’s string methods have been removed completely from Python in Release 3.0. However, because you may still see the module in use in older Python code, a brief look is in order here. The upshot of this legacy is that in Python 2.6, there technically are still two ways to invoke advanced string operations: by calling object methods, or by calling string module functions and passing in the objects as arguments. For instance, given a variable X assigned to a string object, calling an object method: X.method(arguments) is usually equivalent to calling the same operation through the string module (provided that you have already imported the module): string.method(X, arguments) Here’s an example of the method scheme in action: >>> S = 'a+b+c+' >>> x = S.replace('+', 'spam') >>> x 'aspambspamcspam' To access the same operation through the string module in Python 2.6, you need to import the module (at least once in your process) and pass in the object: >>> import string >>> y = string.replace(S, '+', 'spam') >>> y 'aspambspamcspam' 178 | Chapter 7: Strings Because the module approach was the standard for so long, and because strings are such a central component of most programs, you might see both call patterns in Python 2.X code you come across. Again, though, today you should always use method calls instead of the older module calls. There are good reasons for this, besides the fact that the module calls have gone away in Release 3.0. For one thing, the module call scheme requires you to import the string module (methods do not require imports). For another, the module makes calls a few characters longer to type (when you load the module with import, that is, not using from). And, finally, the module runs more slowly than methods (the module maps most calls back to the methods and so incurs an extra call along the way). The original string module itself, without its string method equivalents, is retained in Python 3.0 because it contains additional tools, including predefined string constants and a template object system (a relatively obscure tool omitted here—see the Python library manual for details on template objects). Unless you really want to have to change your 2.6 code to use 3.0, though, you should consider the basic string operation calls in it to be just ghosts from the past. String Formatting Expressions Although you can get a lot done with the string methods and sequence operations we’ve already met, Python also provides a more advanced way to combine string processing tasks—string formatting allows us to perform multiple type-specific substitutions on a string in a single step. It’s never strictly required, but it can be convenient, especially when formatting text to be displayed to a program’s users. Due to the wealth of new ideas in the Python world, string formatting is available in two flavors in Python today: String formatting expressions The original technique, available since Python’s inception; this is based upon the C language’s “printf” model and is used in much existing code. String formatting method calls A newer technique added in Python 2.6 and 3.0; this is more unique to Python and largely overlaps with string formatting expression functionality. Since the method call flavor is new, there is some chance that one or the other of these may become deprecated over time. The expressions are more likely to be deprecated in later Python releases, though this should depend on the future practice of real Python programmers. As they are largely just variations on a theme, though, either technique is valid to use today. Since string formatting expressions are the original in this department, let’s start with them. Python defines the % binary operator to work on strings (you may recall that this is also the remainder of division, or modulus, operator for numbers). When applied to strings, the % operator provides a simple way to format values as strings according to a format String Formatting Expressions | 179 definition. In short, the % operator provides a compact way to code multiple string substitutions all at once, instead of building and concatenating parts individually. To format strings: 1. On the left of the % operator, provide a format string containing one or more embedded conversion targets, each of which starts with a % (e.g., %d). 2. On the right of the % operator, provide the object (or objects, embedded in a tuple) that you want Python to insert into the format string on the left in place of the conversion target (or targets). For instance, in the formatting example we saw earlier in this chapter, the integer 1 replaces the %d in the format string on the left, and the string 'dead' replaces the %s. The result is a new string that reflects these two substitutions: >>> 'That is %d %s bird!' % (1, 'dead') That is 1 dead bird! # Format expression Technically speaking, string formatting expressions are usually optional—you can generally do similar work with multiple concatenations and conversions. However, formatting allows us to combine many steps into a single operation. It’s powerful enough to warrant a few more examples: >>> exclamation = "Ni" >>> "The knights who say %s!" % exclamation 'The knights who say Ni!' >>> "%d %s %d you" % (1, 'spam', 4) '1 spam 4 you' >>> "%s -- %s -- %s" % (42, 3.14159, [1, 2, 3]) '42 -- 3.14159 -- [1, 2, 3]' The first example here plugs the string "Ni" into the target on the left, replacing the %s marker. In the second example, three values are inserted into the target string. Note that when you’re inserting more than one value, you need to group the values on the right in parentheses (i.e., put them in a tuple). The % formatting expression operator expects either a single item or a tuple of one or more items on its right side. The third example again inserts three values—an integer, a floating-point object, and a list object—but notice that all of the targets on the left are %s, which stands for conversion to string. As every type of object can be converted to a string (the one used when printing), every object type works with the %s conversion code. Because of this, unless you will be doing some special formatting, %s is often the only code you need to remember for the formatting expression. Again, keep in mind that formatting always makes a new string, rather than changing the string on the left; because strings are immutable, it must work this way. As before, assign the result to a variable name if you need to retain it. 180 | Chapter 7: Strings Advanced String Formatting Expressions For more advanced type-specific formatting, you can use any of the conversion type codes listed in Table 7-4 in formatting expressions; they appear after the % character in substitution targets. C programmers will recognize most of these because Python string formatting supports all the usual C printf format codes (but returns the result, instead of displaying it, like printf). Some of the format codes in the table provide alternative ways to format the same type; for instance, %e, %f, and %g provide alternative ways to format floating-point numbers. Table 7-4. String formatting type codes Code Meaning s String (or any object’s str(X) string) r s, but uses repr, not str c Character d Decimal (integer) i Integer u Same as d (obsolete: no longer unsigned) o Octal integer x Hex integer X x, but prints uppercase e Floating-point exponent, lowercase E Same as e, but prints uppercase f Floating-point decimal F Floating-point decimal g Floating-point e or f G Floating-point E or F % Literal % In fact, conversion targets in the format string on the expression’s left side support a variety of conversion operations with a fairly sophisticated syntax all their own. The general structure of conversion targets looks like this: %[(name)][flags][width][.precision]typecode The character type codes in Table 7-4 show up at the end of the target string. Between the % and the character code, you can do any of the following: provide a dictionary key; list flags that specify things like left justification (−), numeric sign (+), and zero fills (0); give a total minimum field width and the number of digits after a decimal point; and more. Both width and precision can also be coded as a * to specify that they should take their values from the next item in the input values. String Formatting Expressions | 181 Formatting target syntax is documented in full in the Python standard manuals, but to demonstrate common usage, let’s look at a few examples. This one formats integers by default, and then in a six-character field with left justification and zero padding: >>> x = 1234 >>> res = "integers: ...%d...%−6d...%06d" % (x, x, x) >>> res 'integers: ...1234...1234 ...001234' The %e, %f, and %g formats display floating-point numbers in different ways, as the following interaction demonstrates (%E is the same as %e but the exponent is uppercase): >>> x = 1.23456789 >>> x 1.2345678899999999 >>> '%e | %f | %g' % (x, x, x) '1.234568e+00 | 1.234568 | 1.23457' >>> '%E' % x '1.234568E+00' For floating-point numbers, you can achieve a variety of additional formatting effects by specifying left justification, zero padding, numeric signs, field width, and digits after the decimal point. For simpler tasks, you might get by with simply converting to strings with a format expression or the str built-in function shown earlier: >>> '%−6.2f | %05.2f | %+06.1f' % (x, x, x) '1.23 | 01.23 | +001.2' >>> "%s" % x, str(x) ('1.23456789', '1.23456789') When sizes are not known until runtime, you can have the width and precision computed by specifying them with a * in the format string to force their values to be taken from the next item in the inputs to the right of the % operator—the 4 in the tuple here gives precision: >>> '%f, %.2f, %.*f' % (1/3.0, 1/3.0, 4, 1/3.0) '0.333333, 0.33, 0.3333' If you’re interested in this feature, experiment with some of these examples and operations on your own for more information. Dictionary-Based String Formatting Expressions String formatting also allows conversion targets on the left to refer to the keys in a dictionary on the right and fetch the corresponding values. I haven’t told you much about dictionaries yet, so here’s an example that demonstrates the basics: >>> "%(n)d %(x)s" % {"n":1, "x":"spam"} '1 spam' 182 | Chapter 7: Strings Here, the (n) and (x) in the format string refer to keys in the dictionary literal on the right and fetch their associated values. Programs that generate text such as HTML or XML often use this technique—you can build up a dictionary of values and substitute them all at once with a single formatting expression that uses key-based references: >>> reply = """ Greetings... Hello %(name)s! Your age squared is %(age)s """ >>> values = {'name': 'Bob', 'age': 40} >>> print(reply % values) # Template with substitution targets # Build up values to substitute # Perform substitutions Greetings... Hello Bob! Your age squared is 40 This trick is also used in conjunction with the vars built-in function, which returns a dictionary containing all the variables that exist in the place it is called: >>> food = 'spam' >>> age = 40 >>> vars() {'food': 'spam', 'age': 40, ...many more... } When used on the right of a format operation, this allows the format string to refer to variables by name (i.e., by dictionary key): >>> "%(age)d %(food)s" % vars() '40 spam' We’ll study dictionaries in more depth in Chapter 8. See also Chapter 5 for examples that convert to hexadecimal and octal number strings with the %x and %o formatting target codes. String Formatting Method Calls As mentioned earlier, Python 2.6 and 3.0 introduced a new way to format strings that is seen by some as a bit more Python-specific. Unlike formatting expressions, formatting method calls are not closely based upon the C language’s “printf” model, and they are more verbose and explicit in intent. On the other hand, the new technique still relies on some “printf” concepts, such as type codes and formatting specifications. Moreover, it largely overlaps with (and sometimes requires a bit more code than) formatting expressions, and it can be just as complex in advanced roles. Because of this, there is no best-use recommendation between expressions and method calls today, so most programmers would be well served by a cursory understanding of both schemes. String Formatting Method Calls | 183 The Basics In short, the new string object’s format method in 2.6 and 3.0 (and later) uses the subject string as a template and takes any number of arguments that represent values to be substituted according to the template. Within the subject string, curly braces designate substitution targets and arguments to be inserted either by position (e.g., {1}) or keyword (e.g., {food}). As we’ll learn when we study argument passing in depth in Chapter 18, arguments to functions and methods may be passed by position or keyword name, and Python’s ability to collect arbitrarily many positional and keyword arguments allows for such general method call patterns. In Python 2.6 and 3.0, for example: >>> template = '{0}, {1} and {2}' >>> template.format('spam', 'ham', 'eggs') 'spam, ham and eggs' # By position >>> template = '{motto}, {pork} and {food}' >>> template.format(motto='spam', pork='ham', food='eggs') 'spam, ham and eggs' # By keyword >>> template = '{motto}, {0} and {food}' >>> template.format('ham', motto='spam', food='eggs') 'spam, ham and eggs' # By both Naturally, the string can also be a literal that creates a temporary string, and arbitrary object types can be substituted: >>> '{motto}, {0} and {food}'.format(42, motto=3.14, food=[1, 2]) '3.14, 42 and [1, 2]' Just as with the % expression and other string methods, format creates and returns a new string object, which can be printed immediately or saved for further work (recall that strings are immutable, so format really must make a new object). String formatting is not just for display: >>> X = '{motto}, {0} and {food}'.format(42, motto=3.14, food=[1, 2]) >>> X '3.14, 42 and [1, 2]' >>> X.split(' and ') ['3.14, 42', '[1, 2]'] >>> Y = X.replace('and', 'but under no circumstances') >>> Y '3.14, 42 but under no circumstances [1, 2]' Adding Keys, Attributes, and Offsets Like % formatting expressions, format calls can become more complex to support more advanced usage. For instance, format strings can name object attributes and dictionary keys—as in normal Python syntax, square brackets name dictionary keys and dots denote object attributes of an item referenced by position or keyword. The first of the 184 | Chapter 7: Strings following examples indexes a dictionary on the key “spam” and then fetches the attribute “platform” from the already imported sys module object. The second does the same, but names the objects by keyword instead of position: >>> import sys >>> 'My {1[spam]} runs {0.platform}'.format(sys, {'spam': 'laptop'}) 'My laptop runs win32' >>> 'My {config[spam]} runs {sys.platform}'.format(sys=sys, config={'spam': 'laptop'}) 'My laptop runs win32' Square brackets in format strings can name list (and other sequence) offsets to perform indexing, too, but only single positive offsets work syntactically within format strings, so this feature is not as general as you might think. As with % expressions, to name negative offsets or slices, or to use arbitrary expression results in general, you must run expressions outside the format string itself: >>> somelist = list('SPAM') >>> somelist ['S', 'P', 'A', 'M'] >>> 'first={0[0]}, third={0[2]}'.format(somelist) 'first=S, third=A' >>> 'first={0}, last={1}'.format(somelist[0], somelist[-1]) 'first=S, last=M' # [-1] fails in fmt >>> parts = somelist[0], somelist[-1], somelist[1:3] >>> 'first={0}, last={1}, middle={2}'.format(*parts) "first=S, last=M, middle=['P', 'A']" # [1:3] fails in fmt Adding Specific Formatting Another similarity with % expressions is that more specific layouts can be achieved by adding extra syntax in the format string. For the formatting method, we use a colon after the substitution target’s identification, followed by a format specifier that can name the field size, justification, and a specific type code. Here’s the formal structure of what can appear as a substitution target in a format string: {fieldname!conversionflag:formatspec} In this substitution target syntax: • fieldname is a number or keyword naming an argument, followed by optional “.name” attribute or “[index]” component references. • conversionflag can be r, s, or a to call repr, str, or ascii built-in functions on the value, respectively. String Formatting Method Calls | 185 • formatspec specifies how the value should be presented, including details such as field width, alignment, padding, decimal precision, and so on, and ends with an optional data type code. The formatspec component after the colon character is formally described as follows (brackets denote optional components and are not coded literally): [[fill]align][sign][#][0][width][.precision][typecode] align may be <, >, =, or ^, for left alignment, right alignment, padding after a sign character, or centered alignment, respectively. The formatspec also contains nested {} format strings with field names only, to take values from the arguments list dynamically (much like the * in formatting expressions). See Python’s library manual for more on substitution syntax and a list of the available type codes—they almost completely overlap with those used in % expressions and listed previously in Table 7-4, but the format method also allows a “b” type code used to display integers in binary format (it’s equivalent to using the bin built-in call), allows a “%” type code to display percentages, and uses only “d” for base-10 integers (not “i” or “u”). As an example, in the following {0:10} means the first positional argument in a field 10 characters wide, {1:<10} means the second positional argument left-justified in a 10-character-wide field, and {0.platform:>10} means the platform attribute of the first argument right-justified in a 10-character-wide field: >>> '{0:10} = {1:10}'.format('spam', 123.4567) 'spam = 123.457' >>> '{0:>10} = {1:<10}'.format('spam', 123.4567) ' spam = 123.457 ' >>> '{0.platform:>10} = {1[item]:<10}'.format(sys, dict(item='laptop')) ' win32 = laptop ' Floating-point numbers support the same type codes and formatting specificity in formatting method calls as in % expressions. For instance, in the following {2:g} means the third argument formatted by default according to the “g” floating-point representation, {1:.2f} designates the “f” floating-point format with just 2 decimal digits, and {2:06.2f} adds a field with a width of 6 characters and zero padding on the left: >>> '{0:e}, {1:.3e}, {2:g}'.format(3.14159, 3.14159, 3.14159) '3.141590e+00, 3.142e+00, 3.14159' >>> '{0:f}, {1:.2f}, {2:06.2f}'.format(3.14159, 3.14159, 3.14159) '3.141590, 3.14, 003.14' Hex, octal, and binary formats are supported by the format method as well. In fact, string formatting is an alternative to some of the built-in functions that format integers to a given base: 186 | Chapter 7: Strings >>> '{0:X}, {1:o}, {2:b}'.format(255, 255, 255) 'FF, 377, 11111111' # Hex, octal, binary >>> bin(255), int('11111111', 2), 0b11111111 ('0b11111111', 255, 255) # Other to/from binary >>> hex(255), int('FF', 16), 0xFF ('0xff', 255, 255) # Other to/from hex >>> oct(255), int('377', 8), 0o377, 0377 ('0377', 255, 255, 255) # Other to/from octal # 0377 works in 2.6, not 3.0! Formatting parameters can either be hardcoded in format strings or taken from the arguments list dynamically by nested format syntax, much like the star syntax in formatting expressions: >>> '{0:.2f}'.format(1 / 3.0) '0.33' >>> '%.2f' % (1 / 3.0) '0.33' # Parameters hardcoded >>> '{0:.{1}f}'.format(1 / 3.0, 4) '0.3333' >>> '%.*f' % (4, 1 / 3.0) '0.3333' # Take value from arguments # Ditto for expression Finally, Python 2.6 and 3.0 also provide a new built-in format function, which can be used to format a single item. It’s a more concise alternative to the string format method, and is roughly similar to formatting a single item with the % formatting expression: # String method >>> '{0:.2f}'.format(1.2345) '1.23' >>> format(1.2345, '.2f') '1.23' >>> '%.2f' % 1.2345 '1.23' # Built-in function # Expression Technically, the format built-in runs the subject object’s __format__ method, which the str.format method does internally for each formatted item. It’s still more verbose than the original % expression’s equivalent, though—which leads us to the next section. Comparison to the % Formatting Expression If you study the prior sections closely, you’ll probably notice that at least for positional references and dictionary keys, the string format method looks very much like the % formatting expression, especially in advanced use with type codes and extra formatting syntax. In fact, in common use cases formatting expressions may be easier to code than formatting method calls, especially when using the generic %s print-string substitution target: print('%s=%s' % ('spam', 42)) # 2.X+ format expression print('{0}={1}'.format('spam', 42)) # 3.0 (and 2.6) format method String Formatting Method Calls | 187 As we’ll see in a moment, though, more complex formatting tends to be a draw in terms of complexity (difficult tasks are generally difficult, regardless of approach), and some see the formatting method as largely redundant. On the other hand, the formatting method also offers a few potential advantages. For example, the original % expression can’t handle keywords, attribute references, and binary type codes, although dictionary key references in % format strings can often achieve similar goals. To see how the two techniques overlap, compare the following % expressions to the equivalent format method calls shown earlier: # The basics: with % instead of format() >>> template = '%s, %s, %s' >>> template % ('spam', 'ham', 'eggs') 'spam, ham, eggs' # By position >>> template = '%(motto)s, %(pork)s and %(food)s' >>> template % dict(motto='spam', pork='ham', food='eggs') 'spam, ham and eggs' # By key >>> '%s, %s and %s' % (3.14, 42, [1, 2]) '3.14, 42 and [1, 2]' # Arbitrary types # Adding keys, attributes, and offsets >>> 'My %(spam)s runs %(platform)s' % {'spam': 'laptop', 'platform': sys.platform} 'My laptop runs win32' >>> 'My %(spam)s runs %(platform)s' % dict(spam='laptop', platform=sys.platform) 'My laptop runs win32' >>> somelist = list('SPAM') >>> parts = somelist[0], somelist[-1], somelist[1:3] >>> 'first=%s, last=%s, middle=%s' % parts "first=S, last=M, middle=['P', 'A']" When more complex formatting is applied the two techniques approach parity in terms of complexity, although if you compare the following with the format method call equivalents listed earlier you’ll again find that the % expressions tend to be a bit simpler and more concise: # Adding specific formatting >>> '%-10s = %10s' % ('spam', 123.4567) 'spam = 123.4567' >>> '%10s = %-10s' % ('spam', 123.4567) ' spam = 123.4567 ' >>> '%(plat)10s = %(item)-10s' % dict(plat=sys.platform, item='laptop') ' win32 = laptop ' 188 | Chapter 7: Strings # Floating-point numbers >>> '%e, %.3e, %g' % (3.14159, 3.14159, 3.14159) '3.141590e+00, 3.142e+00, 3.14159' >>> '%f, %.2f, %06.2f' % (3.14159, 3.14159, 3.14159) '3.141590, 3.14, 003.14' # Hex and octal, but not binary >>> '%x, %o' % (255, 255) 'ff, 377' The format method has a handful of advanced features that the % expression does not, but even more involved formatting still seems to be essentially a draw in terms of complexity. For instance, the following shows the same result generated with both techniques, with field sizes and justifications and various argument reference methods: # Hardcoded references in both >>> import sys >>> 'My {1[spam]:<8} runs {0.platform:>8}'.format(sys, {'spam': 'laptop'}) 'My laptop runs win32' >>> 'My %(spam)-8s runs %(plat)8s' % dict(spam='laptop', plat=sys.platform) 'My laptop runs win32' In practice, programs are less likely to hardcode references like this than to execute code that builds up a set of substitution data ahead of time (to collect data to substitute into an HTML template all at once, for instance). When we account for common practice in examples like this, the comparison between the format method and the % expression is even more direct (as we’ll see in Chapter 18, the **data in the method call here is special syntax that unpacks a dictionary of keys and values into individual “name=value” keyword arguments so they can be referenced by name in the format string): # Building data ahead of time in both >>> data = dict(platform=sys.platform, spam='laptop') >>> 'My {spam:<8} runs {platform:>8}'.format(**data) 'My laptop runs win32' >>> 'My %(spam)-8s runs %(platform)8s' % data 'My laptop runs win32' As usual, the Python community will have to decide whether % expressions, format method calls, or a toolset with both techniques proves better over time. Experiment with these techniques on your own to get a feel for what they offer, and be sure to see the Python 2.6 and 3.0 library manuals for more details. String Formatting Method Calls | 189 String format method enhancements in Python 3.1: The upcoming 3.1 release (in alpha form as this chapter was being written) will add a thousand-separator syntax for numbers, which inserts commas between three-digit groups. Add a comma before the type code to make this work, as follows: >>> '{0:d}'.format(999999999999) '999999999999' >>> '{0:,d}'.format(999999999999) '999,999,999,999' Python 3.1 also assigns relative numbers to substitution targets automatically if they are not included explicitly, though using this extension may negate one of the main benefits of the formatting method, as the next section describes: >>> '{:,d}'.format(999999999999) '999,999,999,999' >>> '{:,d} {:,d}'.format(9999999, 8888888) '9,999,999 8,888,888' >>> '{:,.2f}'.format(296999.2567) '296,999.26' This book doesn’t cover 3.1 officially, so you should take this as a preview. Python 3.1 will also address a major performance issue in 3.0 related to the speed of file input/output operations, which made 3.0 impractical for many types of programs. See the 3.1 release notes for more details. See also the formats.py comma-insertion and money-formatting function examples in Chapter 24 for a manual solution that can be imported and used prior to Python 3.1. Why the New Format Method? Now that I’ve gone to such lengths to compare and contrast the two formatting techniques, I need to explain why you might want to consider using the format method variant at times. In short, although the formatting method can sometimes require more code, it also: • • • • Has a few extra features not found in the % expression Can make substitution value references more explicit Trades an operator for an arguably more mnemonic method name Does not support different syntax for single and multiple substitution value cases Although both techniques are available today and the formatting expression is still widely used, the format method might eventually subsume it. But because the choice is currently still yours to make, let’s briefly expand on some of the differences before moving on. 190 | Chapter 7: Strings Extra features The method call supports a few extras that the expression does not, such as binary type codes and (coming in Python 3.1) thousands groupings. In addition, the method call supports key and attribute references directly. As we’ve seen, though, the formatting expression can usually achieve the same effects in other ways: >>> '{0:b}'.format((2 ** 16) −1) '1111111111111111' >>> '%b' % ((2 ** 16) −1) ValueError: unsupported format character 'b' (0x62) at index 1 >>> bin((2 ** 16) −1) '0b1111111111111111' >>> '%s' % bin((2 ** 16) −1)[2:] '1111111111111111' See also the prior examples that compare dictionary-based formatting in the % expression to key and attribute references in the format method; especially in common practice, the two seem largely variations on a theme. Explicit value references One use case where the format method is at least debatably clearer is when there are many values to be substituted into the format string. The lister.py classes example we’ll meet in Chapter 30, for example, substitutes six items into a single string, and in this case the method’s {i} position labels seem easier to read than the expression’s %s: '\n%s<Class %s, address %s:\n%s%s%s>\n' % (...) # Expression '\n{0}<Class {1}, address {2}:\n{3}{4}{5}>\n'.format(...) # Method On the other hand, using dictionary keys in % expressions can mitigate much of this difference. This is also something of a worst-case scenario for formatting complexity, and not very common in practice; more typical use cases seem largely a tossup. Moreover, in Python 3.1 (still in alpha release form as I write these words), numbering substitution values will become optional, thereby subverting this purported benefit altogether: C:\misc> C:\Python31\python >>> 'The {0} side {1} {2}'.format('bright', 'of', 'life') 'The bright side of life' >>> >>> 'The {} side {} {}'.format('bright', 'of', 'life') 'The bright side of life' >>> >>> 'The %s side %s %s' % ('bright', 'of', 'life') 'The bright side of life' # Python 3.1+ String Formatting Method Calls | 191 Using 3.1’s automatic relative numbering like this seems to negate a large part of the method’s advantage. Compare the effect on floating-point formatting, for example— the formatting expression is still more concise, and still seems less cluttered: C:\misc> C:\Python31\python >>> '{0:f}, {1:.2f}, {2:05.2f}'.format(3.14159, 3.14159, 3.14159) '3.141590, 3.14, 03.14' >>> >>> '{:f}, {:.2f}, {:06.2f}'.format(3.14159, 3.14159, 3.14159) '3.141590, 3.14, 003.14' >>> >>> '%f, %.2f, %06.2f' % (3.14159, 3.14159, 3.14159) '3.141590, 3.14, 003.14' Method names and general arguments Given this 3.1 auto-numbering change, the only clearly remaining potential advantages of the formatting method are that it replaces the % operator with a more mnemonic format method name and does not distinguish between single and multiple substitution values. The former may make the method appear simpler to beginners at first glance (“format” may be easier to parse than multiple “%” characters), though this is too subjective to call. The latter difference might be more significant—with the format expression, a single value can be given by itself, but multiple values must be enclosed in a tuple: >>> '%.2f' % 1.2345 '1.23' >>> '%.2f %s' % (1.2345, 99) '1.23 99' Technically, the formatting expression accepts either a single substitution value, or a tuple of one or more items. In fact, because a single item can be given either by itself or within a tuple, a tuple to be formatted must be provided as nested tuples: >>> '%s' % 1.23 '1.23' >>> '%s' % (1.23,) '1.23' >>> '%s' % ((1.23,),) '(1.23,)' The formatting method, on the other hand, tightens this up by accepting general function arguments in both cases: >>> '{0:.2f}'.format(1.2345) '1.23' >>> '{0:.2f} {1}'.format(1.2345, 99) '1.23 99' >>> '{0}'.format(1.23) '1.23' >>> '{0}'.format((1.23,)) '(1.23,)' 192 | Chapter 7: Strings Consequently, it might be less confusing to beginners and cause fewer programming mistakes. This is still a fairly minor issue, though—if you always enclose values in a tuple and ignore the nontupled option, the expression is essentially the same as the method call here. Moreover, the method incurs an extra price in inflated code size to achieve its limited flexibility. Given that the expression has been used extensively throughout Python’s history, it’s not clear that this point justifies breaking existing code for a new tool that is so similar, as the next section argues. Possible future deprecation? As mentioned earlier, there is some risk that Python developers may deprecate the % expression in favor of the format method in the future. In fact, there is a note to this effect in Python 3.0’s manuals. This has not yet occurred, of course, and both formatting techniques are fully available and reasonable to use in Python 2.6 and 3.0 (the versions of Python this book covers). Both techniques are supported in the upcoming Python 3.1 release as well, so deprecation of either seems unlikely for the foreseeable future. Moreover, because formatting expressions are used extensively in almost all existing Python code written to date, most programmers will benefit from being familiar with both techniques for many years to come. If this deprecation ever does occur, though, you may need to recode all your % expressions as format methods, and translate those that appear in this book, in order to use a newer Python release. At the risk of editorializing here, I hope that such a change will be based upon the future common practice of actual Python programmers, not the whims of a handful of core developers—particularly given that the window for Python 3.0’s many incompatible changes is now closed. Frankly, this deprecation would seem like trading one complicated thing for another complicated thing—one that is largely equivalent to the tool it would replace! If you care about migrating to future Python releases, though, be sure to watch for developments on this front over time. General Type Categories Now that we’ve explored the first of Python’s collection objects, the string, let’s pause to define a few general type concepts that will apply to most of the types we look at from here on. With regard to built-in types, it turns out that operations work the same for all the types in the same category, so we’ll only need to define most of these ideas once. We’ve only examined numbers and strings so far, but because they are representative of two of the three major type categories in Python, you already know more about several other types than you might think. General Type Categories | 193 Types Share Operation Sets by Categories As you’ve learned, strings are immutable sequences: they cannot be changed in-place (the immutable part), and they are positionally ordered collections that are accessed by offset (the sequence part). Now, it so happens that all the sequences we’ll study in this part of the book respond to the same sequence operations shown in this chapter at work on strings—concatenation, indexing, iteration, and so on. More formally, there are three major type (and operation) categories in Python: Numbers (integer, floating-point, decimal, fraction, others) Support addition, multiplication, etc. Sequences (strings, lists, tuples) Support indexing, slicing, concatenation, etc. Mappings (dictionaries) Support indexing by key, etc. Sets are something of a category unto themselves (they don’t map keys to values and are not positionally ordered sequences), and we haven’t yet explored mappings on our in-depth tour (dictionaries are discussed in the next chapter). However, many of the other types we will encounter will be similar to numbers and strings. For example, for any sequence objects X and Y: • X + Y makes a new sequence object with the contents of both operands. • X * N makes a new sequence object with N copies of the sequence operand X. In other words, these operations work the same way on any kind of sequence, including strings, lists, tuples, and some user-defined object types. The only difference is that the new result object you get back is of the same type as the operands X and Y—if you concatenate lists, you get back a new list, not a string. Indexing, slicing, and other sequence operations work the same on all sequences, too; the type of the objects being processed tells Python which flavor of the task to perform. Mutable Types Can Be Changed In-Place The immutable classification is an important constraint to be aware of, yet it tends to trip up new users. If an object type is immutable, you cannot change its value in-place; Python raises an error if you try. Instead, you must run code to make a new object containing the new value. The major core types in Python break down as follows: Immutables (numbers, strings, tuples, frozensets) None of the object types in the immutable category support in-place changes, though we can always run expressions to make new objects and assign their results to variables as needed. 194 | Chapter 7: Strings Mutables (lists, dictionaries, sets) Conversely, the mutable types can always be changed in-place with operations that do not create new objects. Although such objects can be copied, in-place changes support direct modification. Generally, immutable types give some degree of integrity by guaranteeing that an object won’t be changed by another part of a program. For a refresher on why this matters, see the discussion of shared object references in Chapter 6. To see how lists, dictionaries, and tuples participate in type categories, we need to move ahead to the next chapter. Chapter Summary In this chapter, we took an in-depth tour of the string object type. We learned about coding string literals, and we explored string operations, including sequence expressions, string method calls, and string formatting with both expressions and method calls. Along the way, we studied a variety of concepts in depth, such as slicing, method call syntax, and triple-quoted block strings. We also defined some core ideas common to a variety of types: sequences, for example, share an entire set of operations. In the next chapter, we’ll continue our types tour with a look at the most general object collections in Python—lists and dictionaries. As you’ll find, much of what you’ve learned here will apply to those types as well. And as mentioned earlier, in the final part of this book we’ll return to Python’s string model to flesh out the details of Unicode text and binary data, which are of interest to some, but not all, Python programmers. Before moving on, though, here’s another chapter quiz to review the material covered here. Test Your Knowledge: Quiz 1. Can the string find method be used to search a list? 2. Can a string slice expression be used on a list? 3. How would you convert a character to its ASCII integer code? How would you convert the other way, from an integer to a character? 4. How might you go about changing a string in Python? 5. Given a string S with the value "s,pa,m", name two ways to extract the two characters in the middle. 6. How many characters are there in the string "a\nb\x1f\000d"? 7. Why might you use the string module instead of string method calls? Test Your Knowledge: Quiz | 195 Test Your Knowledge: Answers 1. No, because methods are always type-specific; that is, they only work on a single data type. Expressions like X+Y and built-in functions like len(X) are generic, though, and may work on a variety of types. In this case, for instance, the in membership expression has a similar effect as the string find, but it can be used to search both strings and lists. In Python 3.0, there is some attempt to group methods by categories (for example, the mutable sequence types list and bytearray have similar method sets), but methods are still more type-specific than other operation sets. 2. Yes. Unlike methods, expressions are generic and apply to many types. In this case, the slice expression is really a sequence operation—it works on any type of sequence object, including strings, lists, and tuples. The only difference is that when you slice a list, you get back a new list. 3. The built-in ord(S) function converts from a one-character string to an integer character code; chr(I) converts from the integer code back to a string. 4. Strings cannot be changed; they are immutable. However, you can achieve a similar effect by creating a new string—by concatenating, slicing, running formatting expressions, or using a method call like replace—and then assigning the result back to the original variable name. 5. You can slice the string using S[2:4], or split on the comma and index the string using S.split(',')[1]. Try these interactively to see for yourself. 6. Six. The string "a\nb\x1f\000d" contains the bytes a, newline (\n), b, binary 31 (a hex escape \x1f), binary 0 (an octal escape \000), and d. Pass the string to the builtin len function to verify this, and print each of its character’s ord results to see the actual byte values. See Table 7-2 for more details. 7. You should never use the string module instead of string object method calls today—it’s deprecated, and its calls are removed completely in Python 3.0. The only reason for using the string module at all is for its other tools, such as predefined constants. You might also see it appear in what is now very old and dusty Python code. 196 | Chapter 7: Strings CHAPTER 8 Lists and Dictionaries This chapter presents the list and dictionary object types, both of which are collections of other objects. These two types are the main workhorses in almost all Python scripts. As you’ll see, both types are remarkably flexible: they can be changed in-place, can grow and shrink on demand, and may contain and be nested in any other kind of object. By leveraging these types, you can build up and process arbitrarily rich information structures in your scripts. Lists The next stop on our built-in object tour is the Python list. Lists are Python’s most flexible ordered collection object type. Unlike strings, lists can contain any sort of object: numbers, strings, and even other lists. Also, unlike strings, lists may be changed in-place by assignment to offsets and slices, list method calls, deletion statements, and more—they are mutable objects. Python lists do the work of most of the collection data structures you might have to implement manually in lower-level languages such as C. Here is a quick look at their main properties. Python lists are: Ordered collections of arbitrary objects From a functional view, lists are just places to collect other objects so you can treat them as groups. Lists also maintain a left-to-right positional ordering among the items they contain (i.e., they are sequences). Accessed by offset Just as with strings, you can fetch a component object out of a list by indexing the list on the object’s offset. Because items in lists are ordered by their positions, you can also do tasks such as slicing and concatenation. 197 Variable-length, heterogeneous, and arbitrarily nestable Unlike strings, lists can grow and shrink in-place (their lengths can vary), and they can contain any sort of object, not just one-character strings (they’re heterogeneous). Because lists can contain other complex objects, they also support arbitrary nesting; you can create lists of lists of lists, and so on. Of the category “mutable sequence” In terms of our type category qualifiers, lists are mutable (i.e., can be changed inplace) and can respond to all the sequence operations used with strings, such as indexing, slicing, and concatenation. In fact, sequence operations work the same on lists as they do on strings; the only difference is that sequence operations such as concatenation and slicing return new lists instead of new strings when applied to lists. Because lists are mutable, however, they also support other operations that strings don’t (such as deletion and index assignment operations, which change the lists in-place). Arrays of object references Technically, Python lists contain zero or more references to other objects. Lists might remind you of arrays of pointers (addresses) if you have a background in some other languages. Fetching an item from a Python list is about as fast as indexing a C array; in fact, lists really are arrays inside the standard Python interpreter, not linked structures. As we learned in Chapter 6, though, Python always follows a reference to an object whenever the reference is used, so your program deals only with objects. Whenever you assign an object to a data structure component or variable name, Python always stores a reference to that same object, not a copy of it (unless you request a copy explicitly). Table 8-1 summarizes common and representative list object operations. As usual, for the full story see the Python standard library manual, or run a help(list) or dir(list) call interactively for a complete list of list methods—you can pass in a real list, or the word list, which is the name of the list data type. Table 8-1. Common list literals and operations Operation Interpretation L = [] An empty list L = [0, 1, 2, 3] Four items: indexes 0..3 L = ['abc', ['def', 'ghi']] Nested sublists L = list('spam') Lists of an iterable’s items, list of successive integers L = list(range(-4, 4)) L[i] L[i][j] L[i:j] len(L) 198 | Chapter 8: Lists and Dictionaries Index, index of index, slice, length Operation Interpretation L1 + L2 Concatenate, repeat L * 3 for x in L: print(x) Iteration, membership 3 in L L.append(4) Methods: growing L.extend([5,6,7]) L.insert(I, X) L.index(1) Methods: searching L.count(X) L.sort() Methods: sorting, reversing, etc. L.reverse() del L[k] Methods, statement: shrinking del L[i:j] L.pop() L.remove(2) L[i:j] = [] L[i] = 1 Index assignment, slice assignment L[i:j] = [4,5,6] L = [x**2 for x in range(5)] List comprehensions and maps (Chapters 14, 20) list(map(ord, 'spam')) When written down as a literal expression, a list is coded as a series of objects (really, expressions that return objects) in square brackets, separated by commas. For instance, the second row in Table 8-1 assigns the variable L to a four-item list. A nested list is coded as a nested square-bracketed series (row 3), and the empty list is just a squarebracket pair with nothing inside (row 1).* Many of the operations in Table 8-1 should look familiar, as they are the same sequence operations we put to work on strings—indexing, concatenation, iteration, and so on. Lists also respond to list-specific method calls (which provide utilities such as sorting, reversing, adding items to the end, etc.), as well as in-place change operations (deleting items, assignment to indexes and slices, and so forth). Lists have these tools for change operations because they are a mutable object type. * In practice, you won’t see many lists written out like this in list-processing programs. It’s more common to see code that processes lists constructed dynamically (at runtime). In fact, although it’s important to master literal syntax, most data structures in Python are built by running program code at runtime. Lists | 199 Lists in Action Perhaps the best way to understand lists is to see them at work. Let’s once again turn to some simple interpreter interactions to illustrate the operations in Table 8-1. Basic List Operations Because they are sequences, lists support many of the same operations as strings. For example, lists respond to the + and * operators much like strings—they mean concatenation and repetition here too, except that the result is a new list, not a string: % python >>> len([1, 2, 3]) 3 >>> [1, 2, 3] + [4, 5, 6] [1, 2, 3, 4, 5, 6] >>> ['Ni!'] * 4 ['Ni!', 'Ni!', 'Ni!', 'Ni!'] # Length # Concatenation # Repetition Although the + operator works the same for lists and strings, it’s important to know that it expects the same sort of sequence on both sides—otherwise, you get a type error when the code runs. For instance, you cannot concatenate a list and a string unless you first convert the list to a string (using tools such as str or % formatting) or convert the string to a list (the list built-in function does the trick): >>> str([1, 2]) + "34" '[1, 2]34' >>> [1, 2] + list("34") [1, 2, '3', '4'] # Same as "[1, 2]" + "34" # Same as [1, 2] + ["3", "4"] List Iteration and Comprehensions More generally, lists respond to all the sequence operations we used on strings in the prior chapter, including iteration tools: >>> 3 in [1, 2, 3] True >>> for x in [1, 2, 3]: ... print(x, end=' ') ... 1 2 3 # Membership # Iteration We will talk more formally about for iteration and the range built-ins in Chapter 13, because they are related to statement syntax. In short, for loops step through items in any sequence from left to right, executing one or more statements for each item. The last items in Table 8-1, list comprehensions and map calls, are covered in more detail in Chapter 14 and expanded on in Chapter 20. Their basic operation is straightforward, though—as introduced in Chapter 4, list comprehensions are a way to build a new list 200 | Chapter 8: Lists and Dictionaries by applying an expression to each item in a sequence, and are close relatives to for loops: # List comprehensions >>> res = [c * 4 for c in 'SPAM'] >>> res ['SSSS', 'PPPP', 'AAAA', 'MMMM'] This expression is functionally equivalent to a for loop that builds up a list of results manually, but as we’ll learn in later chapters, list comprehensions are simpler to code and faster to run today: >>> res = [] >>> for c in 'SPAM': ... res.append(c * 4) ... >>> res ['SSSS', 'PPPP', 'AAAA', 'MMMM'] # List comprehension equivalent As also introduced in Chapter 4, the map built-in function does similar work, but applies a function to items in a sequence and collects all the results in a new list: # map function across sequence >>> list(map(abs, [−1, −2, 0, 1, 2])) [1, 2, 0, 1, 2] Because we’re not quite ready for the full iteration story, we’ll postpone further details for now, but watch for a similar comprehension expression for dictionaries later in this chapter. Indexing, Slicing, and Matrixes Becauselists are sequences, indexing and slicing work the same way for lists as they do for strings. However, the result of indexing a list is whatever type of object lives at the offset you specify, while slicing a list always returns a new list: >>> L = ['spam', 'Spam', 'SPAM!'] >>> L[2] 'SPAM!' >>> L[−2] 'Spam' >>> L[1:] ['Spam', 'SPAM!'] # Offsets start at zero # Negative: count from the right # Slicing fetches sections One note here: because you can nest lists and other object types within lists, you will sometimes need to string together index operations to go deeper into a data structure. For example, one of the simplest ways to represent matrixes (multidimensional arrays) in Python is as lists with nested sublists. Here’s a basic 3 × 3 two-dimensional list-based array: >>> matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] With one index, you get an entire row (really, a nested sublist), and with two, you get an item within the row: Lists in Action | 201 >>> [4, >>> 5 >>> 7 >>> ... ... >>> 5 matrix[1] 5, 6] matrix[1][1] matrix[2][0] matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] matrix[1][1] Notice in the preceding interaction that lists can naturally span multiple lines if you want them to because they are contained by a pair of brackets (more on syntax in the next part of the book). Later in this chapter, you’ll also see a dictionary-based matrix representation. For high-powered numeric work, the NumPy extension mentioned in Chapter 5 provides other ways to handle matrixes. Changing Lists In-Place Because lists are mutable, they support operations that change a list object in-place. That is, the operations in this section all modify the list object directly, without requiring that you make a new copy, as you had to for strings. Because Python deals only in object references, this distinction between changing an object in-place and creating a new object matters—as discussed in Chapter 6, if you change an object in-place, you might impact more than one reference to it at the same time. Index and slice assignments When using a list, you can change its contents by assigning to either a particular item (offset) or an entire section (slice): >>> L = ['spam', 'Spam', 'SPAM!'] >>> L[1] = 'eggs' # Index assignment >>> L ['spam', 'eggs', 'SPAM!'] >>> L[0:2] = ['eat', 'more'] # Slice assignment: delete+insert >>> L # Replaces items 0,1 ['eat', 'more', 'SPAM!'] Both index and slice assignments are in-place changes—they modify the subject list directly, rather than generating a new list object for the result. Index assignment in Python works much as it does in C and most other languages: Python replaces the object reference at the designated offset with a new one. Slice assignment, the last operation in the preceding example, replaces an entire section of a list in a single step. Because it can be a bit complex, it is perhaps best thought of as a combination of two steps: 202 | Chapter 8: Lists and Dictionaries 1. Deletion. The slice you specify to the left of the = is deleted. 2. Insertion. The new items contained in the object to the right of the = are inserted into the list on the left, at the place where the old slice was deleted.† This isn’t what really happens, but it tends to help clarify why the number of items inserted doesn’t have to match the number of items deleted. For instance, given a list L that has the value [1,2,3], the assignment L[1:2]=[4,5] sets L to the list [1,4,5,3]. Python first deletes the 2 (a one-item slice), then inserts the 4 and 5 where the deleted 2 used to be. This also explains why L[1:2]=[] is really a deletion operation—Python deletes the slice (the item at offset 1), and then inserts nothing. In effect, slice assignment replaces an entire section, or “column,” all at once. Because the length of the sequence being assigned does not have to match the length of the slice being assigned to, slice assignment can be used to replace (by overwriting), expand (by inserting), or shrink (by deleting) the subject list. It’s a powerful operation, but frankly, one that you may not see very often in practice. There are usually more straightforward ways to replace, insert, and delete (concatenation and the insert, pop, and remove list methods, for example), which Python programmers tend to prefer in practice. List method calls Like strings, Python list objects also support type-specific method calls, many of which change the subject list in-place: >>> L.append('please') >>> L ['eat', 'more', 'SPAM!', 'please'] >>> L.sort() >>> L ['SPAM!', 'eat', 'more', 'please'] # Append method call: add item at end # Sort list items ('S' < 'e') Methods were introduced in Chapter 7. In brief, they are functions (really, attributes that reference functions) that are associated with particular objects. Methods provide type-specific tools; the list methods presented here, for instance, are generally available only for lists. Perhaps the most commonly used list method is append, which simply tacks a single item (object reference) onto the end of the list. Unlike concatenation, append expects you to pass in a single object, not a list. The effect of L.append(X) is similar to L+[X], but while the former changes L in-place, the latter makes a new list.‡ Another commonly seen method, sort, orders a list in-place; it uses Python standard comparison tests (here, string comparisons), and by default sorts in ascending order. † This description needs elaboration when the value and the slice being assigned overlap: L[2:5]=L[3:6], for instance, works fine because the value to be inserted is fetched before the deletion happens on the left. ‡ Unlike + concatenation, append doesn’t have to generate new objects, so it’s usually faster. You can also mimic append with clever slice assignments: L[len(L):]=[X] is like L.append(X), and L[:0]=[X] is like appending at the front of a list. Both delete an empty slice and insert X, changing L in-place quickly, like append. Lists in Action | 203 You can modify sort behavior by passing in keyword arguments—a special “name=value” syntax in function calls that specifies passing by name and is often used for giving configuration options. In sorts, the key argument gives a one-argument function that returns the value to be used in sorting, and the reverse argument allows sorts to be made in descending instead of ascending order: >>> L = ['abc', 'ABD', 'aBe'] >>> L.sort() >>> L ['ABD', 'aBe', 'abc'] >>> L = ['abc', 'ABD', 'aBe'] >>> L.sort(key=str.lower) >>> L ['abc', 'ABD', 'aBe'] >>> >>> L = ['abc', 'ABD', 'aBe'] >>> L.sort(key=str.lower, reverse=True) >>> L ['aBe', 'ABD', 'abc'] # Sort with mixed case # Normalize to lowercase # Change sort order The sort key argument might also be useful when sorting lists of dictionaries, to pick out a sort key by indexing each dictionary. We’ll study dictionaries later in this chapter, and you’ll learn more about keyword function arguments in Part IV. Comparison and sorts in 3.0: In Python 2.6 and earlier, comparisons of differently typed objects (e.g., a string and a list) work—the language defines a fixed ordering among different types, which is deterministic, if not aesthetically pleasing. That is, the ordering is based on the names of the types involved: all integers are less than all strings, for example, because "int" is less than "str". Comparisons never automatically convert types, except when comparing numeric type objects. In Python 3.0, this has changed: comparison of mixed types raises an exception instead of falling back on the fixed cross-type ordering. Because sorting uses comparisons internally, this means that [1, 2, 'spam'].sort() succeeds in Python 2.X but will raise an exception in Python 3.0 and later. Python 3.0 also no longer supports passing in an arbitrary comparison function to sorts, to implement different orderings. The suggested workaround is to use the key=func keyword argument to code value transformations during the sort, and use the reverse=True keyword argument to change the sort order to descending. These were the typical uses of comparison functions in the past. One warning here: beware that append and sort change the associated list object inplace, but don’t return the list as a result (technically, they both return a value called None). If you say something like L=L.append(X), you won’t get the modified value of L (in fact, you’ll lose the reference to the list altogether!). When you use attributes such as append and sort, objects are changed as a side effect, so there’s no reason to reassign. 204 | Chapter 8: Lists and Dictionaries Partly because of such constraints, sorting is also available in recent Pythons as a builtin function, which sorts any collection (not just lists) and returns a new list for the result (instead of in-place changes): >>> L = ['abc', 'ABD', 'aBe'] >>> sorted(L, key=str.lower, reverse=True) ['aBe', 'ABD', 'abc'] # Sorting built-in >>> L = ['abc', 'ABD', 'aBe'] >>> sorted([x.lower() for x in L], reverse=True) ['abe', 'abd', 'abc'] # Pretransform items: differs! Notice the last example here—we can convert to lowercase prior to the sort with a list comprehension, but the result does not contain the original list’s values as it does with the key argument. The latter is applied temporarily during the sort, instead of changing the values to be sorted. As we move along, we’ll see contexts in which the sorted builtin can sometimes be more useful than the sort method. Like strings, lists have other methods that perform other specialized operations. For instance, reverse reverses the list in-place, and the extend and pop methods insert multiple items at the end of and delete an item from the end of the list, respectively. There is also a reversed built-in function that works much like sorted, but it must be wrapped in a list call because it’s an iterator (more on iterators later): >>> >>> >>> [1, >>> 5 >>> [1, >>> >>> [4, >>> [1, L = [1, 2] L.extend([3,4,5]) L 2, 3, 4, 5] L.pop() L 2, 3, 4] L.reverse() L 3, 2, 1] list(reversed(L)) 2, 3, 4] # Add many items at end # Delete and return last item # In-place reversal method # Reversal built-in with a result In some types of programs, the list pop method used here is often used in conjunction with append to implement a quick last-in-first-out (LIFO) stack structure. The end of the list serves as the top of the stack: >>> >>> >>> >>> [1, >>> 2 >>> [1] L = [] L.append(1) L.append(2) L 2] L.pop() # Push onto stack # Pop off stack L Lists in Action | 205 The pop method also accepts an optional offset of the item to be deleted and returned (the default is the last item). Other list methods remove an item by value (remove), insert an item at an offset (insert), search for an item’s offset (index), and more: >>> L = ['spam', 'eggs', 'ham'] >>> L.index('eggs') 1 >>> L.insert(1, 'toast') >>> L ['spam', 'toast', 'eggs', 'ham'] >>> L.remove('eggs') >>> L ['spam', 'toast', 'ham'] >>> L.pop(1) 'toast' >>> L ['spam', 'ham'] # Index of an object # Insert at position # Delete by value # Delete by position See other documentation sources or experiment with these calls interactively on your own to learn more about list methods. Other common list operations Because lists are mutable, you can use the del statement to delete an item or section in-place: >>> L ['SPAM!', 'eat', 'more', 'please'] >>> del L[0] >>> L ['eat', 'more', 'please'] >>> del L[1:] >>> L ['eat'] # Delete one item # Delete an entire section # Same as L[1:] = [] Because slice assignment is a deletion plus an insertion, you can also delete a section of a list by assigning an empty list to a slice (L[i:j]=[]); Python deletes the slice named on the left, and then inserts nothing. Assigning an empty list to an index, on the other hand, just stores a reference to the empty list in the specified slot, rather than deleting it: >>> L = ['Already', 'got', 'one'] >>> L[1:] = [] >>> L ['Already'] >>> L[0] = [] >>> L [[]] Although all the operations just discussed are typical, there are additional list methods and operations not illustrated here (including methods for inserting and searching). For a comprehensive and up-to-date list of type tools, you should always consult 206 | Chapter 8: Lists and Dictionaries Python’s manuals, Python’s dir and help functions (which we first met in Chapter 4), or one of the reference texts mentioned in the Preface. I’d also like to remind you one more time that all the in-place change operations discussed here work only for mutable objects: they won’t work on strings (or tuples, discussed in Chapter 9), no matter how hard you try. Mutability is an inherent property of each object type. Dictionaries Apart from lists, dictionaries are perhaps the most flexible built-in data type in Python. If you think of lists as ordered collections of objects, you can think of dictionaries as unordered collections; the chief distinction is that in dictionaries, items are stored and fetched by key, instead of by positional offset. Being a built-in type, dictionaries can replace many of the searching algorithms and data structures you might have to implement manually in lower-level languages— indexing a dictionary is a very fast search operation. Dictionaries also sometimes do the work of records and symbol tables used in other languages, can represent sparse (mostly empty) data structures, and much more. Here’s a rundown of their main properties. Python dictionaries are: Accessed by key, not offset Dictionaries are sometimes called associative arrays or hashes. They associate a set of values with keys, so you can fetch an item out of a dictionary using the key under which you originally stored it. You use the same indexing operation to get components in a dictionary as you do in a list, but the index takes the form of a key, not a relative offset. Unordered collections of arbitrary objects Unlike in a list, items stored in a dictionary aren’t kept in any particular order; in fact, Python randomizes their left-to-right order to provide quick lookup. Keys provide the symbolic (not physical) locations of items in a dictionary. Variable-length, heterogeneous, and arbitrarily nestable Like lists, dictionaries can grow and shrink in-place (without new copies being made), they can contain objects of any type, and they support nesting to any depth (they can contain lists, other dictionaries, and so on). Of the category “mutable mapping” Dictionaries can be changed in-place by assigning to indexes (they are mutable), but they don’t support the sequence operations that work on strings and lists. Because dictionaries are unordered collections, operations that depend on a fixed positional order (e.g., concatenation, slicing) don’t make sense. Instead, dictionaries are the only built-in representatives of the mapping type category (objects that map keys to values). Dictionaries | 207 Tables of object references (hash tables) If lists are arrays of object references that support access by position, dictionaries are unordered tables of object references that support access by key. Internally, dictionaries are implemented as hash tables (data structures that support very fast retrieval), which start small and grow on demand. Moreover, Python employs optimized hashing algorithms to find keys, so retrieval is quick. Like lists, dictionaries store object references (not copies). Table 8-2 summarizes some of the most common and representative dictionary operations (again, see the library manual or run a dir(dict) or help(dict) call for a complete list—dict is the name of the type). When coded as a literal expression, a dictionary is written as a series of key:value pairs, separated by commas, enclosed in curly braces.§ An empty dictionary is an empty set of braces, and dictionaries can be nested by writing one as a value inside another dictionary, or within a list or tuple. Table 8-2. Common dictionary literals and operations Operation Interpretation D = {} Empty dictionary D = {'spam': 2, 'eggs': 3} Two-item dictionary D = {'food': {'ham': 1, 'egg': 2}} Nesting D = dict(name='Bob', age=40) Alternative construction techniques: D = dict(zip(keyslist, valslist)) keywords, zipped pairs, key lists D = dict.fromkeys(['a', 'b']) D['eggs'] Indexing by key D['food']['ham'] 'eggs' in D Membership: key present test D.keys() Methods: keys, D.values() values, D.items() keys+values, D.copy() copies, D.get(key, default) defaults, D.update(D2) merge, D.pop(key) delete, etc. len(D) Length: number of stored entries D[key] = 42 Adding/changing keys § As with lists, you won’t often see dictionaries constructed using literals. Lists and dictionaries are grown in different ways, though. As you’ll see in the next section, dictionaries are typically built up by assigning to new keys at runtime; this approach fails for lists (lists are commonly grown with append instead). 208 | Chapter 8: Lists and Dictionaries Operation Interpretation del D[key] Deleting entries by key list(D.keys()) Dictionary views (Python 3.0) D1.keys() & D2.keys() D = {x: x*2 for x in range(10)} Dictionary comprehensions (Python 3.0) Dictionaries in Action As Table 8-2 suggests, dictionaries are indexed by key, and nested dictionary entries are referenced by a series of indexes (keys in square brackets). When Python creates a dictionary, it stores its items in any left-to-right order it chooses; to fetch a value back, you supply the key with which it is associated, not its relative position. Let’s go back to the interpreter to get a feel for some of the dictionary operations in Table 8-2. Basic Dictionary Operations In normal operation, you create dictionaries with literals and store and access items by key with indexing: % python >>> D = {'spam': 2, 'ham': 1, 'eggs': 3} >>> D['spam'] 2 >>> D {'eggs': 3, 'ham': 1, 'spam': 2} # Make a dictionary # Fetch a value by key # Order is scrambled Here, the dictionary is assigned to the variable D; the value of the key 'spam' is the integer 2, and so on. We use the same square bracket syntax to index dictionaries by key as we did to index lists by offset, but here it means access by key, not by position. Notice the end of this example: the left-to-right order of keys in a dictionary will almost always be different from what you originally typed. This is on purpose: to implement fast key lookup (a.k.a. hashing), keys need to be reordered in memory. That’s why operations that assume a fixed left-to-right order (e.g., slicing, concatenation) do not apply to dictionaries; you can fetch values only by key, not by position. The built-in len function works on dictionaries, too; it returns the number of items stored in the dictionary or, equivalently, the length of its keys list. The dictionary in membership operator allows you to test for key existence, and the keys method returns all the keys in the dictionary. The latter of these can be useful for processing dictionaries sequentially, but you shouldn’t depend on the order of the keys list. Because the keys result can be used as a normal list, however, it can always be sorted if order matters (more on sorting and dictionaries later): >>> len(D) 3 >>> 'ham' in D # Number of entries in dictionary # Key membership test alternative Dictionaries in Action | 209 True >>> list(D.keys()) ['eggs', 'ham', 'spam'] # Create a new list of my keys Notice the second expression in this listing. As mentioned earlier, the in membership test used for strings and lists also works on dictionaries—it checks whether a key is stored in the dictionary. Technically, this works because dictionaries define iterators that step through their keys lists. Other types provide iterators that reflect their common uses; files, for example, have iterators that read line by line. We’ll discuss iterators in Chapters 14 and 20. Also note the syntax of the last example in this listing. We have to enclose it in a list call in Python 3.0 for similar reasons—keys in 3.0 returns an iterator, instead of a physical list. The list call forces it to produce all its values at once so we can print them. In 2.6, keys builds and returns an actual list, so the list call isn’t needed to display results. More on this later in this chapter. The order of keys in a dictionary is arbitrary and can change from release to release, so don’t be alarmed if your dictionaries print in a different order than shown here. In fact, the order has changed for me too—I’m running all these examples with Python 3.0, but their keys had a different order in an earlier edition when displayed. You shouldn’t depend on dictionary key ordering, in either programs or books! Changing Dictionaries In-Place Let’s continue with our interactive session. Dictionaries, like lists, are mutable, so you can change, expand, and shrink them in-place without making new dictionaries: simply assign a value to a key to change or create an entry. The del statement works here, too; it deletes the entry associated with the key specified as an index. Notice also the nesting of a list inside a dictionary in this example (the value of the key 'ham'). All collection data types in Python can nest inside each other arbitrarily: >>> D {'eggs': 3, 'ham': 1, 'spam': 2} >>> D['ham'] = ['grill', 'bake', 'fry'] # Change entry >>> D {'eggs': 3, 'ham': ['grill', 'bake', 'fry'], 'spam': 2} >>> del D['eggs'] >>> D {'ham': ['grill', 'bake', 'fry'], 'spam': 2} # Delete entry >>> D['brunch'] = 'Bacon' # Add new entry >>> D {'brunch': 'Bacon', 'ham': ['grill', 'bake', 'fry'], 'spam': 2} 210 | Chapter 8: Lists and Dictionaries As with lists, assigning to an existing index in a dictionary changes its associated value. Unlike with lists, however, whenever you assign a new dictionary key (one that hasn’t been assigned before) you create a new entry in the dictionary, as was done in the previous example for the key 'brunch'. This doesn’t work for lists because Python considers an offset beyond the end of a list out of bounds and throws an error. To expand a list, you need to use tools such as the append method or slice assignment instead. More Dictionary Methods Dictionary methods provide a variety of tools. For instance, the dictionary values and items methods return the dictionary’s values and (key,value) pair tuples, respectively (as with keys, wrap them in a list call in Python 3.0 to collect their values for display): >>> D = {'spam': 2, 'ham': 1, 'eggs': 3} >>> list(D.values()) [3, 1, 2] >>> list(D.items()) [('eggs', 3), ('ham', 1), ('spam', 2)] Such lists are useful in loops that need to step through dictionary entries one by one. Fetching a nonexistent key is normally an error, but the get method returns a default value (None, or a passed-in default) if the key doesn’t exist. It’s an easy way to fill in a default for a key that isn’t present and avoid a missing-key error: >>> D.get('spam') 2 >>> print(D.get('toast')) None >>> D.get('toast', 88) 88 # A key that is there # A key that is missing The update method provides something similar to concatenation for dictionaries, though it has nothing to do with left-to-right ordering (again, there is no such thing in dictionaries). It merges the keys and values of one dictionary into another, blindly overwriting values of the same key: >>> D {'eggs': 3, 'ham': 1, 'spam': 2} >>> D2 = {'toast':4, 'muffin':5} >>> D.update(D2) >>> D {'toast': 4, 'muffin': 5, 'eggs': 3, 'ham': 1, 'spam': 2} Finally, the dictionary pop method deletes a key from a dictionary and returns the value it had. It’s similar to the list pop method, but it takes a key instead of an optional position: # pop a dictionary by key >>> D {'toast': 4, 'muffin': 5, 'eggs': 3, 'ham': 1, 'spam': 2} >>> D.pop('muffin') Dictionaries in Action | 211 5 >>> D.pop('toast') 4 >>> D {'eggs': 3, 'ham': 1, 'spam': 2} # pop a list by position >>> L = ['aa', 'bb', 'cc', 'dd'] >>> L.pop() 'dd' >>> L ['aa', 'bb', 'cc'] >>> L.pop(1) 'bb' >>> L ['aa', 'cc'] # Delete and return from a key # Delete and return from the end # Delete from a specific position Dictionaries also provide a copy method; we’ll discuss this in Chapter 9, as it’s a way to avoid the potential side effects of shared references to the same dictionary. In fact, dictionaries come with many more methods than those listed in Table 8-2; see the Python library manual or other documentation sources for a comprehensive list. A Languages Table Let’s look at a more realistic dictionary example. The following example creates a table that maps programming language names (the keys) to their creators (the values). You fetch creator names by indexing on language names: >>> table = {'Python': 'Guido van Rossum', ... 'Perl': 'Larry Wall', ... 'Tcl': 'John Ousterhout' } >>> >>> language = 'Python' >>> creator = table[language] >>> creator 'Guido van Rossum' >>> for ... ... Tcl Python Perl lang in table: print(lang, '\t', table[lang]) # Same as: for lang in table.keys() John Ousterhout Guido van Rossum Larry Wall The last command uses a for loop, which we haven’t covered in detail yet. If you aren’t familiar with for loops, this command simply iterates through each key in the table and prints a tab-separated list of keys and their values. We’ll learn more about for loops in Chapter 13. Dictionaries aren’t sequences like lists and strings, but if you need to step through the items in a dictionary, it’s easy—calling the dictionary keys method returns all stored 212 | Chapter 8: Lists and Dictionaries keys, which you can iterate through with a for. If needed, you can index from key to value inside the for loop, as was done in this code. In fact, Python also lets you step through a dictionary’s keys list without actually calling the keys method in most for loops. For any dictionary D, saying for key in D: works the same as saying the complete for key in D.keys():. This is really just another instance of the iterators mentioned earlier, which allow the in membership operator to work on dictionaries as well (more on iterators later in this book). Dictionary Usage Notes Dictionaries are fairly straightforward tools once you get the hang of them, but here are a few additional pointers and reminders you should be aware of when using them: • Sequence operations don’t work. Dictionaries are mappings, not sequences; because there’s no notion of ordering among their items, things like concatenation (an ordered joining) and slicing (extracting a contiguous section) simply don’t apply. In fact, Python raises an error when your code runs if you try to do such things. • Assigning to new indexes adds entries. Keys can be created when you write a dictionary literal (in which case they are embedded in the literal itself), or when you assign values to new keys of an existing dictionary object. The end result is the same. • Keys need not always be strings. Our examples so far have used strings as keys, but any other immutable objects (i.e., not lists) work just as well. For instance, you can use integers as keys, which makes the dictionary look much like a list (when indexing, at least). Tuples are sometimes used as dictionary keys too, allowing for compound key values. Class instance objects (discussed in Part VI) can also be used as keys, as long as they have the proper protocol methods; roughly, they need to tell Python that their values are hashable and won’t change, as otherwise they would be useless as fixed keys. Using dictionaries to simulate flexible lists The last point in the prior list is important enough to demonstrate with a few examples. When you use lists, it is illegal to assign to an offset that is off the end of the list: >>> L = [] >>> L[99] = 'spam' Traceback (most recent call last): File "<stdin>", line 1, in ? IndexError: list assignment index out of range Although you can use repetition to preallocate as big a list as you’ll need (e.g., [0]*100), you can also do something that looks similar with dictionaries that does not require such space allocations. By using integer keys, dictionaries can emulate lists that seem to grow on offset assignment: Dictionaries in Action | 213 >>> D = {} >>> D[99] = 'spam' >>> D[99] 'spam' >>> D {99: 'spam'} Here, it looks as if D is a 100-item list, but it’s really a dictionary with a single entry; the value of the key 99 is the string 'spam'. You can access this structure with offsets much like a list, but you don’t have to allocate space for all the positions you might ever need to assign values to in the future. When used like this, dictionaries are like more flexible equivalents of lists. Using dictionaries for sparse data structures In a similar way, dictionary keys are also commonly leveraged to implement sparse data structures—for example, multidimensional arrays where only a few positions have values stored in them: >>> Matrix = {} >>> Matrix[(2, 3, 4)] = 88 >>> Matrix[(7, 8, 9)] = 99 >>> >>> X = 2; Y = 3; Z = 4 >>> Matrix[(X, Y, Z)] 88 >>> Matrix {(2, 3, 4): 88, (7, 8, 9): 99} # ; separates statements Here, we’ve used a dictionary to represent a three-dimensional array that is empty except for the two positions (2,3,4) and (7,8,9). The keys are tuples that record the coordinates of nonempty slots. Rather than allocating a large and mostly empty threedimensional matrix to hold these values, we can use a simple two-item dictionary. In this scheme, accessing an empty slot triggers a nonexistent key exception, as these slots are not physically stored: >>> Matrix[(2,3,6)] Traceback (most recent call last): File "<stdin>", line 1, in ? KeyError: (2, 3, 6) Avoiding missing-key errors Errors for nonexistent key fetches are common in sparse matrixes, but you probably won’t want them to shut down your program. There are at least three ways to fill in a default value instead of getting such an error message—you can test for keys ahead of time in if statements, use a try statement to catch and recover from the exception explicitly, or simply use the dictionary get method shown earlier to provide a default for keys that do not exist: >>> if (2,3,6) in Matrix: ... print(Matrix[(2,3,6)]) 214 | Chapter 8: Lists and Dictionaries # Check for key before fetch # See Chapter 12 for if/else ... ... ... 0 >>> ... ... ... ... 0 >>> 88 >>> 0 else: print(0) try: print(Matrix[(2,3,6)]) except KeyError: print(0) # Try to index # Catch and recover # See Chapter 33 for try/except Matrix.get((2,3,4), 0) # Exists; fetch and return Matrix.get((2,3,6), 0) # Doesn't exist; use default arg Of these, the get method is the most concise in terms of coding requirements; we’ll study the if and try statements in more detail later in this book. Using dictionaries as “records” As you can see, dictionaries can play many roles in Python. In general, they can replace search data structures (because indexing by key is a search operation) and can represent many types of structured information. For example, dictionaries are one of many ways to describe the properties of an item in your program’s domain; that is, they can serve the same role as “records” or “structs” in other languages. The following, for example, fills out a dictionary by assigning to new keys over time: >>> >>> >>> >>> >>> >>> mel rec = {} rec['name'] = 'mel' rec['age'] = 45 rec['job'] = 'trainer/writer' print(rec['name']) Especially when nested, Python’s built-in data types allow us to easily represent structured information. This example again uses a dictionary to capture object properties, but it codes it all at once (rather than assigning to each key separately) and nests a list and a dictionary to represent structured property values: >>> mel = {'name': 'Mark', ... 'jobs': ['trainer', 'writer'], ... 'web': 'www.rmi.net/˜lutz', ... 'home': {'state': 'CO', 'zip':80513}} To fetch components of nested objects, simply string together indexing operations: >>> mel['name'] 'Mark' >>> mel['jobs'] ['trainer', 'writer'] >>> mel['jobs'][1] 'writer' Dictionaries in Action | 215 >>> mel['home']['zip'] 80513 Although we’ll learn in Part VI that classes (which group both data and logic) can be better in this record role, dictionaries are an easy-to-use tool for simpler requirements. Why You Will Care: Dictionary Interfaces Dictionaries aren’t just a convenient way to store information by key in your programs—some Python extensions also present interfaces that look like and work the same as dictionaries. For instance, Python’s interface to DBM access-by-key files looks much like a dictionary that must be opened. Strings are stored and fetched using key indexes: import anydbm file = anydbm.open("filename") # Link to file file['key'] = 'data' # Store data by key data = file['key'] # Fetch data by key In Chapter 27, you’ll see that you can store entire Python objects this way, too, if you replace anydbm in the preceding code with shelve (shelves are access-by-key databases of persistent Python objects). For Internet work, Python’s CGI script support also presents a dictionary-like interface. A call to cgi.FieldStorage yields a dictionary-like object with one entry per input field on the client’s web page: import cgi form = cgi.FieldStorage() # Parse form data if 'name' in form: showReply('Hello, ' + form['name'].value) All of these, like dictionaries, are instances of mappings. Once you learn dictionary interfaces, you’ll find that they apply to a variety of built-in tools in Python. Other Ways to Make Dictionaries Finally, note that because dictionaries are so useful, more ways to build them have emerged over time. In Python 2.3 and later, for example, the last two calls to the dict constructor (really, type name) shown here have the same effect as the literal and keyassignment forms above them: {'name': 'mel', 'age': 45} # Traditional literal expression D = {} D['name'] = 'mel' D['age'] = 45 # Assign by keys dynamically dict(name='mel', age=45) # dict keyword argument form dict([('name', 'mel'), ('age', 45)]) # dict key/value tuples form All four of these forms create the same two-key dictionary, but they are useful in differing circumstances: 216 | Chapter 8: Lists and Dictionaries • The first is handy if you can spell out the entire dictionary ahead of time. • The second is of use if you need to create the dictionary one field at a time on the fly. • The third involves less typing than the first, but it requires all keys to be strings. • The last is useful if you need to build up keys and values as sequences at runtime. We met keyword arguments earlier when sorting; the third form illustrated in this code listing has become especially popular in Python code today, since it has less syntax (and hence there is less opportunity for mistakes). As suggested previously in Table 8-2, the last form in the listing is also commonly used in conjunction with the zip function, to combine separate lists of keys and values obtained dynamically at runtime (parsed out of a data file’s columns, for instance). More on this option in the next section. Provided all the key’s values are the same initially, you can also create a dictionary with this special form—simply pass in a list of keys and an initial value for all of the values (the default is None): >>> dict.fromkeys(['a', 'b'], 0) {'a': 0, 'b': 0} Although you could get by with just literals and key assignments at this point in your Python career, you’ll probably find uses for all of these dictionary-creation forms as you start applying them in realistic, flexible, and dynamic Python programs. The listings in this section document the various ways to create dictionaries in both Python 2.6 and 3.0. However, there is yet another way to create dictionaries, available only in Python 3.0 (and later): the dictionary comprehension expression. To see how this last form looks, we need to move on to the next section. Dictionary Changes in Python 3.0 This chapter has so far focused on dictionary basics that span releases, but the dictionary’s functionality has mutated in Python 3.0. If you are using Python 2.X code, you may come across some dictionary tools that either behave differently or are missing altogether in 3.0. Moreover, 3.0 coders have access to additional dictionary tools not available in 2.X. Specifically, dictionaries in 3.0: • Support a new dictionary comprehension expression, a close cousin to list and set comprehensions • Return iterable views instead of lists for the methods D.keys, D.values, and D.items • Require new coding styles for scanning by sorted keys, because of the prior point • No longer support relative magnitude comparisons directly—compare manually instead • No longer have the D.has_key method—the in membership test is used instead Let’s take a look at what’s new in 3.0 dictionaries. Dictionaries in Action | 217 Dictionary comprehensions As mentioned at the end of the prior section, dictionaries in 3.0 can also be created with dictionary comprehensions. Like the set comprehensions we met in Chapter 5, dictionary comprehensions are available only in 3.0 (not in 2.6). Like the longstanding list comprehensions we met briefly in Chapter 4 and earlier in this chapter, they run an implied loop, collecting the key/value results of expressions on each iteration and using them to fill out a new dictionary. A loop variable allows the comprehension to use loop iteration values along the way. For example, a standard way to initialize a dictionary dynamically in both 2.6 and 3.0 is to zip together its keys and values and pass the result to the dict call. As we’ll learn in more detail in Chapter 13, the zip function is a way to construct a dictionary from key and value lists in a single call. If you cannot predict the set of keys and values in your code, you can always build them up as lists and zip them together: >>> list(zip(['a', 'b', 'c'], [1, 2, 3])) [('a', 1), ('b', 2), ('c', 3)] # Zip together keys and values >>> D = dict(zip(['a', 'b', 'c'], [1, 2, 3])) >>> D {'a': 1, 'c': 3, 'b': 2} # Make a dict from zip result In Python 3.0, you can achieve the same effect with a dictionary comprehension expression. The following builds a new dictionary with a key/value pair for every such pair in the zip result (it reads almost the same in Python, but with a bit more formality): C:\misc> c:\python30\python # Use a dict comprehension >>> D = {k: v for (k, v) in zip(['a', 'b', 'c'], [1, 2, 3])} >>> D {'a': 1, 'c': 3, 'b': 2} Comprehensions actually require more code in this case, but they are also more general than this example implies—we can use them to map a single stream of values to dictionaries as well, and keys can be computed with expressions just like values: >>> D = {x: x ** 2 for x in [1, 2, 3, 4]} >>> D {1: 1, 2: 4, 3: 9, 4: 16} # Or: range(1, 5) >>> D = {c: c * 4 for c in 'SPAM'} # Loop over any iterable >>> D {'A': 'AAAA', 'P': 'PPPP', 'S': 'SSSS', 'M': 'MMMM'} >>> D = {c.lower(): c + '!' for c in ['SPAM', 'EGGS', 'HAM']} >>> D {'eggs': 'EGGS!', 'ham': 'HAM!', 'spam': 'SPAM!'} Dictionary comprehensions are also useful for initializing dictionaries from keys lists, in much the same way as the fromkeys method we met at the end of the preceding section: 218 | Chapter 8: Lists and Dictionaries >>> D = dict.fromkeys(['a', 'b', 'c'], 0) >>> D {'a': 0, 'c': 0, 'b': 0} # Initialize dict from keys >>> D = {k:0 for k in ['a', 'b', 'c']} >>> D {'a': 0, 'c': 0, 'b': 0} # Same, but with a comprehension >>> D = dict.fromkeys('spam') >>> D {'a': None, 'p': None, 's': None, 'm': None} # Other iterators, default value >>> D = {k: None for k in 'spam'} >>> D {'a': None, 'p': None, 's': None, 'm': None} Like related tools, dictionary comprehensions support additional syntax not shown here, including nested loops and if clauses. Unfortunately, to truly understand dictionary comprehensions, we need to also know more about iteration statements and concepts in Python, and we don’t yet have enough information to address that story well. We’ll learn much more about all flavors of comprehensions (list, set, and dictionary) in Chapters 14 and 20, so we’ll defer further details until later. We’ll also study the zip built-in we used in this section in more detail in Chapter 13, when we explore for loops. Dictionary views In 3.0 the dictionary keys, values, and items methods all return view objects, whereas in 2.6 they return actual result lists. View objects are iterables, which simply means objects that generate result items one at a time, instead of producing the result list all at once in memory. Besides being iterable, dictionary views also retain the original order of dictionary components, reflect future changes to the dictionary, and may support set operations. On the other hand, they are not lists, and they do not support operations like indexing or the list sort method; nor do they display their items when printed. We’ll discuss the notion of iterables more formally in Chapter 14, but for our purposes here it’s enough to know that we have to run the results of these three methods through the list built-in if we want to apply list operations or display their values: >>> D = dict(a=1, b=2, c=3) >>> D {'a': 1, 'c': 3, 'b': 2} >>> K = D.keys() >>> K <dict_keys object at 0x026D83C0> >>> list(K) ['a', 'c', 'b'] # Makes a view object in 3.0, not a list # Force a real list in 3.0 if needed >>> V = D.values() # Ditto for values and items views >>> V <dict_values object at 0x026D8260> Dictionaries in Action | 219 >>> list(V) [1, 3, 2] >>> list(D.items()) [('a', 1), ('c', 3), ('b', 2)] >>> K[0] # List operations fail unless converted TypeError: 'dict_keys' object does not support indexing >>> list(K)[0] 'a' Apart from when displaying results at the interactive prompt, you will probably rarely even notice this change, because looping constructs in Python automatically force iterable objects to produce one result on each iteration: >>> for k in D.keys(): print(k) ... a c b # Iterators used automatically in loops In addition, 3.0 dictionaries still have iterators themselves, which return successive keys—as in 2.6, it’s still often not necessary to call keys directly: >>> for key in D: print(key) ... a c b # Still no need to call keys() to iterate Unlike 2.X’s list results, though, dictionary views in 3.0 are not carved in stone when created—they dynamically reflect future changes made to the dictionary after the view object has been created: >>> D = {'a':1, 'b':2, 'c':3} >>> D {'a': 1, 'c': 3, 'b': 2} >>> K = D.keys() >>> V = D.values() >>> list(K) ['a', 'c', 'b'] >>> list(V) [1, 3, 2] # Views maintain same order as dictionary >>> del D['b'] >>> D {'a': 1, 'c': 3} # Change the dictionary in-place >>> list(K) ['a', 'c'] >>> list(V) [1, 3] # Reflected in any current view objects 220 | Chapter 8: Lists and Dictionaries # Not true in 2.X! Dictionary views and sets Also unlike 2.X’s list results, 3.0’s view objects returned by the keys method are setlike and support common set operations such as intersection and union; values views are not, since they aren’t unique, but items results are if their (key, value) pairs are unique and hashable. Given that sets behave much like valueless dictionaries (and are even coded in curly braces like dictionaries in 3.0), this is a logical symmetry. Like dictionary keys, set items are unordered, unique, and immutable. Here is what keys lists look like when used in set operations. In set operations, views may be mixed with other views, sets, and dictionaries (dictionaries are treated the same as their keys views in this context): >>> K | {'x': 4} {'a', 'x', 'c'} # Keys (and some items) views are set-like >>> V & {'x': 4} TypeError: unsupported operand type(s) for &: 'dict_values' and 'dict' >>> V & {'x': 4}.values() TypeError: unsupported operand type(s) for &: 'dict_values' and 'dict_values' >>> D = {'a':1, 'b':2, 'c':3} >>> D.keys() & D.keys() {'a', 'c', 'b'} >>> D.keys() & {'b'} {'b'} >>> D.keys() & {'b': 1} {'b'} >>> D.keys() | {'b', 'c', 'd'} {'a', 'c', 'b', 'd'} # Intersect keys views # Intersect keys and set # Intersect keys and dict # Union keys and set Dictionary items views are set-like too if they are hashable—that is, if they contain only immutable objects: >>> D = {'a': 1} >>> list(D.items()) [('a', 1)] >>> D.items() | D.keys() {('a', 1), 'a'} >>> D.items() | D {('a', 1), 'a'} # Items set-like if hashable # Union view and view # dict treated same as its keys >>> D.items() | {('c', 3), ('d', 4)} {('a', 1), ('d', 4), ('c', 3)} >>> dict(D.items() | {('c', 3), ('d', 4)}) {'a': 1, 'c': 3, 'd': 4} # Set of key/value pairs # dict accepts iterable sets too For more details on set operations in general, see Chapter 5. Now, let’s look at three other quick coding notes for 3.0 dictionaries. Dictionaries in Action | 221 Sorting dictionary keys First of all, because keys does not return a list, the traditional coding pattern for scanning a dictionary by sorted keys in 2.X won’t work in 3.0. You must either convert to a list manually or use the sorted call introduced in Chapter 4 and earlier in this chapter on either a keys view or the dictionary itself: >>> D = {'a':1, 'b':2, 'c':3} >>> D {'a': 1, 'c': 3, 'b': 2} >>> Ks = D.keys() # Sorting a view object doesn't work! >>> Ks.sort() AttributeError: 'dict_keys' object has no attribute 'sort' >>> Ks = list(Ks) >>> Ks.sort() >>> for k in Ks: print(k, D[k]) ... a 1 b 2 c 3 >>> D {'a': 1, 'c': 3, 'b': 2} >>> Ks = D.keys() >>> for k in sorted(Ks): print(k, D[k]) ... a 1 b 2 c 3 >>> D {'a': 1, 'c': 3, 'b': 2} >>> for k in sorted(D): print(k, D[k]) ... a 1 b 2 c 3 # Force it to be a list and then sort # Or you can use sorted() on the keys # sorted() accepts any iterable # sorted() returns its result # Better yet, sort the dict directly # dict iterators return keys Dictionary magnitude comparisons no longer work Secondly, while in Python 2.6 dictionaries may be compared for relative magnitude directly with <, >, and so on, in Python 3.0 this no longer works. However, it can be simulated by comparing sorted keys lists manually: sorted(D1.items()) < sorted(D2.items()) # Like 2.6 D1 < D2 Dictionary equality tests still work in 3.0, though. Since we’ll revisit this in the next chapter in the context of comparisons at large, we’ll defer further details here. 222 | Chapter 8: Lists and Dictionaries The has_key method is dead: long live in! Finally, the widely used dictionary has_key key presence test method is gone in 3.0. Instead, use the in membership expression, or a get with a default test (of these, in is generally preferred): >>> D {'a': 1, 'c': 3, 'b': 2} >>> D.has_key('c') # 2.X only: True/False AttributeError: 'dict' object has no attribute 'has_key' >>> 'c' in D True >>> 'x' in D False >>> if 'c' in D: print('present', D['c']) ... present 3 >>> print(D.get('c')) 3 >>> print(D.get('x')) None >>> if D.get('c') != None: print('present', D['c']) ... present 3 # Preferred in 3.0 # Another option If you work in 2.6 and care about 3.0 compatibility, note that the first two changes (comprehensions and views) can only be coded in 3.0, but the last three (sorted, manual comparisons, and in) can be coded in 2.6 today to ease 3.0 migration in the future. Chapter Summary In this chapter, we explored the list and dictionary types—probably the two most common, flexible, and powerful collection types you will see and use in Python code. We learned that the list type supports positionally ordered collections of arbitrary objects, and that it may be freely nested and grown and shrunk on demand. The dictionary type is similar, but it stores items by key instead of by position and does not maintain any reliable left-to-right order among its items. Both lists and dictionaries are mutable, and so support a variety of in-place change operations not available for strings: for example, lists can be grown by append calls, and dictionaries by assignment to new keys. In the next chapter, we will wrap up our in-depth core object type tour by looking at tuples and files. After that, we’ll move on to statements that code the logic that processes our objects, taking us another step toward writing complete programs. Before we tackle those topics, though, here are some chapter quiz questions to review. Chapter Summary | 223 Test Your Knowledge: Quiz 1. Name two ways to build a list containing five integer zeros. 2. Name two ways to build a dictionary with two keys, 'a' and 'b', each having an associated value of 0. 3. Name four operations that change a list object in-place. 4. Name four operations that change a dictionary object in-place. Test Your Knowledge: Answers 1. A literal expression like [0, 0, 0, 0, 0] and a repetition expression like [0] * 5 will each create a list of five zeros. In practice, you might also build one up with a loop that starts with an empty list and appends 0 to it in each iteration: L.append(0). A list comprehension ([0 for i in range(5)]) could work here, too, but this is more work than you need to do. 2. A literal expression such as {'a': 0, 'b': 0} or a series of assignments like D = {}, D['a'] = 0, and D['b'] = 0 would create the desired dictionary. You can also use the newer and simpler-to-code dict(a=0, b=0) keyword form, or the more flexible dict([('a', 0), ('b', 0)]) key/value sequences form. Or, because all the values are the same, you can use the special form dict.fromkeys('ab', 0). In 3.0, you can also use a dictionary comprehension: {k:0 for k in 'ab'}. 3. The append and extend methods grow a list in-place, the sort and reverse methods order and reverse lists, the insert method inserts an item at an offset, the remove and pop methods delete from a list by value and by position, the del statement deletes an item or slice, and index and slice assignment statements replace an item or entire section. Pick any four of these for the quiz. 4. Dictionaries are primarily changed by assignment to a new or existing key, which creates or changes the key’s entry in the table. Also, the del statement deletes a key’s entry, the dictionary update method merges one dictionary into another inplace, and D.pop(key) removes a key and returns the value it had. Dictionaries also have other, more exotic in-place change methods not listed in this chapter, such as setdefault; see reference sources for more details. 224 | Chapter 8: Lists and Dictionaries CHAPTER 9 Tuples, Files, and Everything Else This chapter rounds out our in-depth look at the core object types in Python by exploring the tuple, a collection of other objects that cannot be changed, and the file, an interface to external files on your computer. As you’ll see, the tuple is a relatively simple object that largely performs operations you’ve already learned about for strings and lists. The file object is a commonly used and full-featured tool for processing files; the basic overview of files here is supplemented by larger examples in later chapters. This chapter also concludes this part of the book by looking at properties common to all the core object types we’ve met—the notions of equality, comparisons, object copies, and so on. We’ll also briefly explore other object types in the Python toolbox; as you’ll see, although we’ve covered all the primary built-in types, the object story in Python is broader than I’ve implied thus far. Finally, we’ll close this part of the book by taking a look at a set of common object type pitfalls and exploring some exercises that will allow you to experiment with the ideas you’ve learned. Tuples The last collection type in our survey is the Python tuple. Tuples construct simple groups of objects. They work exactly like lists, except that tuples can’t be changed inplace (they’re immutable) and are usually written as a series of items in parentheses, not square brackets. Although they don’t support as many methods, tuples share most of their properties with lists. Here’s a quick look at the basics. Tuples are: Ordered collections of arbitrary objects Like strings and lists, tuples are positionally ordered collections of objects (i.e., they maintain a left-to-right order among their contents); like lists, they can embed any kind of object. Accessed by offset Like strings and lists, items in a tuple are accessed by offset (not by key); they support all the offset-based access operations, such as indexing and slicing. 225 Of the category “immutable sequence” Like strings and lists, tuples are sequences; they support many of the same operations. However, like strings, tuples are immutable; they don’t support any of the in-place change operations applied to lists. Fixed-length, heterogeneous, and arbitrarily nestable Because tuples are immutable, you cannot change the size of a tuple without making a copy. On the other hand, tuples can hold any type of object, including other compound objects (e.g., lists, dictionaries, other tuples), and so support arbitrary nesting. Arrays of object references Like lists, tuples are best thought of as object reference arrays; tuples store access points to other objects (references), and indexing a tuple is relatively quick. Table 9-1 highlights common tuple operations. A tuple is written as a series of objects (technically, expressions that generate objects), separated by commas and normally enclosed in parentheses. An empty tuple is just a parentheses pair with nothing inside. Table 9-1. Common tuple literals and operations Operation Interpretation () An empty tuple T = (0,) A one-item tuple (not an expression) T = (0, 'Ni', 1.2, 3) A four-item tuple T = 0, 'Ni', 1.2, 3 Another four-item tuple (same as prior line) T = ('abc', ('def', 'ghi')) Nested tuples T = tuple('spam') Tuple of items in an iterable T[i] Index, index of index, slice, length T[i][j] T[i:j] len(T) T1 + T2 Concatenate, repeat T * 3 for x in T: print(x) Iteration, membership 'spam' in T [x ** 2 for x in T] T.index('Ni') Methods in 2.6 and 3.0: search, count T.count('Ni') 226 | Chapter 9: Tuples, Files, and Everything Else Tuples in Action As usual, let’s start an interactive session to explore tuples at work. Notice in Table 9-1 that tuples do not have all the methods that lists have (e.g., an append call won’t work here). They do, however, support the usual sequence operations that we saw for both strings and lists: >>> (1, 2) + (3, 4) (1, 2, 3, 4) # Concatenation >>> (1, 2) * 4 (1, 2, 1, 2, 1, 2, 1, 2) # Repetition >>> T = (1, 2, 3, 4) >>> T[0], T[1:3] (1, (2, 3)) # Indexing, slicing Tuple syntax peculiarities: Commas and parentheses The second and fourth entries in Table 9-1 merit a bit more explanation. Because parentheses can also enclose expressions (see Chapter 5), you need to do something special to tell Python when a single object in parentheses is a tuple object and not a simple expression. If you really want a single-item tuple, simply add a trailing comma after the single item, before the closing parenthesis: >>> x = (40) >>> x 40 >>> y = (40,) >>> y (40,) # An integer! # A tuple containing an integer As a special case, Python also allows you to omit the opening and closing parentheses for a tuple in contexts where it isn’t syntactically ambiguous to do so. For instance, the fourth line of Table 9-1 simply lists four items separated by commas. In the context of an assignment statement, Python recognizes this as a tuple, even though it doesn’t have parentheses. Now, some people will tell you to always use parentheses in your tuples, and some will tell you to never use parentheses in tuples (and still others have lives, and won’t tell you what to do with your tuples!). The only significant places where the parentheses are required are when a tuple is passed as a literal in a function call (where parentheses matter), and when one is listed in a Python 2.X print statement (where commas are significant). For beginners, the best advice is that it’s probably easier to use the parentheses than it is to figure out when they are optional. Many programmers (myself included) also find that parentheses tend to aid script readability by making the tuples more explicit, but your mileage may vary. Tuples | 227 Conversions, methods, and immutability Apart from literal syntax differences, tuple operations (the middle rows in Table 9-1) are identical to string and list operations. The only differences worth noting are that the +, *, and slicing operations return new tuples when applied to tuples, and that tuples don’t provide the same methods you saw for strings, lists, and dictionaries. If you want to sort a tuple, for example, you’ll usually have to either first convert it to a list to gain access to a sorting method call and make it a mutable object, or use the newer sorted built-in that accepts any sequence object (and more): >>> T = ('cc', 'aa', 'dd', 'bb') >>> tmp = list(T) >>> tmp.sort() >>> tmp ['aa', 'bb', 'cc', 'dd'] >>> T = tuple(tmp) >>> T ('aa', 'bb', 'cc', 'dd') >>> sorted(T) ['aa', 'bb', 'cc', 'dd'] # Make a list from a tuple's items # Sort the list # Make a tuple from the list's items # Or use the sorted built-in Here, the list and tuple built-in functions are used to convert the object to a list and then back to a tuple; really, both calls make new objects, but the net effect is like a conversion. List comprehensions can also be used to convert tuples. The following, for example, makes a list from a tuple, adding 20 to each item along the way: >>> T = (1, 2, 3, 4, 5) >>> L = [x + 20 for x in T] >>> L [21, 22, 23, 24, 25] List comprehensions are really sequence operations—they always build new lists, but they may be used to iterate over any sequence objects, including tuples, strings, and other lists. As we’ll see later in the book, they even work on some things that are not physically stored sequences—any iterable objects will do, including files, which are automatically read line by line. Although tuples don’t have the same methods as lists and strings, they do have two of their own as of Python 2.6 and 3.0—index and count works as they do for lists, but they are defined for tuple objects: >>> >>> 1 >>> 3 >>> 3 T = (1, 2, 3, 2, 4, 2) T.index(2) # Tuple methods in 2.6 and 3.0 # Offset of first appearance of 2 T.index(2, 2) # Offset of appearance after offset 2 T.count(2) # How many 2s are there? 228 | Chapter 9: Tuples, Files, and Everything Else Prior to 2.6 and 3.0, tuples have no methods at all—this was an old Python convention for immutable types, which was violated years ago on grounds of practicality with strings, and more recently with both numbers and tuples. Also, note that the rule about tuple immutability applies only to the top level of the tuple itself, not to its contents. A list inside a tuple, for instance, can be changed as usual: >>> T = (1, [2, 3], 4) >>> T[1] = 'spam' # This fails: can't change tuple itself TypeError: object doesn't support item assignment >>> T[1][0] = 'spam' >>> T (1, ['spam', 3], 4) # This works: can change mutables inside For most programs, this one-level-deep immutability is sufficient for common tuple roles. Which, coincidentally, brings us to the next section. Why Lists and Tuples? This seems to be the first question that always comes up when teaching beginners about tuples: why do we need tuples if we have lists? Some of the reasoning may be historic; Python’s creator is a mathematician by training, and he has been quoted as seeing a tuple as a simple association of objects and a list as a data structure that changes over time. In fact, this use of the word “tuple” derives from mathematics, as does its frequent use for a row in a relational database table. The best answer, however, seems to be that the immutability of tuples provides some integrity—you can be sure a tuple won’t be changed through another reference elsewhere in a program, but there’s no such guarantee for lists. Tuples, therefore, serve a similar role to “constant” declarations in other languages, though the notion of constantness is associated with objects in Python, not variables. Tuples can also be used in places that lists cannot—for example, as dictionary keys (see the sparse matrix example in Chapter 8). Some built-in operations may also require or imply tuples, not lists, though such operations have often been generalized in recent years. As a rule of thumb, lists are the tool of choice for ordered collections that might need to change; tuples can handle the other cases of fixed associations. Files You may already be familiar with the notion of files, which are named storage compartments on your computer that are managed by your operating system. The last major built-in object type that we’ll examine on our object types tour provides a way to access those files inside Python programs. Files | 229 In short, the built-in open function creates a Python file object, which serves as a link to a file residing on your machine. After calling open, you can transfer strings of data to and from the associated external file by calling the returned file object’s methods. Compared to the types you’ve seen so far, file objects are somewhat unusual. They’re not numbers, sequences, or mappings, and they don’t respond to expression operators; they export only methods for common file-processing tasks. Most file methods are concerned with performing input from and output to the external file associated with a file object, but other file methods allow us to seek to a new position in the file, flush output buffers, and so on. Table 9-2 summarizes common file operations. Table 9-2. Common file operations Operation Interpretation output = open(r'C:\spam', 'w') Create output file ('w' means write) input = open('data', 'r') Create input file ('r' means read) input = open('data') Same as prior line ('r' is the default) aString = input.read() Read entire file into a single string aString = input.read(N) Read up to next N characters (or bytes) into a string aString = input.readline() Read next line (including \n newline) into a string aList = input.readlines() Read entire file into list of line strings (with \n) output.write(aString) Write a string of characters (or bytes) into file output.writelines(aList) Write all line strings in a list into file output.close() Manual close (done for you when file is collected) output.flush() Flush output buffer to disk without closing anyFile.seek(N) Change file position to offset N for next operation for line in open('data'): use line File iterators read line by line open('f.txt', encoding='latin-1') Python 3.0 Unicode text files (str strings) open('f.bin', 'rb') Python 3.0 binary bytes files (bytes strings) Opening Files To open a file, a program calls the built-in open function, with the external filename first, followed by a processing mode. The mode is typically the string 'r' to open for text input (the default), 'w' to create and open for text output, or 'a' to open for appending text to the end. The processing mode argument can specify additional options: • Adding a b to the mode string allows for binary data (end-of-line translations and 3.0 Unicode encodings are turned off). 230 | Chapter 9: Tuples, Files, and Everything Else • Adding a + opens the file for both input and output (i.e., you can both read and write to the same file object, often in conjunction with seek operations to reposition in the file). Both arguments to open must be Python strings, and an optional third argument can be used to control output buffering—passing a zero means that output is unbuffered (it is transferred to the external file immediately on a write method call). The external filename argument may include a platform-specific and absolute or relative directory path prefix; without a directory path, the file is assumed to exist in the current working directory (i.e., where the script runs). We’ll cover file fundamentals and explore some basic examples here, but we won’t go into all file-processing mode options; as usual, consult the Python library manual for additional details. Using Files Once you make a file object with open, you can call its methods to read from or write to the associated external file. In all cases, file text takes the form of strings in Python programs; reading a file returns its text in strings, and text is passed to the write methods as strings. Reading and writing methods come in multiple flavors; Table 9-2 lists the most common. Here are a few fundamental usage notes: File iterators are best for reading lines Though the reading and writing methods in the table are common, keep in mind that probably the best way to read lines from a text file today is to not read the file at all—as we’ll see in Chapter 14, files also have an iterator that automatically reads one line at a time in a for loop, list comprehension, or other iteration context. Content is strings, not objects Notice in Table 9-2 that data read from a file always comes back to your script as a string, so you’ll have to convert it to a different type of Python object if a string is not what you need. Similarly, unlike with the print operation, Python does not add any formatting and does not convert objects to strings automatically when you write data to a file—you must send an already formatted string. Because of this, the tools we have already met to convert objects to and from strings (e.g., int, float, str, and the string formatting expression and method) come in handy when dealing with files. Python also includes advanced standard library tools for handling generic object storage (such as the pickle module) and for dealing with packed binary data in files (such as the struct module). We’ll see both of these at work later in this chapter. close is usually optional Calling the file close method terminates your connection to the external file. As discussed in Chapter 6, in Python an object’s memory space is automatically reclaimed as soon as the object is no longer referenced anywhere in the program. When file objects are reclaimed, Python also automatically closes the files if they are still open (this also happens when a program shuts down). This means you Files | 231 don’t always need to manually close your files, especially in simple scripts that don’t run for long. On the other hand, including manual close calls can’t hurt and is usually a good idea in larger systems. Also, strictly speaking, this auto-close-oncollection feature of files is not part of the language definition, and it may change over time. Consequently, manually issuing file close method calls is a good habit to form. (For an alternative way to guarantee automatic file closes, also see this section’s later discussion of the file object’s context manager, used with the new with/as statement in Python 2.6 and 3.0.) Files are buffered and seekable. The prior paragraph’s notes about closing files are important, because closing both frees up operating system resources and flushes output buffers. By default, output files are always buffered, which means that text you write may not be transferred from memory to disk immediately—closing a file, or running its flush method, forces the buffered data to disk. You can avoid buffering with extra open arguments, but it may impede performance. Python files are also random-access on a byte offset basis—their seek method allows your scripts to jump around to read and write at specific locations. Files in Action Let’s work through a simple example that demonstrates file-processing basics. The following code begins by opening a new text file for output, writing two lines (strings terminated with a newline marker, \n), and closing the file. Later, the example opens the same file again in input mode and reads the lines back one at a time with readline. Notice that the third readline call returns an empty string; this is how Python file methods tell you that you’ve reached the end of the file (empty lines in the file come back as strings containing just a newline character, not as empty strings). Here’s the complete interaction: >>> >>> 16 >>> 18 >>> myfile = open('myfile.txt', 'w') myfile.write('hello text file\n') # Open for text output: create/empty # Write a line of text: string myfile.write('goodbye text file\n') myfile.close() >>> myfile = open('myfile.txt') >>> myfile.readline() 'hello text file\n' >>> myfile.readline() 'goodbye text file\n' >>> myfile.readline() '' # Flush output buffers to disk # Open for text input: 'r' is default # Read the lines back # Empty string: end of file Notice that file write calls return the number of characters written in Python 3.0; in 2.6 they don’t, so you won’t see these numbers echoed interactively. This example writes each line of text, including its end-of-line terminator, \n, as a string; write 232 | Chapter 9: Tuples, Files, and Everything Else methods don’t add the end-of-line character for us, so we must include it to properly terminate our lines (otherwise the next write will simply extend the current line in the file). If you want to display the file’s content with end-of-line characters interpreted, read the entire file into a string all at once with the file object’s read method and print it: >>> open('myfile.txt').read() 'hello text file\ngoodbye text file\n' # Read all at once into string >>> print(open('myfile.txt').read()) hello text file goodbye text file # User-friendly display And if you want to scan a text file line by line, file iterators are often your best option: >>> for line in open('myfile'): ... print(line, end='') ... hello text file goodbye text file # Use file iterators, not reads When coded this way, the temporary file object created by open will automatically read and return one line on each loop iteration. This form is usually easiest to code, good on memory use, and may be faster than some other options (depending on many variables, of course). Since we haven’t reached statements or iterators yet, though, you’ll have to wait until Chapter 14 for a more complete explanation of this code. Text and binary files in Python 3.0 Strictly speaking, the example in the prior section uses text files. In both Python 3.0 and 2.6, file type is determined by the second argument to open, the mode string—an included “b” means binary. Python has always supported both text and binary files, but in Python 3.0 there is a sharper distinction between the two: • Text files represent content as normal str strings, perform Unicode encoding and decoding automatically, and perform end-of-line translation by default. • Binary files represent content as a special bytes string type and allow programs to access file content unaltered. In contrast, Python 2.6 text files handle both 8-bit text and binary data, and a special string type and file interface (unicode strings and codecs.open) handles Unicode text. The differences in Python 3.0 stem from the fact that simple and Unicode text have been merged in the normal string type—which makes sense, given that all text is Unicode, including ASCII and other 8-bit encodings. Because most programmers deal only with ASCII text, they can get by with the basic text file interface used in the prior example, and normal strings. All strings are technically Unicode in 3.0, but ASCII users will not generally notice. In fact, files and strings work the same in 3.0 and 2.6 if your script’s scope is limited to such simple forms of text. Files | 233 If you need to handle internationalized applications or byte-oriented data, though, the distinction in 3.0 impacts your code (usually for the better). In general, you must use bytes strings for binary files, and normal str strings for text files. Moreover, because text files implement Unicode encodings, you cannot open a binary data file in text mode—decoding its content to Unicode text will likely fail. Let’s look at an example. When you read a binary data file you get back a bytes object— a sequence of small integers that represent absolute byte values (which may or may not correspond to characters), which looks and feels almost exactly like a normal string: >>> data = open('data.bin', 'rb').read() >>> data b'\x00\x00\x00\x07spam\x00\x08' >>> data[4:8] b'spam' >>> data[0] 115 >>> bin(data[0]) '0b1110011' # Open binary file: rb=read binary # bytes string holds binary data # Act like strings # But really are small 8-bit integers # Python 3.0 bin() function In addition, binary files do not perform any end-of-line translation on data; text files by default map all forms to and from \n when written and read and implement Unicode encodings on transfers. Since Unicode and binary data is of marginal interest to many Python programmers, we’ll postpone the full story until Chapter 36. For now, let’s move on to some more substantial file examples. Storing and parsing Python objects in files Our next example writes a variety of Python objects into a text file on multiple lines. Notice that it must convert objects to strings using conversion tools. Again, file data is always strings in our scripts, and write methods do not do any automatic to-string formatting for us (for space, I’m omitting byte-count return values from write methods from here on): >>> >>> >>> >>> >>> >>> >>> >>> >>> >>> X, Y, Z = 43, 44, 45 S = 'Spam' D = {'a': 1, 'b': 2} L = [1, 2, 3] # Native Python objects # Must be strings to store in file F = open('datafile.txt', 'w') F.write(S + '\n') F.write('%s,%s,%s\n' % (X, Y, Z)) F.write(str(L) + '$' + str(D) + '\n') F.close() # Create output file # Terminate lines with \n # Convert numbers to strings # Convert and separate with $ Once we have created our file, we can inspect its contents by opening it and reading it into a string (a single operation). Notice that the interactive echo gives the exact byte contents, while the print operation interprets embedded end-of-line characters to render a more user-friendly display: >>> chars = open('datafile.txt').read() >>> chars 234 | Chapter 9: Tuples, Files, and Everything Else # Raw string display "Spam\n43,44,45\n[1, 2, 3]${'a': 1, 'b': 2}\n" >>> print(chars) # User-friendly display Spam 43,44,45 [1, 2, 3]${'a': 1, 'b': 2} We now have to use other conversion tools to translate from the strings in the text file to real Python objects. As Python never converts strings to numbers (or other types of objects) automatically, this is required if we need to gain access to normal object tools like indexing, addition, and so on: >>> F = open('datafile.txt') >>> line = F.readline() >>> line 'Spam\n' >>> line.rstrip() 'Spam' # Open again # Read one line # Remove end-of-line For this first line, we used the string rstrip method to get rid of the trailing end-of-line character; a line[:−1] slice would work, too, but only if we can be sure all lines end in the \n character (the last line in a file sometimes does not). So far, we’ve read the line containing the string. Now let’s grab the next line, which contains numbers, and parse out (that is, extract) the objects on that line: >>> line = F.readline() >>> line '43,44,45\n' >>> parts = line.split(',') >>> parts ['43', '44', '45\n'] # Next line from file # It's a string here # Split (parse) on commas We used the string split method here to chop up the line on its comma delimiters; the result is a list of substrings containing the individual numbers. We still must convert from strings to integers, though, if we wish to perform math on these: >>> int(parts[1]) 44 >>> numbers = [int(P) for P in parts] >>> numbers [43, 44, 45] # Convert from string to int # Convert all in list at once As we have learned, int translates a string of digits into an integer object, and the list comprehension expression introduced in Chapter 4 can apply the call to each item in our list all at once (you’ll find more on list comprehensions later in this book). Notice that we didn’t have to run rstrip to delete the \n at the end of the last part; int and some other converters quietly ignore whitespace around digits. Finally, to convert the stored list and dictionary in the third line of the file, we can run them through eval, a built-in function that treats a string as a piece of executable program code (technically, a string containing a Python expression): >>> line = F.readline() >>> line Files | 235 "[1, 2, 3]${'a': 1, 'b': 2}\n" >>> parts = line.split('$') >>> parts ['[1, 2, 3]', "{'a': 1, 'b': 2}\n"] >>> eval(parts[0]) [1, 2, 3] >>> objects = [eval(P) for P in parts] >>> objects [[1, 2, 3], {'a': 1, 'b': 2}] # Split (parse) on $ # Convert to any object type # Do same for all in list Because the end result of all this parsing and converting is a list of normal Python objects instead of strings, we can now apply list and dictionary operations to them in our script. Storing native Python objects with pickle Using eval to convert from strings to objects, as demonstrated in the preceding code, is a powerful tool. In fact, sometimes it’s too powerful. eval will happily run any Python expression—even one that might delete all the files on your computer, given the necessary permissions! If you really want to store native Python objects, but you can’t trust the source of the data in the file, Python’s standard library pickle module is ideal. The pickle module is an advanced tool that allows us to store almost any Python object in a file directly, with no to- or from-string conversion requirement on our part. It’s like a super-general data formatting and parsing utility. To store a dictionary in a file, for instance, we pickle it directly: >>> >>> >>> >>> >>> D = {'a': 1, 'b': 2} F = open('datafile.pkl', 'wb') import pickle pickle.dump(D, F) F.close() # Pickle any object to file Then, to get the dictionary back later, we simply use pickle again to re-create it: >>> F = open('datafile.pkl', 'rb') >>> E = pickle.load(F) >>> E {'a': 1, 'b': 2} # Load any object from file We get back an equivalent dictionary object, with no manual splitting or converting required. The pickle module performs what is known as object serialization—converting objects to and from strings of bytes—but requires very little work on our part. In fact, pickle internally translates our dictionary to a string form, though it’s not much to look at (and may vary if we pickle in other data protocol modes): >>> open('datafile.pkl', 'rb').read() # Format is prone to change! b'\x80\x03}q\x00(X\x01\x00\x00\x00aq\x01K\x01X\x01\x00\x00\x00bq\x02K\x02u.' Because pickle can reconstruct the object from this format, we don’t have to deal with that ourselves. For more on the pickle module, see the Python standard library manual, or import pickle and pass it to help interactively. While you’re exploring, also take a look at the shelve module. shelve is a tool that uses pickle to store Python objects in an access-by-key filesystem, which is beyond our scope here (though you will get to see 236 | Chapter 9: Tuples, Files, and Everything Else an example of shelve in action in Chapter 27, and other pickle examples in Chapters 30 and 36). Note that I opened the file used to store the pickled object in binary mode; binary mode is always required in Python 3.0, because the pickler creates and uses a bytes string object, and these objects imply binarymode files (text-mode files imply str strings in 3.0). In earlier Pythons it’s OK to use text-mode files for protocol 0 (the default, which creates ASCII text), as long as text mode is used consistently; higher protocols require binary-mode files. Python 3.0’s default protocol is 3 (binary), but it creates bytes even for protocol 0. See Chapter 36, Python’s library manual, or reference books for more details on this. Python 2.6 also has a cPickle module, which is an optimized version of pickle that can be imported directly for speed. Python 3.0 renames this module _pickle and uses it automatically in pickle—scripts simply import pickle and let Python optimize itself. Storing and parsing packed binary data in files One other file-related note before we move on: some advanced applications also need to deal with packed binary data, created perhaps by a C language program. Python’s standard library includes a tool to help in this domain—the struct module knows how to both compose and parse packed binary data. In a sense, this is another dataconversion tool that interprets strings in files as binary data. To create a packed binary data file, for example, open it in 'wb' (write binary) mode, and pass struct a format string and some Python objects. The format string used here means pack as a 4-byte integer, a 4-character string, and a 2-byte integer, all in bigendian form (other format codes handle padding bytes, floating-point numbers, and more): >>> F = open('data.bin', 'wb') >>> import struct >>> data = struct.pack('>i4sh', 7, 'spam', 8) >>> data b'\x00\x00\x00\x07spam\x00\x08' >>> F.write(data) >>> F.close() # Open binary output file # Make packed binary data # Write byte string Python creates a binary bytes data string, which we write out to the file normally—this one consists mostly of nonprintable characters printed in hexadecimal escapes, and is the same binary file we met earlier. To parse the values out to normal Python objects, we simply read the string back and unpack it using the same format string. Python extracts the values into normal Python objects—integers and a string: >>> F = open('data.bin', 'rb') >>> data = F.read() >>> data b'\x00\x00\x00\x07spam\x00\x08' # Get packed binary data Files | 237 >>> values = struct.unpack('>i4sh', data) >>> values (7, 'spam', 8) # Convert to Python objects Binary data files are advanced and somewhat low-level tools that we won’t cover in more detail here; for more help, see Chapter 36, consult the Python library manual, or import struct and pass it to the help function interactively. Also note that the binary file-processing modes 'wb' and 'rb' can be used to process a simpler binary file such as an image or audio file as a whole without having to unpack its contents. File context managers You’ll also want to watch for Chapter 33’s discussion of the file’s context manager support, new in Python 3.0 and 2.6. Though more a feature of exception processing than files themselves, it allows us to wrap file-processing code in a logic layer that ensures that the file will be closed automatically on exit, instead of relying on the autoclose on garbage collection: with open(r'C:\misc\data.txt') as myfile: for line in myfile: ...use line here... # See Chapter 33 for details The try/finally statement we’ll look at in Chapter 33 can provide similar functionality, but at some cost in extra code—three extra lines, to be precise (though we can often avoid both options and let Python close files for us automatically): myfile = open(r'C:\misc\data.txt') try: for line in myfile: ...use line here... finally: myfile.close() Since both these options require more information than we have yet obtained, we’ll postpone details until later in this book. Other File Tools There are additional, more advanced file methods shown in Table 9-2, and even more that are not in the table. For instance, as mentioned earlier, seek resets your current position in a file (the next read or write happens at that position), flush forces buffered output to be written out to disk (by default, files are always buffered), and so on. The Python standard library manual and the reference books described in the Preface provide complete lists of file methods; for a quick look, run a dir or help call interactively, passing in an open file object (in Python 2.6 but not 3.0, you can pass in the name file instead). For more file-processing examples, watch for the sidebar “Why You Will Care: File Scanners” on page 340. It sketches common file-scanning loop code patterns with statements we have not covered enough yet to use here. 238 | Chapter 9: Tuples, Files, and Everything Else Also, note that although the open function and the file objects it returns are your main interface to external files in a Python script, there are additional file-like tools in the Python toolset. Also available, to name a few, are: Standard streams Preopened file objects in the sys module, such as sys.stdout (see “Print Operations” on page 297) Descriptor files in the os module Integer file handles that support lower-level tools such as file locking Sockets, pipes, and FIFOs File-like objects used to synchronize processes or communicate over networks Access-by-key files known as “shelves” Used to store unaltered Python objects directly, by key (used in Chapter 27) Shell command streams Tools such as os.popen and subprocess.Popen that support spawning shell commands and reading and writing to their standard streams The third-party open source domain offers even more file-like tools, including support for communicating with serial ports in the PySerial extension and interactive programs in the pexpect system. See more advanced Python texts and the Web at large for additional information on file-like tools. Version skew note: In Python 2.5 and earlier, the built-in name open is essentially a synonym for the name file, and files may technically be opened by calling either open or file (though open is generally preferred for opening). In Python 3.0, the name file is no longer available, because of its redundancy with open. Python 2.6 users may also use the name file as the file object type, in order to customize files with object-oriented programming (described later in this book). In Python 3.0, files have changed radically. The classes used to implement file objects live in the standard library module io. See this module’s documentation or code for the classes it makes available for customization, and run a type(F) call on open files F for hints. Type Categories Revisited Now that we’ve seen all of Python’s core built-in types in action, let’s wrap up our object types tour by reviewing some of the properties they share. Table 9-3 classifies all the major types we’ve seen so far according to the type categories introduced earlier. Here are some points to remember: Type Categories Revisited | 239 • Objects share operations according to their category; for instance, strings, lists, and tuples all share sequence operations such as concatenation, length, and indexing. • Only mutable objects (lists, dictionaries, and sets) may be changed in-place; you cannot change numbers, strings, or tuples in-place. • Files export only methods, so mutability doesn’t really apply to them—their state may be changed when they are processed, but this isn’t quite the same as Python core type mutability constraints. • “Numbers” in Table 9-3 includes all number types: integer (and the distinct long integer in 2.6), floating-point, complex, decimal, and fraction. • “Strings” in Table 9-3 includes str, as well as bytes in 3.0 and unicode in 2.6; the bytearray string type in 3.0 is mutable. • Sets are something like the keys of a valueless dictionary, but they don’t map to values and are not ordered, so sets are neither a mapping nor a sequence type; frozenset is an immutable variant of set. • In addition to type category operations, as of Python 2.6 and 3.0 all the types in Table 9-3 have callable methods, which are generally specific to their type. Table 9-3. Object classifications Object type Category Mutable? Numbers (all) Numeric No Strings Sequence No Lists Sequence Yes Dictionaries Mapping Yes Tuples Sequence No Files Extension N/A Sets Set Yes frozenset Set No bytearray (3.0) Sequence Yes Why You Will Care: Operator Overloading In Part VI of this book, we’ll see that objects we implement with classes can pick and choose from these categories arbitrarily. For instance, if we want to provide a new kind of specialized sequence object that is consistent with built-in sequences, we can code a class that overloads things like indexing and concatenation: class MySequence: def __getitem__(self, index): # Called on self[index], others def __add__(self, other): # Called on self + other 240 | Chapter 9: Tuples, Files, and Everything Else and so on. We can also make the new object mutable or not by selectively implementing methods called for in-place change operations (e.g., __setitem__ is called on self[index]=value assignments). Although it’s beyond this book’s scope, it’s also possible to implement new objects in an external language like C as C extension types. For these, we fill in C function pointer slots to choose between number, sequence, and mapping operation sets. Object Flexibility This part of the book introduced a number of compound object types (collections with components). In general: • Lists, dictionaries, and tuples can hold any kind of object. • Lists, dictionaries, and tuples can be arbitrarily nested. • Lists and dictionaries can dynamically grow and shrink. Because they support arbitrary structures, Python’s compound object types are good at representing complex information in programs. For example, values in dictionaries may be lists, which may contain tuples, which may contain dictionaries, and so on. The nesting can be as deep as needed to model the data to be processed. Let’s look at an example of nesting. The following interaction defines a tree of nested compound sequence objects, shown in Figure 9-1. To access its components, you may include as many index operations as required. Python evaluates the indexes from left to right, and fetches a reference to a more deeply nested object at each step. Figure 9-1 may be a pathologically complicated data structure, but it illustrates the syntax used to access nested objects in general: >>> L = ['abc', [(1, 2), ([3], 4)], 5] >>> L[1] [(1, 2), ([3], 4)] >>> L[1][1] ([3], 4) >>> L[1][1][0] [3] >>> L[1][1][0][0] 3 References Versus Copies Chapter 6 mentioned that assignments always store references to objects, not copies of those objects. In practice, this is usually what you want. Because assignments can generate multiple references to the same object, though, it’s important to be aware that changing a mutable object in-place may affect other references to the same object References Versus Copies | 241 Figure 9-1. A nested object tree with the offsets of its components, created by running the literal expression ['abc', [(1, 2), ([3], 4)], 5]. Syntactically nested objects are internally represented as references (i.e., pointers) to separate pieces of memory. elsewhere in your program. If you don’t want such behavior, you’ll need to tell Python to copy the object explicitly. We studied this phenomenon in Chapter 6, but it can become more subtle when larger objects come into play. For instance, the following example creates a list assigned to X, and another list assigned to L that embeds a reference back to list X. It also creates a dictionary D that contains another reference back to list X: >>> X = [1, 2, 3] >>> L = ['a', X, 'b'] >>> D = {'x':X, 'y':2} # Embed references to X's object At this point, there are three references to the first list created: from the name X, from inside the list assigned to L, and from inside the dictionary assigned to D. The situation is illustrated in Figure 9-2. Because lists are mutable, changing the shared list object from any of the three references also changes what the other two reference: >>> X[1] = 'surprise' # Changes all three references! >>> L ['a', [1, 'surprise', 3], 'b'] >>> D {'x': [1, 'surprise', 3], 'y': 2} References are a higher-level analog of pointers in other languages. Although you can’t grab hold of the reference itself, it’s possible to store the same reference in more than one place (variables, lists, and so on). This is a feature—you can pass a large object 242 | Chapter 9: Tuples, Files, and Everything Else Figure 9-2. Shared object references: because the list referenced by variable X is also referenced from within the objects referenced by L and D, changing the shared list from X makes it look different from L and D, too. around a program without generating expensive copies of it along the way. If you really do want copies, however, you can request them: • • • • Slice expressions with empty limits (L[:]) copy sequences. The dictionary and set copy method (X.copy()) copies a dictionary or set. Some built-in functions, such as list, make copies (list(L)). The copy standard library module makes full copies. For example, say you have a list and a dictionary, and you don’t want their values to be changed through other variables: >>> L = [1,2,3] >>> D = {'a':1, 'b':2} To prevent this, simply assign copies to the other variables, not references to the same objects: >>> A = L[:] >>> B = D.copy() # Instead of A = L (or list(L)) # Instead of B = D (ditto for sets) This way, changes made from the other variables will change the copies, not the originals: >>> A[1] = 'Ni' >>> B['c'] = 'spam' >>> >>> L, D ([1, 2, 3], {'a': 1, 'b': 2}) >>> A, B ([1, 'Ni', 3], {'a': 1, 'c': 'spam', 'b': 2}) In terms of our original example, you can avoid the reference side effects by slicing the original list instead of simply naming it: References Versus Copies | 243 >>> X = [1, 2, 3] >>> L = ['a', X[:], 'b'] >>> D = {'x':X[:], 'y':2} # Embed copies of X's object This changes the picture in Figure 9-2—L and D will now point to different lists than X. The net effect is that changes made through X will impact only X, not L and D; similarly, changes to L or D will not impact X. One final note on copies: empty-limit slices and the dictionary copy method only make top-level copies; that is, they do not copy nested data structures, if any are present. If you need a complete, fully independent copy of a deeply nested data structure, use the standard copy module: include an import copy statement and say X = copy.deep copy(Y) to fully copy an arbitrarily nested object Y. This call recursively traverses objects to copy all their parts. This is a much more rare case, though (which is why you have to say more to make it go). References are usually what you will want; when they are not, slices and copy methods are usually as much copying as you’ll need to do. Comparisons, Equality, and Truth All Python objects also respond to comparisons: tests for equality, relative magnitude, and so on. Python comparisons always inspect all parts of compound objects until a result can be determined. In fact, when nested objects are present, Python automatically traverses data structures to apply comparisons recursively from left to right, and as deeply as needed. The first difference found along the way determines the comparison result. For instance, a comparison of list objects compares all their components automatically: >>> L1 >>> L2 >>> L1 (True, = [1, ('a', 3)] = [1, ('a', 3)] == L2, L1 is L2 False) # Same value, unique objects # Equivalent? Same object? Here, L1 and L2 are assigned lists that are equivalent but distinct objects. Because of the nature of Python references (studied in Chapter 6), there are two ways to test for equality: • The == operator tests value equivalence. Python performs an equivalence test, comparing all nested objects recursively. • The is operator tests object identity. Python tests whether the two are really the same object (i.e., live at the same address in memory). In the preceding example, L1 and L2 pass the == test (they have equivalent values because all their components are equivalent) but fail the is check (they reference two different objects, and hence two different pieces of memory). Notice what happens for short strings, though: >>> S1 = 'spam' >>> S2 = 'spam' 244 | Chapter 9: Tuples, Files, and Everything Else >>> S1 == S2, S1 is S2 (True, True) Here, we should again have two distinct objects that happen to have the same value: == should be true, and is should be false. But because Python internally caches and reuses some strings as an optimization, there really is just a single string 'spam' in memory, shared by S1 and S2; hence, the is identity test reports a true result. To trigger the normal behavior, we need to use longer strings: >>> S1 >>> S2 >>> S1 (True, = 'a longer string' = 'a longer string' == S2, S1 is S2 False) Of course, because strings are immutable, the object caching mechanism is irrelevant to your code—strings can’t be changed in-place, regardless of how many variables refer to them. If identity tests seem confusing, see Chapter 6 for a refresher on object reference concepts. As a rule of thumb, the == operator is what you will want to use for almost all equality checks; is is reserved for highly specialized roles. We’ll see cases where these operators are put to use later in the book. Relative magnitude comparisons are also applied recursively to nested data structures: >>> L1 = [1, ('a', 3)] >>> L2 = [1, ('a', 2)] >>> L1 < L2, L1 == L2, L1 > L2 (False, False, True) # Less, equal, greater: tuple of results Here, L1 is greater than L2 because the nested 3 is greater than 2. The result of the last line is really a tuple of three objects—the results of the three expressions typed (an example of a tuple without its enclosing parentheses). In general, Python compares types as follows: • • • • Numbers are compared by relative magnitude. Strings are compared lexicographically, character by character ("abc" < "ac"). Lists and tuples are compared by comparing each component from left to right. Dictionaries compare as equal if their sorted (key, value) lists are equal. Relative magnitude comparisons are not supported for dictionaries in Python 3.0, but they work in 2.6 and earlier as though comparing sorted (key, value) lists. • Nonnumeric mixed-type comparisons (e.g., 1 < 'spam') are errors in Python 3.0. They are allowed in Python 2.6, but use a fixed but arbitrary ordering rule. By proxy, this also applies to sorts, which use comparisons internally: nonnumeric mixed-type collections cannot be sorted in 3.0. In general, comparisons of structured objects proceed as though you had written the objects as literals and compared all their parts one at a time from left to right. In later chapters, we’ll see other object types that can change the way they get compared. Comparisons, Equality, and Truth | 245 Python 3.0 Dictionary Comparisons The second to last point in the preceding section merits illustration. In Python 2.6 and earlier, dictionaries support magnitude comparisons, as though you were comparing sorted key/value lists: C:\misc> c:\python26\python >>> D1 = {'a':1, 'b':2} >>> D2 = {'a':1, 'b':3} >>> D1 == D2 False >>> D1 < D2 True In Python 3.0, magnitude comparisons for dictionaries are removed because they incur too much overhead when equality is desired (equality uses an optimized scheme in 3.0 that doesn’t literally compare sorted key/value lists). The alternative in 3.0 is to either write loops to compare values by key or compare the sorted key/value lists manually— the items dictionary methods and sorted built-in suffice: C:\misc> c:\python30\python >>> D1 = {'a':1, 'b':2} >>> D2 = {'a':1, 'b':3} >>> D1 == D2 False >>> D1 < D2 TypeError: unorderable types: dict() < dict() >>> list(D1.items()) [('a', 1), ('b', 2)] >>> sorted(D1.items()) [('a', 1), ('b', 2)] >>> sorted(D1.items()) < sorted(D2.items()) True >>> sorted(D1.items()) > sorted(D2.items()) False In practice, most programs requiring this behavior will develop more efficient ways to compare data in dictionaries than either this workaround or the original behavior in Python 2.6. The Meaning of True and False in Python Notice that the test results returned in the last two examples represent true and false values. They print as the words True and False, but now that we’re using logical tests like these in earnest, I should be a bit more formal about what these names really mean. In Python, as in most programming languages, an integer 0 represents false, and an integer 1 represents true. In addition, though, Python recognizes any empty data structure as false and any nonempty data structure as true. More generally, the notions of 246 | Chapter 9: Tuples, Files, and Everything Else true and false are intrinsic properties of every object in Python—each object is either true or false, as follows: • Numbers are true if nonzero. • Other objects are true if nonempty. Table 9-4 gives examples of true and false objects in Python. Table 9-4. Example object truth values Object Value "spam" True "" False [] False {} False 1 True 0.0 False None False As one application, because objects are true or false themselves, it’s common to see Python programmers code tests like if X:, which, assuming X is a string, is the same as if X != '':. In other words, you can test the object itself, instead of comparing it to an empty object. (More on if statements in Part III.) The None object As shown in the last item in Table 9-4, Python also provides a special object called None, which is always considered to be false. None was introduced in Chapter 4; it is the only value of a special data type in Python and typically serves as an empty placeholder (much like a NULL pointer in C). For example, recall that for lists you cannot assign to an offset unless that offset already exists (the list does not magically grow if you make an out-of-bounds assignment). To preallocate a 100-item list such that you can add to any of the 100 offsets, you can fill it with None objects: >>> L = [None] * 100 >>> >>> L [None, None, None, None, None, None, None, ... ] This doesn’t limit the size of the list (it can still grow and shrink later), but simply presets an initial size to allow for future index assignments. You could initialize a list with zeros the same way, of course, but best practice dictates using None if the list’s contents are not yet known. Comparisons, Equality, and Truth | 247 Keep in mind that None does not mean “undefined.” That is, None is something, not nothing (despite its name!)—it is a real object and piece of memory, given a built-in name by Python. Watch for other uses of this special object later in the book; it is also the default return value of functions, as we’ll see in Part IV. The bool type Also keep in mind that the Python Boolean type bool, introduced in Chapter 5, simply augments the notions of true and false in Python. As we learned in Chapter 5, the builtin words True and False are just customized versions of the integers 1 and 0—it’s as if these two words have been preassigned to 1 and 0 everywhere in Python. Because of the way this new type is implemented, this is really just a minor extension to the notions of true and false already described, designed to make truth values more explicit: • When used explicitly in truth test code, the words True and False are equivalent to 1 and 0, but they make the programmer’s intent clearer. • Results of Boolean tests run interactively print as the words True and False, instead of as 1 and 0, to make the type of result clearer. You are not required to use only Boolean types in logical statements such as if; all objects are still inherently true or false, and all the Boolean concepts mentioned in this chapter still work as described if you use other types. Python also provides a bool builtin function that can be used to test the Boolean value of an object (i.e., whether it is True—that is, nonzero or nonempty): >>> bool(1) True >>> bool('spam') True >>> bool({}) False In practice, though, you’ll rarely notice the Boolean type produced by logic tests, because Boolean results are used automatically by if statements and other selection tools. We’ll explore Booleans further when we study logical statements in Chapter 12. Python’s Type Hierarchies Figure 9-3 summarizes all the built-in object types available in Python and their relationships. We’ve looked at the most prominent of these; most of the other kinds of objects in Figure 9-3 correspond to program units (e.g., functions and modules) or exposed interpreter internals (e.g., stack frames and compiled code). The main point to notice here is that everything in a Python system is an object type and may be processed by your Python programs. For instance, you can pass a class to a function, assign it to a variable, stuff it in a list or dictionary, and so on. 248 | Chapter 9: Tuples, Files, and Everything Else Figure 9-3. Python’s major built-in object types, organized by categories. Everything is a type of object in Python, even the type of an object! Python’s Type Hierarchies | 249 Type Objects In fact, even types themselves are an object type in Python: the type of an object is an object of type type (say that three times fast!). Seriously, a call to the built-in function type(X) returns the type object of object X. The practical application of this is that type objects can be used for manual type comparisons in Python if statements. However, for reasons introduced in Chapter 4, manual type testing is usually not the right thing to do in Python, since it limits your code’s flexibility. One note on type names: as of Python 2.2, each core type has a new built-in name added to support type customization through object-oriented subclassing: dict, list, str, tuple, int, float, complex, bytes, type, set, and more (in Python 2.6 but not 3.0, file is also a type name and a synonym for open). Calls to these names are really object constructor calls, not simply conversion functions, though you can treat them as simple functions for basic usage. In addition, the types standard library module in Python 3.0 provides additional type names for types that are not available as built-ins (e.g., the type of a function; in Python 2.6 but not 3.0, this module also includes synonyms for built-in type names), and it is possible to do type tests with the isinstance function. For example, all of the following type tests are true: type([1]) == type([]) type([1]) == list isinstance([1], list) # Type of another list # List type name # List or customization thereof import types def f(): pass type(f) == types.FunctionType # types has names for other types Because types can be subclassed in Python today, the isinstance technique is generally recommended. See Chapter 31 for more on subclassing built-in types in Python 2.2 and later. Also in Chapter 31, we will explore how type(X) and type-testing in general apply to instances of user-defined classes. In short, in Python 3.0 and for new-style classes in Python 2.6, the type of a class instance is the class from which the instance was made. For classic classes in Python 2.6 and earlier, all class instances are of the type “instance,” and we must compare instance __class__ attributes to compare their types meaningfully. Since we’re not ready for classes yet, we’ll postpone the rest of this story until Chapter 31. Other Types in Python Besides the core objects studied in this part of the book, and the program-unit objects such as functions, modules, and classes that we’ll meet later, a typical Python installation has dozens of additional object types available as linked-in C extensions or 250 | Chapter 9: Tuples, Files, and Everything Else Python classes—regular expression objects, DBM files, GUI widgets, network sockets, and so on. The main difference between these extra tools and the built-in types we’ve seen so far is that the built-ins provide special language creation syntax for their objects (e.g., 4 for an integer, [1,2] for a list, the open function for files, and def and lambda for functions). Other tools are generally made available in standard library modules that you must first import to use. For instance, to make a regular expression object, you import re and call re.compile(). See Python’s library reference for a comprehensive guide to all the tools available to Python programs. Built-in Type Gotchas That’s the end of our look at core data types. We’ll wrap up this part of the book with a discussion of common problems that seem to bite new users (and the occasional expert), along with their solutions. Some of this is a review of ideas we’ve already covered, but these issues are important enough to warn about again here. Assignment Creates References, Not Copies Because this is such a central concept, I’ll mention it again: you need to understand what’s going on with shared references in your program. For instance, in the following example, the list object assigned to the name L is referenced from L and from inside the list assigned to the name M. Changing L in-place changes what M references, too: >>> L = [1, 2, 3] >>> M = ['X', L, 'Y'] >>> M ['X', [1, 2, 3], 'Y'] >>> L[1] = 0 >>> M ['X', [1, 0, 3], 'Y'] # Embed a reference to L # Changes M too This effect usually becomes important only in larger programs, and shared references are often exactly what you want. If they’re not, you can avoid sharing objects by copying them explicitly. For lists, you can always make a top-level copy by using an emptylimits slice: >>> L = [1, 2, 3] >>> M = ['X', L[:], 'Y'] >>> L[1] = 0 >>> L [1, 0, 3] >>> M ['X', [1, 2, 3], 'Y'] # Embed a copy of L # Changes only L, not M Built-in Type Gotchas | 251 Remember, slice limits default to 0 and the length of the sequence being sliced; if both are omitted, the slice extracts every item in the sequence and so makes a top-level copy (a new, unshared object). Repetition Adds One Level Deep Repeating a sequence is like adding it to itself a number of times. However, when mutable sequences are nested, the effect might not always be what you expect. For instance, in the following example X is assigned to L repeated four times, whereas Y is assigned to a list containing L repeated four times: >>> L = [4, 5, 6] >>> X = L * 4 >>> Y = [L] * 4 # Like [4, 5, 6] + [4, 5, 6] + ... # [L] + [L] + ... = [L, L,...] >>> X [4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6] >>> Y [[4, 5, 6], [4, 5, 6], [4, 5, 6], [4, 5, 6]] Because L was nested in the second repetition, Y winds up embedding references back to the original list assigned to L, and so is open to the same sorts of side effects noted in the last section: >>> L[1] = 0 # Impacts Y but not X >>> X [4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6] >>> Y [[4, 0, 6], [4, 0, 6], [4, 0, 6], [4, 0, 6]] The same solutions to this problem apply here as in the previous section, as this is really just another way to create the shared mutable object reference case. If you remember that repetition, concatenation, and slicing copy only the top level of their operand objects, these sorts of cases make much more sense. Beware of Cyclic Data Structures We actually encountered this concept in a prior exercise: if a collection object contains a reference to itself, it’s called a cyclic object. Python prints a [...] whenever it detects a cycle in the object, rather than getting stuck in an infinite loop: >>> L = ['grail'] >>> L.append(L) >>> L ['grail', [...]] # Append reference to same object # Generates cycle in object: [...] Besides understanding that the three dots in square brackets represent a cycle in the object, this case is worth knowing about because it can lead to gotchas—cyclic structures may cause code of your own to fall into unexpected loops if you don’t anticipate them. For instance, some programs keep a list or dictionary of already visited items and 252 | Chapter 9: Tuples, Files, and Everything Else check it to determine whether they’re in a cycle. See the solutions to the “Test Your Knowledge: Part I Exercises” in Appendix B for more on this problem, and check out the reloadall.py program in Chapter 24 for a solution. Don’t use cyclic references unless you really need to. There are good reasons to create cycles, but unless you have code that knows how to handle them, you probably won’t want to make your objects reference themselves very often in practice. Immutable Types Can’t Be Changed In-Place You can’t change an immutable object in-place. Instead, you construct a new object with slicing, concatenation, and so on, and assign it back to the original reference, if needed: T = (1, 2, 3) T[2] = 4 # Error! T = T[:2] + (4,) # OK: (1, 2, 4) That might seem like extra coding work, but the upside is that the previous gotchas can’t happen when you’re using immutable objects such as tuples and strings; because they can’t be changed in-place, they are not open to the sorts of side effects that lists are. Chapter Summary This chapter explored the last two major core object types—the tuple and the file. We learned that tuples support all the usual sequence operations, have just a few methods, and do not allow any in-place changes because they are immutable. We also learned that files are returned by the built-in open function and provide methods for reading and writing data. We explored how to translate Python objects to and from strings for storing in files, and we looked at the pickle and struct modules for advanced roles (object serialization and binary data). Finally, we wrapped up by reviewing some properties common to all object types (e.g., shared references) and went through a list of common mistakes (“gotchas”) in the object type domain. In the next part, we’ll shift gears, turning to the topic of statement syntax in Python— we’ll explore all of Python’s basic procedural statements in the chapters that follow. The next chapter kicks off that part of the book with an introduction to Python’s general syntax model, which is applicable to all statement types. Before moving on, though, take the chapter quiz, and then work through the end-of-part lab exercises to review type concepts. Statements largely just create and process objects, so make sure you’ve mastered this domain by working through all the exercises before reading on. Chapter Summary | 253 Test Your Knowledge: Quiz 1. How can you determine how large a tuple is? Why is this tool located where it is? 2. Write an expression that changes the first item in a tuple. (4, 5, 6) should become (1, 5, 6) in the process. 3. What is the default for the processing mode argument in a file open call? 4. What module might you use to store Python objects in a file without converting them to strings yourself? 5. How might you go about copying all parts of a nested structure at once? 6. When does Python consider an object true? 7. What is your quest? Test Your Knowledge: Answers 1. The built-in len function returns the length (number of contained items) for any container object in Python, including tuples. It is a built-in function instead of a type method because it applies to many different types of objects. In general, builtin functions and expressions may span many object types; methods are specific to a single object type, though some may be available on more than one type (index, for example, works on lists and tuples). 2. Because they are immutable, you can’t really change tuples in-place, but you can generate a new tuple with the desired value. Given T = (4, 5, 6), you can change the first item by making a new tuple from its parts by slicing and concatenating: T = (1,) + T[1:]. (Recall that single-item tuples require a trailing comma.) You could also convert the tuple to a list, change it in-place, and convert it back to a tuple, but this is more expensive and is rarely required in practice—simply use a list if you know that the object will require in-place changes. 3. The default for the processing mode argument in a file open call is 'r', for reading text input. For input text files, simply pass in the external file’s name. 4. The pickle module can be used to store Python objects in a file without explicitly converting them to strings. The struct module is related, but it assumes the data is to be in packed binary format in the file. 5. Import the copy module, and call copy.deepcopy(X) if you need to copy all parts of a nested structure X. This is also rarely seen in practice; references are usually the desired behavior, and shallow copies (e.g., aList[:], aDict.copy()) usually suffice for most copies. 254 | Chapter 9: Tuples, Files, and Everything Else 6. An object is considered true if it is either a nonzero number or a nonempty collection object. The built-in words True and False are essentially predefined to have the same meanings as integer 1 and 0, respectively. 7. Acceptable answers include “To learn Python,” “To move on to the next part of the book,” or “To seek the Holy Grail.” Test Your Knowledge: Part II Exercises This session asks you to get your feet wet with built-in object fundamentals. As before, a few new ideas may pop up along the way, so be sure to flip to the answers in Appendix B when you’re done (or when you’re not, if necessary). If you have limited time, I suggest starting with exercises 10 and 11 (the most practical of the bunch), and then working from first to last as time allows. This is all fundamental material, though, so try to do as many of these as you can. 1. The basics. Experiment interactively with the common type operations found in the various operation tables in this part of the book. To get started, bring up the Python interactive interpreter, type each of the following expressions, and try to explain what’s happening in each case. Note that the semicolon in some of these is being used as a statement separator, to squeeze multiple statements onto a single line: for example, X=1;X assigns and then prints a variable (more on statement syntax in the next part of the book). Also remember that a comma between expressions usually builds a tuple, even if there are no enclosing parentheses: X,Y,Z is a three-item tuple, which Python prints back to you in parentheses. 2 ** 16 2 / 5, 2 / 5.0 "spam" + "eggs" S = "ham" "eggs " + S S * 5 S[:0] "green %s and %s" % ("eggs", S) 'green {0} and {1}'.format('eggs', S) ('x',)[0] ('x', 'y')[1] L = [1,2,3] + [4,5,6] L, L[:], L[:0], L[−2], L[−2:] ([1,2,3] + [4,5,6])[2:4] [L[2], L[3]] L.reverse(); L L.sort(); L L.index(4) {'a':1, 'b':2}['b'] D = {'x':1, 'y':2, 'z':3} Test Your Knowledge: Part II Exercises | 255 D['w'] = 0 D['x'] + D['w'] D[(1,2,3)] = 4 list(D.keys()), list(D.values()), (1,2,3) in D [[]], ["",[],(),{},None] 2. Indexing and slicing. At the interactive prompt, define a list named L that contains four strings or numbers (e.g., L=[0,1,2,3]). Then, experiment with some boundary cases; you may not ever see these cases in real programs, but they are intended to make you think about the underlying model, and some may be useful in less artificial forms: a. What happens when you try to index out of bounds (e.g., L[4])? b. What about slicing out of bounds (e.g., L[−1000:100])? c. Finally, how does Python handle it if you try to extract a sequence in reverse, with the lower bound greater than the higher bound (e.g., L[3:1])? Hint: try assigning to this slice (L[3:1]=['?']), and see where the value is put. Do you think this may be the same phenomenon you saw when slicing out of bounds? 3. Indexing, slicing, and del. Define another list L with four items, and assign an empty list to one of its offsets (e.g., L[2]=[]). What happens? Then, assign an empty list to a slice (L[2:3]=[]). What happens now? Recall that slice assignment deletes the slice and inserts the new value where it used to be. The del statement deletes offsets, keys, attributes, and names. Use it on your list to delete an item (e.g., del L[0]). What happens if you delete an entire slice (del L[1:])? What happens when you assign a nonsequence to a slice (L[1:2]=1)? 4. Tuple assignment. Type the following lines: >>> X = 'spam' >>> Y = 'eggs' >>> X, Y = Y, X What do you think is happening to X and Y when you type this sequence? 5. Dictionary keys. Consider the following code fragments: >>> D = {} >>> D[1] = 'a' >>> D[2] = 'b' You’ve learned that dictionaries aren’t accessed by offsets, so what’s going on here? Does the following shed any light on the subject? (Hint: strings, integers, and tuples share which type category?) >>> D[(1, 2, 3)] = 'c' >>> D {1: 'a', 2: 'b', (1, 2, 3): 'c'} 256 | Chapter 9: Tuples, Files, and Everything Else 6. Dictionary indexing. Create a dictionary named D with three entries, for keys 'a', 'b', and 'c'. What happens if you try to index a nonexistent key (D['d'])? What does Python do if you try to assign to a nonexistent key 'd' (e.g., D['d']='spam')? How does this compare to out-of-bounds assignments and references for lists? Does this sound like the rule for variable names? 7. Generic operations. Run interactive tests to answer the following questions: a. What happens when you try to use the + operator on different/mixed types (e.g., string + list, list + tuple)? b. Does + work when one of the operands is a dictionary? c. Does the append method work for both lists and strings? How about using the keys method on lists? (Hint: what does append assume about its subject object?) d. Finally, what type of object do you get back when you slice or concatenate two lists or two strings? 8. String indexing. Define a string S of four characters: S = "spam". Then type the following expression: S[0][0][0][0][0]. Any clue as to what’s happening this time? (Hint: recall that a string is a collection of characters, but Python characters are one-character strings.) Does this indexing expression still work if you apply it to a list such as ['s', 'p', 'a', 'm']? Why? 9. Immutable types. Define a string S of four characters again: S = "spam". Write an assignment that changes the string to "slam", using only slicing and concatenation. Could you perform the same operation using just indexing and concatenation? How about index assignment? 10. Nesting. Write a data structure that represents your personal information: name (first, middle, last), age, job, address, email address, and phone number. You may build the data structure with any combination of built-in object types you like (lists, tuples, dictionaries, strings, numbers). Then, access the individual components of your data structures by indexing. Do some structures make more sense than others for this object? 11. Files. Write a script that creates a new output file called myfile.txt and writes the string "Hello file world!" into it. Then write another script that opens myfile.txt and reads and prints its contents. Run your two scripts from the system command line. Does the new file show up in the directory where you ran your scripts? What if you add a different directory path to the filename passed to open? Note: file write methods do not add newline characters to your strings; add an explicit \n at the end of the string if you want to fully terminate the line in the file. Test Your Knowledge: Part II Exercises | 257 PART III Statements and Syntax CHAPTER 10 Introducing Python Statements Now that you’re familiar with Python’s core built-in object types, this chapter begins our exploration of its fundamental statement forms. As in the previous part, we’ll begin here with a general introduction to statement syntax, and we’ll follow up with more details about specific statements in the next few chapters. In simple terms, statements are the things you write to tell Python what your programs should do. If programs “do things with stuff,” statements are the way you specify what sort of things a program does. Python is a procedural, statement-based language; by combining statements, you specify a procedure that Python performs to satisfy a program’s goals. Python Program Structure Revisited Another way to understand the role of statements is to revisit the concept hierarchy introduced in Chapter 4, which talked about built-in objects and the expressions used to manipulate them. This chapter climbs the hierarchy to the next level: 1. 2. 3. 4. Programs are composed of modules. Modules contain statements. Statements contain expressions. Expressions create and process objects. At its core, Python syntax is composed of statements and expressions. Expressions process objects and are embedded in statements. Statements code the larger logic of a program’s operation—they use and direct expressions to process the objects we studied in the preceding chapters. Moreover, statements are where objects spring into existence (e.g., in expressions within assignment statements), and some statements create entirely new kinds of objects (functions, classes, and so on). Statements always exist in modules, which themselves are managed with statements. 261 Python’s Statements Table 10-1 summarizes Python’s statement set. This part of the book deals with entries in the table from the top through break and continue. You’ve informally been introduced to a few of the statements in Table 10-1 already; this part of the book will fill in details that were skipped earlier, introduce the rest of Python’s procedural statement set, and cover the overall syntax model. Statements lower in Table 10-1 that have to do with larger program units—functions, classes, modules, and exceptions—lead to larger programming ideas, so they will each have a section of their own. More focused statements (like del, which deletes various components) are covered elsewhere in the book, or in Python’s standard manuals. Table 10-1. Python 3.0 statements Statement Role Example Assignment Creating references a, *b = 'good', 'bad', 'ugly' Calls and other expressions Running functions log.write("spam, ham") print calls Printing objects print('The Killer', joke) if/elif/else Selecting actions if "python" in text: print(text) for/else Sequence iteration for x in mylist: print(x) while/else General loops while X > Y: print('hello') pass Empty placeholder while True: pass break Loop exit while True: if exittest(): break continue Loop continue while True: if skiptest(): continue def Functions and methods def f(a, b, c=1, *d): print(a+b+c+d[0]) return Functions results def f(a, b, c=1, *d): return a+b+c+d[0] yield Generator functions def gen(n): for i in n: yield i*2 global Namespaces x = 'old' def function(): global x, y; x = 'new' nonlocal Namespaces (3.0+) def outer(): x = 'old' def function(): nonlocal x; x = 'new' import Module access import sys from Attribute access from sys import stdin class Building objects class Subclass(Superclass): staticData = [] def method(self): pass 262 | Chapter 10: Introducing Python Statements Statement Role Example try/except/ finally Catching exceptions try: raise Triggering exceptions raise EndSearch(location) assert Debugging checks assert X > Y, 'X too small' with/as Context managers (2.6+) with open('data') as myfile: process(myfile) del Deleting references del del del del action() except: print('action error') data[k] data[i:j] obj.attr variable Table 10-1 reflects the statement forms in Python 3.0—units of code that each have a specific syntax and purpose. Here are a few fine points about its content: • Assignment statements come in a variety of syntax flavors, described in Chapter 11: basic, sequence, augmented, and more. • print is technically neither a reserved word nor a statement in 3.0, but a built-in function call; because it will nearly always be run as an expression statement, though (that is, on a line by itself), it’s generally thought of as a statement type. We’ll study print operations in Chapter 11 the next chapter. • yield is actually an expression instead of a statement too, as of 2.5; like print, it’s typically used in a line by itself and so is included in this table, but scripts occasionally assign or otherwise use its result, as we’ll see in Chapter 20. As an expression, yield is also a reserved word, unlike print. Most of this table applies to Python 2.6, too, except where it doesn’t—if you are using Python 2.6 or older, here are a few notes for your Python, too: • In 2.6, nonlocal is not available; as we’ll see in Chapter 17, there are alternative ways to achieve this statement’s writeable state-retention effect. • In 2.6, print is a statement instead of a built-in function call, with specific syntax covered in Chapter 11. • In 2.6, the 3.0 exec code execution built-in function is a statement, with specific syntax; since it supports enclosing parentheses, though, you can generally use its 3.0 call form in 2.6 code. • In 2.5, the try/except and try/finally statements were merged: the two were formerly separate statements, but we can now say both except and finally in the same try statement. • In 2.5, with/as is an optional extension, and it is not available unless you explicitly turn it on by running the statement from __future__ import with_statement (see Chapter 33). Python Program Structure Revisited | 263 A Tale of Two ifs Before we delve into the details of any of the concrete statements in Table 10-1, I want to begin our look at Python statement syntax by showing you what you are not going to type in Python code so you can compare and contrast it with other syntax models you might have seen in the past. Consider the following if statement, coded in a C-like language: if (x > y) { x = 1; y = 2; } This might be a statement in C, C++, Java, JavaScript, or Perl. Now, look at the equivalent statement in the Python language: if x > y: x = 1 y = 2 The first thing that may pop out at you is that the equivalent Python statement is less, well, cluttered—that is, there are fewer syntactic components. This is by design; as a scripting language, one of Python’s goals is to make programmers’ lives easier by requiring less typing. More specifically, when you compare the two syntax models, you’ll notice that Python adds one new thing to the mix, and that three items that are present in the C-like language are not present in Python code. What Python Adds The one new syntax component in Python is the colon character (:). All Python compound statements (i.e., statements that have statements nested inside them) follow the same general pattern of a header line terminated in a colon, followed by a nested block of code usually indented underneath the header line, like this: Header line: Nested statement block The colon is required, and omitting it is probably the most common coding mistake among new Python programmers—it’s certainly one I’ve witnessed thousands of times in Python training classes. In fact, if you are new to Python, you’ll almost certainly forget the colon character very soon. Most Python-friendly editors make this mistake easy to spot, and including it eventually becomes an unconscious habit (so much so that you may start typing colons in your C++ code, too, generating many entertaining error messages from your C++ compiler!). 264 | Chapter 10: Introducing Python Statements What Python Removes Although Python requires the extra colon character, there are three things programmers in C-like languages must include that you don’t generally have to in Python. Parentheses are optional The first of these is the set of parentheses around the tests at the top of the statement: if (x < y) The parentheses here are required by the syntax of many C-like languages. In Python, though, they are not—we simply omit the parentheses, and the statement works the same way: if x < y Technically speaking, because every expression can be enclosed in parentheses, including them will not hurt in this Python code, and they are not treated as an error if present. But don’t do that: you’ll be wearing out your keyboard needlessly, and broadcasting to the world that you’re an ex-C programmer still learning Python (I was once, too). The Python way is to simply omit the parentheses in these kinds of statements altogether. End of line is end of statement The second and more significant syntax component you won’t find in Python code is the semicolon. You don’t need to terminate statements with semicolons in Python the way you do in C-like languages: x = 1; In Python, the general rule is that the end of a line automatically terminates the statement that appears on that line. In other words, you can leave off the semicolons, and it works the same way: x = 1 There are some ways to work around this rule, as you’ll see in a moment. But, in general, you write one statement per line for the vast majority of Python code, and no semicolon is required. Here, too, if you are pining for your C programming days (if such a state is possible...) you can continue to use semicolons at the end of each statement—the language lets you get away with them if they are present. But don’t do that either (really!); again, doing so tells the world that you’re still a C programmer who hasn’t quite made the switch to Python coding. The Pythonic style is to leave off the semicolons altogether. A Tale of Two ifs | 265 End of indentation is end of block The third and final syntax component that Python removes, and the one that may seem the most unusual to soon-to-be-ex-C programmers (until they’ve used it for 10 minutes and realize it’s actually a feature), is that you do not type anything explicit in your code to syntactically mark the beginning and end of a nested block of code. You don’t need to include begin/end, then/endif, or braces around the nested block, as you do in Clike languages: if (x > y) { x = 1; y = 2; } Instead, in Python, we consistently indent all the statements in a given single nested block the same distance to the right, and Python uses the statements’ physical indentation to determine where the block starts and stops: if x > y: x = 1 y = 2 By indentation, I mean the blank whitespace all the way to the left of the two nested statements here. Python doesn’t care how you indent (you may use either spaces or tabs), or how much you indent (you may use any number of spaces or tabs). In fact, the indentation of one nested block can be totally different from that of another. The syntax rule is only that for a given single nested block, all of its statements must be indented the same distance to the right. If this is not the case, you will get a syntax error, and your code will not run until you repair its indentation to be consistent. Why Indentation Syntax? The indentation rule may seem unusual at first glance to programmers accustomed to C-like languages, but it is a deliberate feature of Python, and it’s one of the main ways that Python almost forces programmers to produce uniform, regular, and readable code. It essentially means that you must line up your code vertically, in columns, according to its logical structure. The net effect is to make your code more consistent and readable (unlike much of the code written in C-like languages). To put that more strongly, aligning your code according to its logical structure is a major part of making it readable, and thus reusable and maintainable, by yourself and others. In fact, even if you never use Python after reading this book, you should get into the habit of aligning your code for readability in any block-structured language. Python forces the issue by making this a part of its syntax, but it’s an important thing to do in any programming language, and it has a huge impact on the usefulness of your code. Your experience may vary, but when I was still doing development on a full-time basis, I was mostly paid to work on large old C++ programs that had been worked on by many programmers over the years. Almost invariably, each programmer had his or her 266 | Chapter 10: Introducing Python Statements own style for indenting code. For example, I’d often be asked to change a while loop coded in the C++ language that began like this: while (x > 0) { Before we even get into indentation, there are three or four ways that programmers can arrange these braces in a C-like language, and organizations often have political debates and write standards manuals to address the options (which seems more than a little off-topic for the problem to be solved by programming). Ignoring that, here’s the scenario I often encountered in C++ code. The first person who worked on the code indented the loop four spaces: while (x > 0) { --------; --------; That person eventually moved on to management, only to be replaced by someone who liked to indent further to the right: while (x > 0) { --------; --------; --------; --------; That person later moved on to other opportunities, and someone else picked up the code who liked to indent less: while (x > 0) { --------; --------; --------; --------; --------; --------; } And so on. Eventually, the block is terminated by a closing brace (}), which of course makes this “block-structured code” (he says, sarcastically). In any block-structured language, Python or otherwise, if nested blocks are not indented consistently, they become very difficult for the reader to interpret, change, or reuse, because the code no longer visually reflects its logical meaning. Readability matters, and indentation is a major component of readability. Here is another example that may have burned you in the past if you’ve done much programming in a C-like language. Consider the following statement in C: if (x) if (y) statement1; else statement2; A Tale of Two ifs | 267 Which if does the else here go with? Surprisingly, the else is paired with the nested if statement (if (y)), even though it looks visually as though it is associated with the outer if (x). This is a classic pitfall in the C language, and it can lead to the reader completely misinterpreting the code and changing it incorrectly in ways that might not be uncovered until the Mars rover crashes into a giant rock! This cannot happen in Python—because indentation is significant, the way the code looks is the way it will work. Consider an equivalent Python statement: if x: if y: statement1 else: statement2 In this example, the if that the else lines up with vertically is the one it is associated with logically (the outer if x). In a sense, Python is a WYSIWYG language—what you see is what you get because the way code looks is the way it runs, regardless of who coded it. If this still isn’t enough to underscore the benefits of Python’s syntax, here’s another anecdote. Early in my career, I worked at a successful company that developed systems software in the C language, where consistent indentation is not required. Even so, when we checked our code into source control at the end of the day, this company ran an automated script that analyzed the indentation used in the code. If the script noticed that we’d indented our code inconsistently, we received an automated email about it the next morning—and so did our managers! The point is that even when a language doesn’t require it, good programmers know that consistent use of indentation has a huge impact on code readability and quality. The fact that Python promotes this to the level of syntax is seen by most as a feature of the language. Also keep in mind that nearly every programmer-friendly text editor has built-in support for Python’s syntax model. In the IDLE Python GUI, for example, lines of code are automatically indented when you are typing a nested block; pressing the Backspace key backs up one level of indentation, and you can customize how far to the right IDLE indents statements in a nested block. There is no universal standard on this: four spaces or one tab per level is common, but it’s up to you to decide how and how much you wish to indent. Indent further to the right for further nested blocks, and less to close the prior block. As a rule of thumb, you probably shouldn’t mix tabs and spaces in the same block in Python, unless you do so consistently; use tabs or spaces in a given block, but not both (in fact, Python 3.0 now issues an error for inconsistent use of tabs and spaces, as we’ll see in Chapter 12). But you probably shouldn’t mix tabs or spaces in indentation in any structured language—such code can cause major readability issues if the next programmer has his or her editor set to display tabs differently than yours. C-like languages 268 | Chapter 10: Introducing Python Statements might let coders get away with this, but they shouldn’t: the result can be a mangled mess. I can’t stress enough that regardless of which language you code in, you should be indenting consistently for readability. In fact, if you weren’t taught to do this earlier in your career, your teachers did you a disservice. Most programmers—especially those who must read others’ code—consider it a major asset that Python elevates this to the level of syntax. Moreover, generating tabs instead of braces is no more difficult in practice for tools that must output Python code. In general, if you do what you should be doing in a C-like language anyhow, but get rid of the braces, your code will satisfy Python’s syntax rules. A Few Special Cases As mentioned previously, in Python’s syntax model: • The end of a line terminates the statement on that line (without semicolons). • Nested statements are blocked and associated by their physical indentation (without braces). Those rules cover almost all Python code you’ll write or see in practice. However, Python also provides some special-purpose rules that allow customization of both statements and nested statement blocks. Statement rule special cases Although statements normally appear one per line, it is possible to squeeze more than one statement onto a single line in Python by separating them with semicolons: a = 1; b = 2; print(a + b) # Three statements on one line This is the only place in Python where semicolons are required: as statement separators. This only works, though, if the statements thus combined are not themselves compound statements. In other words, you can chain together only simple statements, like assignments, prints, and function calls. Compound statements must still appear on lines of their own (otherwise, you could squeeze an entire program onto one line, which probably would not make you very popular among your coworkers!). The other special rule for statements is essentially the inverse: you can make a single statement span across multiple lines. To make this work, you simply have to enclose part of your statement in a bracketed pair—parentheses (()), square brackets ([]), or curly braces ({}). Any code enclosed in these constructs can cross multiple lines: your statement doesn’t end until Python reaches the line containing the closing part of the pair. For instance, to continue a list literal: mlist = [111, 222, 333] A Tale of Two ifs | 269 Because the code is enclosed in a square brackets pair, Python simply drops down to the next line until it encounters the closing bracket. The curly braces surrounding dictionaries (as well as set literals and dictionary and set comprehensions in 3.0) allow them to span lines this way too, and parentheses handle tuples, function calls, and expressions. The indentation of the continuation lines does not matter, though common sense dictates that the lines should be aligned somehow for readability. Parentheses are the catchall device—because any expression can be wrapped up in them, simply inserting a left parenthesis allows you to drop down to the next line and continue your statement: X = (A + B + C + D) This technique works with compound statements, too, by the way. Anywhere you need to code a large expression, simply wrap it in parentheses to continue it on the next line: if (A == 1 and B == 2 and C == 3): print('spam' * 3) An older rule also allows for continuation lines when the prior line ends in a backslash: X = A + B + \ C + D # An error-prone alternative This alternative technique is dated, though, and is frowned on today because it’s difficult to notice and maintain the backslashes, and it’s fairly brittle—there can be no spaces after the backslash, and omitting it can have unexpected effects if the next line is mistaken to be a new statement. It’s also another throwback to the C language, where it is commonly used in “#define” macros; again, when in Pythonland, do as Pythonistas do, not as C programmers do. Block rule special case As mentioned previously, statements in a nested block of code are normally associated by being indented the same amount to the right. As one special case here, the body of a compound statement can instead appear on the same line as the header in Python, after the colon: if x > y: print(x) This allows us to code single-line if statements, single-line loops, and so on. Here again, though, this will work only if the body of the compound statement itself does not contain any compound statements. That is, only simple statements—assignments, prints, function calls, and the like—are allowed after the colon. Larger statements must still appear on lines by themselves. Extra parts of compound statements (such as the else part of an if, which we’ll meet later) must also be on separate lines of their own. The body can consist of multiple simple statements separated by semicolons, but this tends to be frowned upon. 270 | Chapter 10: Introducing Python Statements In general, even though it’s not always required, if you keep all your statements on individual lines and always indent your nested blocks, your code will be easier to read and change in the future. Moreover, some code profiling and coverage tools may not be able to distinguish between multiple statements squeezed onto a single line or the header and body of a one-line compound statement. It is almost always to your advantage to keep things simple in Python. To see a prime and common exception to one of these rules in action, however (the use of a single-line if statement to break out of a loop), let’s move on to the next section and write some real code. A Quick Example: Interactive Loops We’ll see all these syntax rules in action when we tour Python’s specific compound statements in the next few chapters, but they work the same everywhere in the Python language. To get started, let’s work through a brief, realistic example that demonstrates the way that statement syntax and statement nesting come together in practice, and introduces a few statements along the way. A Simple Interactive Loop Suppose you’re asked to write a Python program that interacts with a user in a console window. Maybe you’re accepting inputs to send to a database, or reading numbers to be used in a calculation. Regardless of the purpose, you need to code a loop that reads one or more inputs from a user typing on a keyboard, and prints back a result for each. In other words, you need to write a classic read/evaluate/print loop program. In Python, typical boilerplate code for such an interactive loop might look like this: while True: reply = input('Enter text:') if reply == 'stop': break print(reply.upper()) This code makes use of a few new ideas: • The code leverages the Python while loop, Python’s most general looping statement. We’ll study the while statement in more detail later, but in short, it consists of the word while, followed by an expression that is interpreted as a true or false result, followed by a nested block of code that is repeated while the test at the top is true (the word True here is considered always true). • The input built-in function we met earlier in the book is used here for general console input—it prints its optional argument string as a prompt and returns the user’s typed reply as a string. • A single-line if statement that makes use of the special rule for nested blocks also appears here: the body of the if appears on the header line after the colon instead A Quick Example: Interactive Loops | 271 of being indented on a new line underneath it. This would work either way, but as it’s coded, we’ve saved an extra line. • Finally, the Python break statement is used to exit the loop immediately—it simply jumps out of the loop statement altogether, and the program continues after the loop. Without this exit statement, the while would loop forever, as its test is always true. In effect, this combination of statements essentially means “read a line from the user and print it in uppercase until the user enters the word ‘stop.’” There are other ways to code such a loop, but the form used here is very common in Python code. Notice that all three lines nested under the while header line are indented the same amount—because they line up vertically in a column this way, they are the block of code that is associated with the while test and repeated. Either the end of the source file or a lesser-indented statement will terminate the loop body block. When run, here is the sort of interaction we get from this code: Enter text:spam SPAM Enter text:42 42 Enter text:stop Version skew note: This example is coded for Python 3.0. If you are working in Python 2.6 or earlier, the code works the same, but you should use raw_input instead of input, and you can omit the outer parentheses in print statements. In 3.0 the former was renamed, and the latter is a built-in function instead of a statement (more on prints in the next chapter). Doing Math on User Inputs Our script works, but now suppose that instead of converting a text string to uppercase, we want to do some math with numeric input—squaring it, for example, perhaps in some misguided effort to discourage users who happen to be obsessed with youth. We might try statements like these to achieve the desired effect: >>> reply = '20' >>> reply ** 2 ...error text omitted... TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int' This won’t quite work in our script, though, because (as discussed in the prior part of the book) Python won’t convert object types in expressions unless they are all numeric, and input from a user is always returned to our script as a string. We cannot raise a string of digits to a power unless we convert it manually to an integer: 272 | Chapter 10: Introducing Python Statements >>> int(reply) ** 2 400 Armed with this information, we can now recode our loop to perform the necessary math. Type the following in a file to test it: while True: reply = input('Enter text:') if reply == 'stop': break print(int(reply) ** 2) print('Bye') This script uses a single-line if statement to exit on “stop” as before, but it also converts inputs to perform the required math. This version also adds an exit message at the bottom. Because the print statement in the last line is not indented as much as the nested block of code, it is not considered part of the loop body and will run only once, after the loop is exited: Enter text:2 4 Enter text:40 1600 Enter text:stop Bye One note here: I’m assuming that this code is stored in and run from a script file. If you are entering this code interactively, be sure to include a blank line (i.e., press Enter twice) before the final print statement, to terminate the loop. The final print doesn’t quite make sense in interactive mode, though (you’ll have to code it after interacting with the loop!). Handling Errors by Testing Inputs So far so good, but notice what happens when the input is invalid: Enter text:xxx ...error text omitted... ValueError: invalid literal for int() with base 10: 'xxx' The built-in int function raises an exception here in the face of a mistake. If we want our script to be robust, we can check the string’s content ahead of time with the string object’s isdigit method: >>> S = '123' >>> T = 'xxx' >>> S.isdigit(), T.isdigit() (True, False) This also gives us an excuse to further nest the statements in our example. The following new version of our interactive script uses a full-blown if statement to work around the exception on errors: while True: reply = input('Enter text:') A Quick Example: Interactive Loops | 273 if reply == 'stop': break elif not reply.isdigit(): print('Bad!' * 8) else: print(int(reply) ** 2) print('Bye') We’ll study the if statement in more detail in Chapter 12, but it’s a fairly lightweight tool for coding logic in scripts. In its full form, it consists of the word if followed by a test and an associated block of code, one or more optional elif (“else if”) tests and code blocks, and an optional else part, with an associated block of code at the bottom to serve as a default. Python runs the block of code associated with the first test that is true, working from top to bottom, or the else part if all tests are false. The if, elif, and else parts in the preceding example are associated as part of the same statement because they all line up vertically (i.e., share the same level of indentation). The if statement spans from the word if to the start of the print statement on the last line of the script. In turn, the entire if block is part of the while loop because all of it is indented under the loop’s header line. Statement nesting is natural once you get the hang of it. When we run our new script, its code catches errors before they occur and prints an (arguably silly) error message to demonstrate: Enter text:5 25 Enter text:xyz Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad! Enter text:10 100 Enter text:stop Handling Errors with try Statements The preceding solution works, but as you’ll see later in the book, the most general way to handle errors in Python is to catch and recover from them completely using the Python try statement. We’ll explore this statement in depth in Part VII of this book, but as a preview, using a try here can lead to code that some would claim is simpler than the prior version: while True: reply = input('Enter text:') if reply == 'stop': break try: num = int(reply) except: print('Bad!' * 8) else: print(int(reply) ** 2) print('Bye') 274 | Chapter 10: Introducing Python Statements This version works exactly like the previous one, but we’ve replaced the explicit error check with code that assumes the conversion will work and wraps it up in an exception handler for cases when it doesn’t. This try statement is composed of the word try, followed by the main block of code (the action we are trying to run), followed by an except part that gives the exception handler code and an else part to be run if no exception is raised in the try part. Python first runs the try part, then runs either the except part (if an exception occurs) or the else part (if no exception occurs). In terms of statement nesting, because the words try, except, and else are all indented to the same level, they are all considered part of the same single try statement. Notice that the else part is associated with the try here, not the if. As we’ve seen, else can appear in if statements in Python, but it can also appear in try statements and loops— its indentation tells you what statement it is a part of. In this case, the try statement spans from the word try through the code indented under the word else, because the else is indented to the same level as try. The if statement in this code is a one-liner and ends after the break. Again, we’ll come back to the try statement later in this book. For now, be aware that because try can be used to intercept any error, it reduces the amount of error-checking code you have to write, and it’s a very general approach to dealing with unusual cases. If we wanted to support input of floating-point numbers instead of just integers, for example, using try would be much easier than manual error testing—we could simply run a float call and catch its exceptions, instead of trying to analyze all possible floatingpoint syntax. Nesting Code Three Levels Deep Let’s look at one last mutation of our script. Nesting can take us even further if we need it to—we could, for example, branch to one of a set of alternatives based on the relative magnitude of a valid input: while True: reply = input('Enter text:') if reply == 'stop': break elif not reply.isdigit(): print('Bad!' * 8) else: num = int(reply) if num < 20: print('low') else: print(num ** 2) print('Bye') A Quick Example: Interactive Loops | 275 This version includes an if statement nested in the else clause of another if statement, which is in turn nested in the while loop. When code is conditional, or repeated like this, we simply indent it further to the right. The net effect is like that of the prior versions, but we’ll now print “low” for numbers less than 20: Enter text:19 low Enter text:20 400 Enter text:spam Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad! Enter text:stop Bye Chapter Summary That concludes our quick look at Python statement syntax. This chapter introduced the general rules for coding statements and blocks of code. As you’ve learned, in Python we normally code one statement per line and indent all the statements in a nested block the same amount (indentation is part of Python’s syntax). However, we also looked at a few exceptions to these rules, including continuation lines and single-line tests and loops. Finally, we put these ideas to work in an interactive script that demonstrated a handful of statements and showed statement syntax in action. In the next chapter, we’ll start to dig deeper by going over each of Python’s basic procedural statements in depth. As you’ll see, though, all statements follow the same general rules introduced here. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. 7. 8. What three things are required in a C-like language but omitted in Python? How is a statement normally terminated in Python? How are the statements in a nested block of code normally associated in Python? How can you make a single statement span multiple lines? How can you code a compound statement on a single line? Is there any valid reason to type a semicolon at the end of a statement in Python? What is a try statement for? What is the most common coding mistake among Python beginners? 276 | Chapter 10: Introducing Python Statements Test Your Knowledge: Answers 1. C-like languages require parentheses around the tests in some statements, semicolons at the end of each statement, and braces around a nested block of code. 2. The end of a line terminates the statement that appears on that line. Alternatively, if more than one statement appears on the same line, they can be terminated with semicolons; similarly, if a statement spans many lines, you must terminate it by closing a bracketed syntactic pair. 3. The statements in a nested block are all indented the same number of tabs or spaces. 4. A statement can be made to span many lines by enclosing part of it in parentheses, square brackets, or curly braces; the statement ends when Python sees a line that contains the closing part of the pair. 5. The body of a compound statement can be moved to the header line after the colon, but only if the body consists of only noncompound statements. 6. Only when you need to squeeze more than one statement onto a single line of code. Even then, this only works if all the statements are noncompound, and it’s discouraged because it can lead to code that is difficult to read. 7. The try statement is used to catch and recover from exceptions (errors) in a Python script. It’s usually an alternative to manually checking for errors in your code. 8. Forgetting to type the colon character at the end of the header line in a compound statement is the most common beginner’s mistake. If you haven’t made it yet, you probably will soon! Test Your Knowledge: Answers | 277 CHAPTER 11 Assignments, Expressions, and Prints Now that we’ve had a quick introduction to Python statement syntax, this chapter begins our in-depth tour of specific Python statements. We’ll begin with the basics: assignment statements, expression statements, and print operations. We’ve already seen all of these in action, but here we’ll fill in important details we’ve skipped so far. Although they’re fairly simple, as you’ll see, there are optional variations for each of these statement types that will come in handy once you begin writing real Python programs. Assignment Statements We’ve been using the Python assignment statement for a while to assign objects to names. In its basic form, you write the target of an assignment on the left of an equals sign, and the object to be assigned on the right. The target on the left may be a name or object component, and the object on the right can be an arbitrary expression that computes an object. For the most part, assignments are straightforward, but here are a few properties to keep in mind: • Assignments create object references. As discussed in Chapter 6, Python assignments store references to objects in names or data structure components. They always create references to objects instead of copying the objects. Because of that, Python variables are more like pointers than data storage areas. • Names are created when first assigned. Python creates a variable name the first time you assign it a value (i.e., an object reference), so there’s no need to predeclare names ahead of time. Some (but not all) data structure slots are created when assigned, too (e.g., dictionary entries, some object attributes). Once assigned, a name is replaced with the value it references whenever it appears in an expression. • Names must be assigned before being referenced. It’s an error to use a name to which you haven’t yet assigned a value. Python raises an exception if you try, rather than returning some sort of ambiguous default value; if it returned a default instead, it would be more difficult for you to spot typos in your code. 279 • Some operations perform assignments implicitly. In this section we’re concerned with the = statement, but assignment occurs in many contexts in Python. For instance, we’ll see later that module imports, function and class definitions, for loop variables, and function arguments are all implicit assignments. Because assignment works the same everywhere it pops up, all these contexts simply bind names to object references at runtime. Assignment Statement Forms Although assignment is a general and pervasive concept in Python, we are primarily interested in assignment statements in this chapter. Table 11-1 illustrates the different assignment statement forms in Python. Table 11-1. Assignment statement forms Operation Interpretation spam = 'Spam' Basic form spam, ham = 'yum', 'YUM' Tuple assignment (positional) [spam, ham] = ['yum', 'YUM'] List assignment (positional) a, b, c, d = 'spam' Sequence assignment, generalized a, *b = 'spam' Extended sequence unpacking (Python 3.0) spam = ham = 'lunch' Multiple-target assignment spams += 42 Augmented assignment (equivalent to spams = spams + 42) The first form in Table 11-1 is by far the most common: binding a name (or data structure component) to a single object. In fact, you could get all your work done with this basic form alone. The other table entries represent special forms that are all optional, but that programmers often find convenient in practice: Tuple- and list-unpacking assignments The second and third forms in the table are related. When you code a tuple or list on the left side of the =, Python pairs objects on the right side with targets on the left by position and assigns them from left to right. For example, in the second line of Table 11-1, the name spam is assigned the string 'yum', and the name ham is bound to the string 'YUM'. In this case Python internally makes a tuple of the items on the right, which is why this is called tuple-unpacking assignment. Sequence assignments In recent versions of Python, tuple and list assignments have been generalized into instances of what we now call sequence assignment—any sequence of names can be assigned to any sequence of values, and Python assigns the items one at a time by position. We can even mix and match the types of the sequences involved. The fourth line in Table 11-1, for example, pairs a tuple of names with a string of characters: a is assigned 's', b is assigned 'p', and so on. 280 | Chapter 11: Assignments, Expressions, and Prints Extended sequence unpacking In Python 3.0, a new form of sequence assignment allows us to be more flexible in how we select portions of a sequence to assign. The fifth line in Table 11-1, for example, matches a with the first character in the string on the right and b with the rest: a is assigned 's', and b is assigned 'pam'. This provides a simpler alternative to assigning the results of manual slicing operations. Multiple-target assignments The sixth line in Table 11-1 shows the multiple-target form of assignment. In this form, Python assigns a reference to the same object (the object farthest to the right) to all the targets on the left. In the table, the names spam and ham are both assigned references to the same string object, 'lunch'. The effect is the same as if we had coded ham = 'lunch' followed by spam = ham, as ham evaluates to the original string object (i.e., not a separate copy of that object). Augmented assignments The last line in Table 11-1 is an example of augmented assignment—a shorthand that combines an expression and an assignment in a concise way. Saying spam += 42, for example, has the same effect as spam = spam + 42, but the augmented form requires less typing and is generally quicker to run. In addition, if the subject is mutable and supports the operation, an augmented assignment may run even quicker by choosing an in-place update operation instead of an object copy. There is one augmented assignment statement for every binary expression operator in Python. Sequence Assignments We’ve already used basic assignments in this book. Here are a few simple examples of sequence-unpacking assignments in action: % python >>> nudge = 1 >>> wink = 2 >>> A, B = nudge, wink >>> A, B (1, 2) >>> [C, D] = [nudge, wink] >>> C, D (1, 2) # Tuple assignment # Like A = nudge; B = wink # List assignment Notice that we really are coding two tuples in the third line in this interaction—we’ve just omitted their enclosing parentheses. Python pairs the values in the tuple on the right side of the assignment operator with the variables in the tuple on the left side and assigns the values one at a time. Tuple assignment leads to a common coding trick in Python that was introduced in a solution to the exercises at the end of Part II. Because Python creates a temporary tuple that saves the original values of the variables on the right while the statement runs, Assignment Statements | 281 unpacking assignments are also a way to swap two variables’ values without creating a temporary variable of your own—the tuple on the right remembers the prior values of the variables automatically: >>> >>> >>> >>> (2, nudge = 1 wink = 2 nudge, wink = wink, nudge nudge, wink 1) # Tuples: swaps values # Like T = nudge; nudge = wink; wink = T In fact, the original tuple and list assignment forms in Python have been generalized to accept any type of sequence on the right as long as it is of the same length as the sequence on the left. You can assign a tuple of values to a list of variables, a string of characters to a tuple of variables, and so on. In all cases, Python assigns items in the sequence on the right to variables in the sequence on the left by position, from left to right: >>> [a, b, c] = (1, 2, 3) >>> a, c (1, 3) >>> (a, b, c) = "ABC" >>> a, c ('A', 'C') # Assign tuple of values to list of names # Assign string of characters to tuple Technically speaking, sequence assignment actually supports any iterable object on the right, not just any sequence. This is a more general concept that we will explore in Chapters 14 and 20. Advanced sequence assignment patterns Although we can mix and match sequence types around the = symbol, we must have the same number of items on the right as we have variables on the left, or we’ll get an error. Python 3.0 allows us to be more general with extended unpacking syntax, described in the next section. But normally, and always in Python 2.X, the number of items in the assignment target and subject must match: >>> string = 'SPAM' >>> a, b, c, d = string >>> a, d ('S', 'M') >>> a, b, c = string ...error text omitted... ValueError: too many values to unpack # Same number on both sides # Error if not To be more general, we can slice. There are a variety of ways to employ slicing to make this last case work: >>> a, b, c = string[0], string[1], string[2:] >>> a, b, c ('S', 'P', 'AM') # Index and slice >>> a, b, c = list(string[:2]) + [string[2:]] >>> a, b, c # Slice and concatenate 282 | Chapter 11: Assignments, Expressions, and Prints ('S', 'P', 'AM') >>> a, b = string[:2] >>> c = string[2:] >>> a, b, c ('S', 'P', 'AM') # Same, but simpler >>> (a, b), c = string[:2], string[2:] >>> a, b, c ('S', 'P', 'AM') # Nested sequences As the last example in this interaction demonstrates, we can even assign nested sequences, and Python unpacks their parts according to their shape, as expected. In this case, we are assigning a tuple of two items, where the first item is a nested sequence (a string), exactly as though we had coded it this way: # Paired by shape and position >>> ((a, b), c) = ('SP', 'AM') >>> a, b, c ('S', 'P', 'AM') Python pairs the first string on the right ('SP') with the first tuple on the left ((a, b)) and assigns one character at a time, before assigning the entire second string ('AM') to the variable c all at once. In this event, the sequence-nesting shape of the object on the left must match that of the object on the right. Nested sequence assignment like this is somewhat advanced, and rare to see, but it can be convenient for picking out the parts of data structures with known shapes. For example, we’ll see in Chapter 13 that this technique also works in for loops, because loop items are assigned to the target given in the loop header: for (a, b, c) in [(1, 2, 3), (4, 5, 6)]: ... # Simple tuple assignment for ((a, b), c) in [((1, 2), 3), ((4, 5), 6)]: ... # Nested tuple assignment In a note in Chapter 18, we’ll also see that this nested tuple (really, sequence) unpacking assignment form works for function argument lists in Python 2.6 (though not in 3.0), because function arguments are passed by assignment as well: def f(((a, b), c)): f(((1, 2), 3)) # For arguments too in Python 2.6, but not 3.0 Sequence-unpacking assignments also give rise to another common coding idiom in Python—assigning an integer series to a set of variables: >>> red, green, blue = range(3) >>> red, blue (0, 2) This initializes the three names to the integer codes 0, 1, and 2, respectively (it’s Python’s equivalent of the enumerated data types you may have seen in other languages). To make sense of this, you need to know that the range built-in function generates a list of successive integers: Assignment Statements | 283 >>> range(3) [0, 1, 2] # Use list(range(3)) in Python 3.0 Because range is commonly used in for loops, we’ll say more about it in Chapter 13. Another place you may see a tuple assignment at work is for splitting a sequence into its front and the rest in loops like this: >>> L = [1, 2, 3, 4] >>> while L: ... front, L = L[0], L[1:] ... print(front, L) ... 1 [2, 3, 4] 2 [3, 4] 3 [4] 4 [] # See next section for 3.0 alternative The tuple assignment in the loop here could be coded as the following two lines instead, but it’s often more convenient to string them together: ... ... front = L[0] L = L[1:] Notice that this code is using the list as a sort of stack data structure, which can often also be achieved with the append and pop methods of list objects; here, front = L.pop(0) would have much the same effect as the tuple assignment statement, but it would be an in-place change. We’ll learn more about while loops, and other (often better) ways to step through a sequence with for loops, in Chapter 13. Extended Sequence Unpacking in Python 3.0 The prior section demonstrated how to use manual slicing to make sequence assignments more general. In Python 3.0 (but not 2.6), sequence assignment has been generalized to make this easier. In short, a single starred name, *X, can be used in the assignment target in order to specify a more general matching against the sequence— the starred name is assigned a list, which collects all items in the sequence not assigned to other names. This is especially handy for common coding patterns such as splitting a sequence into its “front” and “rest”, as in the preceding section’s last example. Extended unpacking in action Let’s look at an example. As we’ve seen, sequence assignments normally require exactly as many names in the target on the left as there are items in the subject on the right. We get an error if the lengths disagree (unless we manually sliced on the right, as shown in the prior section): C:\misc> c:\python30\python >>> seq = [1, 2, 3, 4] >>> a, b, c, d = seq >>> print(a, b, c, d) 1 2 3 4 284 | Chapter 11: Assignments, Expressions, and Prints >>> a, b = seq ValueError: too many values to unpack In Python 3.0, though, we can use a single starred name in the target to match more generally. In the following continuation of our interactive session, a matches the first item in the sequence, and b matches the rest: >>> >>> 1 >>> [2, a, *b = seq a b 3, 4] When a starred name is used, the number of items in the target on the left need not match the length of the subject sequence. In fact, the starred name can appear anywhere in the target. For instance, in the next interaction b matches the last item in the sequence, and a matches everything before the last: >>> >>> [1, >>> 4 *a, b = seq a 2, 3] b When the starred name appears in the middle, it collects everything between the other names listed. Thus, in the following interaction a and c are assigned the first and last items, and b gets everything in between them: >>> >>> 1 >>> [2, >>> 4 a, *b, c = seq a b 3] c More generally, wherever the starred name shows up, it will be assigned a list that collects every unassigned name at that position: >>> >>> 1 >>> 2 >>> [3, a, b, *c = seq a b c 4] Naturally, like normal sequence assignment, extended sequence unpacking syntax works for any sequence types, not just lists. Here it is unpacking characters in a string: >>> a, *b = 'spam' >>> a, b ('s', ['p', 'a', 'm']) >>> a, *b, c = 'spam' Assignment Statements | 285 >>> a, b, c ('s', ['p', 'a'], 'm') This is similar in spirit to slicing, but not exactly the same—a sequence unpacking assignment always returns a list for multiple matched items, whereas slicing returns a sequence of the same type as the object sliced: >>> S = 'spam' >>> S[0], S[1:] ('s', 'pam') # Slices are type-specific, * assignment always returns a list >>> S[0], S[1:3], S[3] ('s', 'pa', 'm') Given this extension in 3.0, as long as we’re processing a list the last example of the prior section becomes even simpler, since we don’t have to manually slice to get the first and rest of the items: >>> L = [1, 2, 3, 4] >>> while L: ... front, *L = L ... print(front, L) ... 1 [2, 3, 4] 2 [3, 4] 3 [4] 4 [] # Get first, rest without slicing Boundary cases Although extended sequence unpacking is flexible, some boundary cases are worth noting. First, the starred name may match just a single item, but is always assigned a list: >>> seq [1, 2, 3, 4] >>> a, b, c, *d = seq >>> print(a, b, c, d) 1 2 3 [4] Second, if there is nothing left to match the starred name, it is assigned an empty list, regardless of where it appears. In the following, a, b, c, and d have matched every item in the sequence, but Python assigns e an empty list instead of treating this as an error case: >>> a, b, c, d, *e = seq >>> print(a, b, c, d, e) 1 2 3 4 [] >>> a, b, *e, c, d = seq >>> print(a, b, c, d, e) 1 2 3 4 [] 286 | Chapter 11: Assignments, Expressions, and Prints Finally, errors can still be triggered if there is more than one starred name, if there are too few values and no star (as before), and if the starred name is not itself coded inside a sequence: >>> a, *b, c, *d = seq SyntaxError: two starred expressions in assignment >>> a, b = seq ValueError: too many values to unpack >>> *a = seq SyntaxError: starred assignment target must be in a list or tuple >>> *a, = seq >>> a [1, 2, 3, 4] A useful convenience Keep in mind that extended sequence unpacking assignment is just a convenience. We can usually achieve the same effects with explicit indexing and slicing (and in fact must in Python 2.X), but extended unpacking is simpler to code. The common “first, rest” splitting coding pattern, for example, can be coded either way, but slicing involves extra work: >>> seq [1, 2, 3, 4] >>> a, *b = seq >>> a, b (1, [2, 3, 4]) # First, rest >>> a, b = seq[0], seq[1:] >>> a, b (1, [2, 3, 4]) # First, rest: traditional The also common “rest, last” splitting pattern can similarly be coded either way, but the new extended unpacking syntax requires noticeably fewer keystrokes: >>> *a, b = seq >>> a, b ([1, 2, 3], 4) # Rest, last >>> a, b = seq[:-1], seq[-1] >>> a, b ([1, 2, 3], 4) # Rest, last: traditional Because it is not only simpler but, arguably, more natural, extended sequence unpacking syntax will likely become widespread in Python code over time. Assignment Statements | 287 Application to for loops Because the loop variable in the for loop statement can be any assignment target, extended sequence assignment works here too. We met the for loop iteration tool briefly in Part II and will study it formally in Chapter 13. In Python 3.0, extended assignments may show up after the word for, where a simple variable name is more commonly used: for (a, *b, c) in [(1, 2, 3, 4), (5, 6, 7, 8)]: ... When used in this context, on each iteration Python simply assigns the next tuple of values to the tuple of names. On the first loop, for example, it’s as if we’d run the following assignment statement: a, *b, c = (1, 2, 3, 4) # b gets [2, 3] The names a, b, and c can be used within the loop’s code to reference the extracted components. In fact, this is really not a special case at all, but just an instance of general assignment at work. As we saw earlier in this chapter, we can do the same thing with simple tuple assignment in both Python 2.X and 3.X: for (a, b, c) in [(1, 2, 3), (4, 5, 6)]: # a, b, c = (1, 2, 3), ... And we can always emulate 3.0’s extended assignment behavior in 2.6 by manually slicing: for all in [(1, 2, 3, 4), (5, 6, 7, 8)]: a, b, c = all[0], all[1:3], all[3] Since we haven’t learned enough to get more detailed about the syntax of for loops, we’ll return to this topic in Chapter 13. Multiple-Target Assignments A multiple-target assignment simply assigns all the given names to the object all the way to the right. The following, for example, assigns the three variables a, b, and c to the string 'spam': >>> a = b = c = 'spam' >>> a, b, c ('spam', 'spam', 'spam') This form is equivalent to (but easier to code than) these three assignments: >>> c = 'spam' >>> b = c >>> a = b Multiple-target assignment and shared references Keep in mind that there is just one object here, shared by all three variables (they all wind up pointing to the same object in memory). This behavior is fine for immutable types—for example, when initializing a set of counters to zero (recall that variables 288 | Chapter 11: Assignments, Expressions, and Prints must be assigned before they can be used in Python, so you must initialize counters to zero before you can start adding to them): >>> >>> >>> (0, a = b = 0 b = b + 1 a, b 1) Here, changing b only changes b because numbers do not support in-place changes. As long as the object assigned is immutable, it’s irrelevant if more than one name references it. As usual, though, we have to be more cautious when initializing variables to an empty mutable object such as a list or dictionary: >>> a = b = [] >>> b.append(42) >>> a, b ([42], [42]) This time, because a and b reference the same object, appending to it in-place through b will impact what we see through a as well. This is really just another example of the shared reference phenomenon we first met in Chapter 6. To avoid the issue, initialize mutable objects in separate statements instead, so that each creates a distinct empty object by running a distinct literal expression: >>> a = [] >>> b = [] >>> b.append(42) >>> a, b ([], [42]) Augmented Assignments Beginning with Python 2.0, the set of additional assignment statement formats listed in Table 11-2 became available. Known as augmented assignments, and borrowed from the C language, these formats are mostly just shorthand. They imply the combination of a binary expression and an assignment. For instance, the following two formats are now roughly equivalent: # Traditional form # Newer augmented form X = X + Y X += Y Table 11-2. Augmented assignment statements X += Y X &= Y X -= Y X |= Y X *= Y X ^= Y X /= Y X >>= Y X %= Y X <<= Y X **= Y X //= Y Augmented assignment works on any type that supports the implied binary expression. For example, here are two ways to add 1 to a name: Assignment Statements | 289 >>> >>> >>> 2 >>> >>> 3 x = 1 x = x + 1 x x += 1 x # Traditional # Augmented When applied to a string, the augmented form performs concatenation instead. Thus, the second line here is equivalent to typing the longer S = S + "SPAM": >>> S = "spam" >>> S += "SPAM" >>> S 'spamSPAM' # Implied concatenation As shown in Table 11-2, there are analogous augmented assignment forms for every Python binary expression operator (i.e., each operator with values on the left and right side). For instance, X *= Y multiplies and assigns, X >>= Y shifts right and assigns, and so on. X //= Y (for floor division) was added in version 2.2. Augmented assignments have three advantages:* • There’s less for you to type. Need I say more? • The left side only has to be evaluated once. In X += Y, X may be a complicated object expression. In the augmented form, it only has to be evaluated once. However, in the long form, X = X + Y, X appears twice and must be run twice. Because of this, augmented assignments usually run faster. • The optimal technique is automatically chosen. That is, for objects that support in-place changes, the augmented forms automatically perform in-place change operations instead of slower copies. The last point here requires a bit more explanation. For augmented assignments, inplace operations may be applied for mutable objects as an optimization. Recall that lists can be extended in a variety of ways. To add a single item to the end of a list, we can concatenate or call append: >>> >>> >>> [1, >>> >>> [1, L = [1, 2] L = L + [3] L 2, 3] L.append(4) L 2, 3, 4] # Concatenate: slower # Faster, but in-place * C/C++ programmers take note: although Python now supports statements like X += Y, it still does not have C’s auto-increment/decrement operators (e.g., X++, −−X). These don’t quite map to the Python object model because Python has no notion of in-place changes to immutable objects like numbers. 290 | Chapter 11: Assignments, Expressions, and Prints And to add a set of items to the end, we can either concatenate again or call the list extend method:† >>> >>> [1, >>> >>> [1, L = L + [5, 6] L 2, 3, 4, 5, 6] L.extend([7, 8]) L 2, 3, 4, 5, 6, 7, 8] # Concatenate: slower # Faster, but in-place In both cases, concatenation is less prone to the side effects of shared object references but will generally run slower than the in-place equivalent. Concatenation operations must create a new object, copy in the list on the left, and then copy in the list on the right. By contrast, in-place method calls simply add items at the end of a memory block. When we use augmented assignment to extend a list, we can forget these details—for example, Python automatically calls the quicker extend method instead of using the slower concatenation operation implied by +: >>> L += [9, 10] # Mapped to L.extend([9, 10]) >>> L [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] Augmented assignment and shared references This behavior is usually what we want, but notice that it implies that the += is an inplace change for lists; thus, it is not exactly like + concatenation, which always makes a new object. As for all shared reference cases, this difference might matter if other names reference the object being changed: >>> L = [1, 2] >>> M = L >>> L = L + [3, 4] >>> L, M ([1, 2, 3, 4], [1, 2]) >>> L = [1, 2] >>> M = L >>> L += [3, 4] >>> L, M ([1, 2, 3, 4], [1, 2, 3, 4]) # L and M reference the same object # Concatenation makes a new object # Changes L but not M # But += really means extend # M sees the in-place change too! This only matters for mutables like lists and dictionaries, and it is a fairly obscure case (at least, until it impacts your code!). As always, make copies of your mutable objects if you need to break the shared reference structure. † As suggested in Chapter 6, we can also use slice assignment (e.g., L[len(L):] = [11,12,13]), but this works roughly the same as the simpler list extend method. Assignment Statements | 291 Variable Name Rules Now that we’ve explored assignment statements, it’s time to get more formal about the use of variable names. In Python, names come into existence when you assign values to them, but there are a few rules to follow when picking names for things in your programs: Syntax: (underscore or letter) + (any number of letters, digits, or underscores) Variable names must start with an underscore or letter, which can be followed by any number of letters, digits, or underscores. _spam, spam, and Spam_1 are legal names, but 1_Spam, spam$, and @#! are not. Case matters: SPAM is not the same as spam Python always pays attention to case in programs, both in names you create and in reserved words. For instance, the names X and x refer to two different variables. For portability, case also matters in the names of imported module files, even on platforms where the filesystems are case-insensitive. Reserved words are off-limits Names you define cannot be the same as words that mean special things in the Python language. For instance, if you try to use a variable name like class, Python will raise a syntax error, but klass and Class work fine. Table 11-3 lists the words that are currently reserved (and hence off-limits for names of your own) in Python. Table 11-3. Python 3.0 reserved words False class finally is return None continue for lambda try True def from nonlocal while and del global not with as elif if or yield assert else import pass break except in raise Table 11-3 is specific to Python 3.0. In Python 2.6, the set of reserved words differs slightly: • print is a reserved word, because printing is a statement, not a built-in (more on this later in this chapter). • exec is a reserved word, because it is a statement, not a built-in function. • nonlocal is not a reserved word because this statement is not available. In older Pythons the story is also more or less the same, with a few variations: 292 | Chapter 11: Assignments, Expressions, and Prints • with and as were not reserved until 2.6, when context managers were officially enabled. • yield was not reserved until Python 2.3, when generator functions were enabled. • yield morphed from statement to expression in 2.5, but it’s still a reserved word, not a built-in function. As you can see, most of Python’s reserved words are all lowercase. They are also all truly reserved—unlike names in the built-in scope that you will meet in the next part of this book, you cannot redefine reserved words by assignment (e.g., and = 1 results in a syntax error).‡ Besides being of mixed case, the first three entries in Table 11-3, True, False, and None, are somewhat unusual in meaning—they also appear in the built-in scope of Python described in Chapter 17, and they are technically names assigned to objects. They are truly reserved in all other senses, though, and cannot be used for any other purpose in your script other than that of the objects they represent. All the other reserved words are hardwired into Python’s syntax and can appear only in the specific contexts for which they are intended. Furthermore, because module names in import statements become variables in your scripts, variable name constraints extend to your module filenames too. For instance, you can code files called and.py and my-code.py and run them as top-level scripts, but you cannot import them: their names without the “.py” extension become variables in your code and so must follow all the variable rules just outlined. Reserved words are off-limits, and dashes won’t work, though underscores will. We’ll revisit this idea in Part V of this book. Python’s Deprecation Protocol It is interesting to note how reserved word changes are gradually phased into the language. When a new feature might break existing code, Python normally makes it an option and begins issuing “deprecation” warnings one or more releases before the feature is officially enabled. The idea is that you should have ample time to notice the warnings and update your code before migrating to the new release. This is not true for major new releases like 3.0 (which breaks existing code freely), but it is generally true in other cases. For example, yield was an optional extension in Python 2.2, but is a standard keyword as of 2.3. It is used in conjunction with generator functions. This was one of a small handful of instances where Python broke with backward compatibility. Still, yield was phased in over time: it began generating deprecation warnings in 2.2 and was not enabled until 2.3. ‡ In the Jython Java-based implementation of Python, though, user-defined variable names can sometimes be the same as Python reserved words. See Chapter 2 for an overview of the Jython system. Assignment Statements | 293 Similarly, in Python 2.6, the words with and as become new reserved words for use in context managers (a newer form of exception handling). These two words are not reserved in 2.5, unless the context manager feature is turned on manually with a from__future__import (discussed later in this book). When used in 2.5, with and as generate warnings about the upcoming change—except in the version of IDLE in Python 2.5, which appears to have enabled this feature for you (that is, using these words as variable names does generate errors in 2.5, but only in its version of the IDLE GUI). Naming conventions Besides these rules, there is also a set of naming conventions—rules that are not required but are followed in normal practice. For instance, because names with two leading and trailing underscores (e.g., __name__) generally have special meaning to the Python interpreter, you should avoid this pattern for your own names. Here is a list of the conventions Python follows: • Names that begin with a single underscore (_X) are not imported by a from module import * statement (described in Chapter 22). • Names that have two leading and trailing underscores (__X__) are system-defined names that have special meaning to the interpreter. • Names that begin with two underscores and do not end with two more (__X) are localized (“mangled”) to enclosing classes (see the discussion of pseudoprivate attributes in Chapter 30). • The name that is just a single underscore (_) retains the result of the last expression when working interactively. In addition to these Python interpreter conventions, there are various other conventions that Python programmers usually follow. For instance, later in the book we’ll see that class names commonly start with an uppercase letter and module names with a lowercase letter, and that the name self, though not reserved, usually has a special role in classes. In Chapter 17 we’ll also study another, larger category of names known as the built-ins, which are predefined but not reserved (and so can be reassigned: open = 42 works, though sometimes you might wish it didn’t!). Names have no type, but objects do This is mostly review, but remember that it’s crucial to keep Python’s distinction between names and objects clear. As described in Chapter 6, objects have a type (e.g., integer, list) and may be mutable or not. Names (a.k.a. variables), on the other hand, are always just references to objects; they have no notion of mutability and have no associated type information, apart from the type of the object they happen to reference at a given point in time. 294 | Chapter 11: Assignments, Expressions, and Prints Thus, it’s OK to assign the same name to different kinds of objects at different times: >>> x = 0 >>> x = "Hello" >>> x = [1, 2, 3] # x bound to an integer object # Now it's a string # And now it's a list In later examples, you’ll see that this generic nature of names can be a decided advantage in Python programming. In Chapter 17, you’ll also learn that names also live in something called a scope, which defines where they can be used; the place where you assign a name determines where it is visible.§ For additional naming suggestions, see the previous section “Naming conventions” of Python’s semi-official style guide, known as PEP 8. This guide is available at http://www.python.org/dev/peps/pep-0008, or via a web search for “Python PEP 8.” Technically, this document formalizes coding standards for Python library code. Though useful, the usual caveats about coding standards apply here. For one thing, PEP 8 comes with more detail than you are probably ready for at this point in the book. And frankly, it has become more complex, rigid, and subjective than it needs to be—some of its suggestions are not at all universally accepted or followed by Python programmers doing real work. Moreover, some of the most prominent companies using Python today have adopted coding standards of their own that differ. PEP 8 does codify useful rule-of-thumb Python knowledge, though, and it’s a great read for Python beginners, as long as you take its recommendations as guidelines, not gospel. Expression Statements In Python, you can use an expression as a statement, too—that is, on a line by itself. But because the result of the expression won’t be saved, it usually makes sense to do so only if the expression does something useful as a side effect. Expressions are commonly used as statements in two situations: For calls to functions and methods Some functions and methods do lots of work without returning a value. Such functions are sometimes called procedures in other languages. Because they don’t return values that you might be interested in retaining, you can call these functions with expression statements. § If you’ve used a more restrictive language like C++, you may be interested to know that there is no notion of C++’s const declaration in Python; certain objects may be immutable, but names can always be assigned. Python also has ways to hide names in classes and modules, but they’re not the same as C++’s declarations (if hiding attributes matters to you, see the coverage of _X module names in Chapter 24, __X class names in Chapter 30, and the Private and Public class decorators example in Chapter 38). Expression Statements | 295 For printing values at the interactive prompt Python echoes back the results of expressions typed at the interactive command line. Technically, these are expression statements, too; they serve as a shorthand for typing print statements. Table 11-4 lists some common expression statement forms in Python. Calls to functions and methods are coded with zero or more argument objects (really, expressions that evaluate to objects) in parentheses, after the function/method name. Table 11-4. Common Python expression statements Operation Interpretation spam(eggs, ham) Function calls spam.ham(eggs) Method calls spam Printing variables in the interactive interpreter print(a, b, c, sep='') Printing operations in Python 3.0 yield x ** 2 Yielding expression statements The last two entries in Table 11-4 are somewhat special cases—as we’ll see later in this chapter, printing in Python 3.0 is a function call usually coded on a line by itself, and the yield operation in generator functions (discussed in Chapter 20) is often coded as a statement as well. Both are really just instances of expression statements. For instance, though you normally run a print call on a line by itself as an expression statement, it returns a value like any other function call (its return value is None, the default return value for functions that don’t return anything meaningful): >>> x = print('spam') spam >>> print(x) None # print is a function call expression in 3.0 # But it is coded as an expression statement Also keep in mind that although expressions can appear as statements in Python, statements cannot be used as expressions. For example, Python doesn’t allow you to embed assignment statements (=) in other expressions. The rationale for this is that it avoids common coding mistakes; you can’t accidentally change a variable by typing = when you really mean to use the == equality test. You’ll see how to code around this when you meet the Python while loop in Chapter 13. Expression Statements and In-Place Changes This brings up a mistake that is common in Python work. Expression statements are often used to run list methods that change a list in-place: >>> >>> >>> [1, L = [1, 2] L.append(3) L 2, 3] # Append is an in-place change 296 | Chapter 11: Assignments, Expressions, and Prints However, it’s not unusual for Python newcomers to code such an operation as an assignment statement instead, intending to assign L to the larger list: >>> L = L.append(4) >>> print(L) None # But append returns None, not L # So we lose our list! This doesn’t quite work, though. Calling an in-place change operation such as append, sort, or reverse on a list always changes the list in-place, but these methods do not return the list they have changed; instead, they return the None object. Thus, if you assign such an operation’s result back to the variable name, you effectively lose the list (and it is probably garbage collected in the process!). The moral of the story is, don’t do this. We’ll revisit this phenomenon in the section “Common Coding Gotchas” on page 387 at the end of this part of the book because it can also appear in the context of some looping statements we’ll meet in later chapters. Print Operations In Python, print prints things—it’s simply a programmer-friendly interface to the standard output stream. Technically, printing converts one or more objects to their textual representations, adds some minor formatting, and sends the resulting text to either standard output or another file-like stream. In a bit more detail, print is strongly bound up with the notions of files and streams in Python: File object methods In Chapter 9, we learned about file object methods that write text (e.g., file.write(str)). Printing operations are similar, but more focused—whereas file write methods write strings to arbitrary files, print writes objects to the stdout stream by default, with some automatic formatting added. Unlike with file methods, there is no need to convert objects to strings when using print operations. Standard output stream The standard output stream (often known as stdout) is simply a default place to send a program’s text output. Along with the standard input and error streams, it’s one of three data connections created when your script starts. The standard output stream is usually mapped to the window where you started your Python program, unless it’s been redirected to a file or pipe in your operating system’s shell. Because the standard output stream is available in Python as the stdout file object in the built-in sys module (i.e., sys.stdout), it’s possible to emulate print with file write method calls. However, print is noticeably easier to use and makes it easy to print text to other files and streams. Print Operations | 297 Printing is also one of the most visible places where Python 3.0 and 2.6 have diverged. In fact, this divergence is usually the first reason that most 2.X code won’t run unchanged under 3.X. Specifically, the way you code print operations depends on which version of Python you use: • In Python 3.X, printing is a built-in function, with keyword arguments for special modes. • In Python 2.X, printing is a statement with specific syntax all its own. Because this book covers both 3.0 and 2.6, we will look at each form in turn here. If you are fortunate enough to be able to work with code written for just one version of Python, feel free to pick the section that is relevant to you; however, as your circumstances may change, it probably won’t hurt to be familiar with both cases. The Python 3.0 print Function Strictly speaking, printing is not a separate statement form in 3.0. Instead, it is simply an instance of the expression statement we studied in the preceding section. The print built-in function is normally called on a line of its own, because it doesn’t return any value we care about (technically, it returns None). Because it is a normal function, though, printing in 3.0 uses standard function-call syntax, rather than a special statement form. Because it provides special operation modes with keyword arguments, this form is both more general and supports future enhancements better. By comparison, Python 2.6 print statements have somewhat ad-hoc syntax to support extensions such as end-of-line suppression and target files. Further, the 2.6 statement does not support separator specification at all; in 2.6, you wind up building strings ahead of time more often than you do in 3.0. Call format Syntactically, calls to the 3.0 print function have the following form: print([object, ...][, sep=' '][, end='\n'][, file=sys.stdout]) In this formal notation, items in square brackets are optional and may be omitted in a given call, and values after = give argument defaults. In English, this built-in function prints the textual representation of one or more objects separated by the string sep and followed by the string end to the stream file. The sep, end, and file parts, if present, must be given as keyword arguments—that is, you must use a special “name=value” syntax to pass the arguments by name instead of position. Keyword arguments are covered in depth in Chapter 18, but they’re straightforward to use. The keyword arguments sent to this call may appear in any left-to-right order following the objects to be printed, and they control the print operation: 298 | Chapter 11: Assignments, Expressions, and Prints • sep is a string inserted between each object’s text, which defaults to a single space if not passed; passing an empty string suppresses separators altogether. • end is a string added at the end of the printed text, which defaults to a \n newline character if not passed. Passing an empty string avoids dropping down to the next output line at the end of the printed text—the next print will keep adding to the end of the current output line. • file specifies the file, standard stream, or other file-like object to which the text will be sent; it defaults to the sys.stdout standard output stream if not passed. Any object with a file-like write(string) method may be passed, but real files should be already opened for output. The textual representation of each object to be printed is obtained by passing the object to the str built-in call; as we’ve seen, this built-in returns a “user friendly” display string for any object.‖ With no arguments at all, the print function simply prints a newline character to the standard output stream, which usually displays a blank line. The 3.0 print function in action Printing in 3.0 is probably simpler than some of its details may imply. To illustrate, let’s run some quick examples. The following prints a variety of object types to the default standard output stream, with the default separator and end-of-line formatting added (these are the defaults because they are the most common use case): C:\misc> c:\python30\python >>> >>> print() >>> x = 'spam' >>> y = 99 >>> z = ['eggs'] >>> >>> print(x, y, z) spam 99 ['eggs'] # Display a blank line # Print 3 objects per defaults There’s no need to convert objects to strings here, as would be required for file write methods. By default, print calls add a space between the objects printed. To suppress this, send an empty string to the sep keyword argument, or send an alternative separator of your choosing: >>> print(x, y, z, sep='') spam99['eggs'] >>> >>> print(x, y, z, sep=', ') spam, 99, ['eggs'] # Suppress separator # Custom separator ‖ Technically, printing uses the equivalent of str in the internal implementation of Python, but the effect is the same. Besides this to-string conversion role, str is also the name of the string data type and can be used to decode Unicode strings from raw bytes with an extra encoding argument, as we’ll learn in Chapter 36; this latter role is an advanced usage that we can safely ignore here. Print Operations | 299 Also by default, print adds an end-of-line character to terminate the output line. You can suppress this and avoid the line break altogether by passing an empty string to the end keyword argument, or you can pass a different terminator of your own (include a \n character to break the line manually): >>> print(x, y, z, end='') spam 99 ['eggs']>>> >>> >>> print(x, y, z, end=''); print(x, y, z) spam 99 ['eggs']spam 99 ['eggs'] >>> print(x, y, z, end='...\n') spam 99 ['eggs']... >>> # Suppress line break # Two prints, same output line # Custom line end You can also combine keyword arguments to specify both separators and end-of-line strings—they may appear in any order but must appear after all the objects being printed: >>> print(x, y, z, sep='...', end='!\n') spam...99...['eggs']! >>> print(x, y, z, end='!\n', sep='...') spam...99...['eggs']! # Multiple keywords # Order doesn't matter Here is how the file keyword argument is used—it directs the printed text to an open output file or other compatible object for the duration of the single print (this is really a form of stream redirection, a topic we will revisit later in this section): >>> print(x, y, z, sep='...', file=open('data.txt', 'w')) >>> print(x, y, z) spam 99 ['eggs'] >>> print(open('data.txt').read()) spam...99...['eggs'] # Print to a file # Back to stdout # Display file text Finally, keep in mind that the separator and end-of-line options provided by print operations are just conveniences. If you need to display more specific formatting, don’t print this way, Instead, build up a more complex string ahead of time or within the print itself using the string tools we met in Chapter 7, and print the string all at once: >>> text = '%s: %-.4f, %05d' % ('Result', 3.14159, 42) >>> print(text) Result: 3.1416, 00042 >>> print('%s: %-.4f, %05d' % ('Result', 3.14159, 42)) Result: 3.1416, 00042 As we’ll see in the next section, almost everything we’ve just seen about the 3.0 print function also applies directly to 2.6 print statements—which makes sense, given that the function was intended to both emulate and improve upon 2.6 printing support. The Python 2.6 print Statement As mentioned earlier, printing in Python 2.6 uses a statement with unique and specific syntax, rather than a built-in function. In practice, though, 2.6 printing is mostly a variation on a theme; with the exception of separator strings (which are supported in 300 | Chapter 11: Assignments, Expressions, and Prints 3.0 but not 2.6), everything we can do with the 3.0 print function has a direct translation to the 2.6 print statement. Statement forms Table 11-5 lists the print statement’s forms in Python 2.6 and gives their Python 3.0 print function equivalents for reference. Notice that the comma is significant in print statements—it separates objects to be printed, and a trailing comma suppresses the end-of-line character normally added at the end of the printed text (not to be confused with tuple syntax!). The >> syntax, normally used as a bitwise right-shift operation, is used here as well, to specify a target output stream other than the sys.stdout default. Table 11-5. Python 2.6 print statement forms Python 2.6 statement Python 3.0 equivalent Interpretation print x, y print(x, y) Print objects’ textual forms to sys.stdout; add a space between the items and an end-of-line at the end print x, y, print(x, y, end='') Same, but don’t add end-of-line at end of text print >> afile, x, y print(x, y, file=afile) Send text to myfile.write, not to sys.stdout.write The 2.6 print statement in action Although the 2.6 print statement has more unique syntax than the 3.0 function, it’s similarly easy to use. Let’s turn to some basic examples again. By default, the 2.6 print statement adds a space between the items separated by commas and adds a line break at the end of the current output line: C:\misc> c:\python26\python >>> >>> x = 'a' >>> y = 'b' >>> print x, y a b This formatting is just a default; you can choose to use it or not. To suppress the line break so you can add more text to the current line later, end your print statement with a comma, as shown in the second line of Table 11-5 (the following is two statements on one line, separated by a semicolon): >>> print x, y,; print x, y a b a b Print Operations | 301 To suppress the space between items, again, don’t print this way. Instead, build up an output string using the string concatenation and formatting tools covered in Chapter 7, and print the string all at once: >>> print x + y ab >>> print '%s...%s' % (x, y) a...b As you can see, apart from their special syntax for usage modes, 2.6 print statements are roughly as simple to use as 3.0’s function. The next section uncovers the way that files are specified in 2.6 prints. Print Stream Redirection In both Python 3.0 and 2.6, printing sends text to the standard output stream by default. However, it’s often useful to send it elsewhere—to a text file, for example, to save results for later use or testing purposes. Although such redirection can be accomplished in system shells outside Python itself, it turns out to be just as easy to redirect a script’s streams from within the script. The Python “hello world” program Let’s start off with the usual (and largely pointless) language benchmark—the “hello world” program. To print a “hello world” message in Python, simply print the string per your version’s print operation: >>> print('hello world') hello world # Print a string object in 3.0 >>> print 'hello world' hello world # Print a string object in 2.6 Because expression results are echoed on the interactive command line, you often don’t even need to use a print statement there—simply type the expressions you’d like to have printed, and their results are echoed back: >>> 'hello world' 'hello world' # Interactive echoes This code isn’t exactly an earth-shattering piece of software mastery, but it serves to illustrate printing behavior. Really, the print operation is just an ergonomic feature of Python—it provides a simple interface to the sys.stdout object, with a bit of default formatting. In fact, if you enjoy working harder than you must, you can also code print operations this way: >>> import sys >>> sys.stdout.write('hello world\n') hello world 302 | Chapter 11: Assignments, Expressions, and Prints # Printing the hard way This code explicitly calls the write method of sys.stdout—an attribute preset when Python starts up to an open file object connected to the output stream. The print operation hides most of those details, providing a simple tool for simple printing tasks. Manual stream redirection So, why did I just show you the hard way to print? The sys.stdout print equivalent turns out to be the basis of a common technique in Python. In general, print and sys.stdout are directly related as follows. This statement: print(X, Y) # Or, in 2.6: print X, Y is equivalent to the longer: import sys sys.stdout.write(str(X) + ' ' + str(Y) + '\n') which manually performs a string conversion with str, adds a separator and newline with +, and calls the output stream’s write method. Which would you rather code? (He says, hoping to underscore the programmer-friendly nature of prints....) Obviously, the long form isn’t all that useful for printing by itself. However, it is useful to know that this is exactly what print operations do because it is possible to reassign sys.stdout to something different from the standard output stream. In other words, this equivalence provides a way of making your print operations send their text to other places. For example: import sys sys.stdout = open('log.txt', 'a') ... print(x, y, x) # Redirects prints to a file # Shows up in log.txt Here, we reset sys.stdout to a manually opened file named log.txt, located in the script’s working directory and opened in append mode (so we add to its current content). After the reset, every print operation anywhere in the program will write its text to the end of the file log.txt instead of to the original output stream. The print operations are happy to keep calling sys.stdout’s write method, no matter what sys.stdout happens to refer to. Because there is just one sys module in your process, assigning sys.stdout this way will redirect every print anywhere in your program. In fact, as this chapter’s upcoming sidebar about print and stdout will explain, you can even reset sys.stdout to an object that isn’t a file at all, as long as it has the expected interface: a method named write to receive the printed text string argument. When that object is a class, printed text can be routed and processed arbitrarily per a write method you code yourself. This trick of resetting the output stream is primarily useful for programs originally coded with print statements. If you know that output should go to a file to begin with, you can always call file write methods instead. To redirect the output of a print-based Print Operations | 303 program, though, resetting sys.stdout provides a convenient alternative to changing every print statement or using system shell-based redirection syntax. Automatic stream redirection This technique of redirecting printed text by assigning sys.stdout is commonly used in practice. One potential problem with the last section’s code, though, is that there is no direct way to restore the original output stream should you need to switch back after printing to a file. Because sys.stdout is just a normal file object, you can always save it and restore it if needed:# C:\misc> c:\python30\python >>> import sys >>> temp = sys.stdout >>> sys.stdout = open('log.txt', 'a') >>> print('spam') >>> print(1, 2, 3) >>> sys.stdout.close() >>> sys.stdout = temp >>> print('back here') back here >>> print(open('log.txt').read()) spam 1 2 3 # Save for restoring later # Redirect prints to a file # Prints go to file, not here # Flush output to disk # Restore original stream # Prints show up here again # Result of earlier prints As you can see, though, manual saving and restoring of the original output stream like this involves quite a bit of extra work. Because this crops up fairly often, a print extension is available to make it unnecessary. In 3.0, the file keyword allows a single print call to send its text to a file’s write method, without actually resetting sys.stdout. Because the redirection is temporary, normal print calls keep printing to the original output stream. In 2.6, a print statement that begins with a >> followed by an output file object (or other compatible object) has the same effect. For example, the following again sends printed text to a file named log.txt: log = open('log.txt', 'a') print(x, y, z, file=log) print(a, b, c) # 3.0 # Print to a file-like object # Print to original stdout log = open('log.txt', 'a') print >> log, x, y, z print a, b, c # 2.6 # Print to a file-like object # Print to original stdout These redirected forms of print are handy if you need to print to both files and the standard output stream in the same program. If you use these forms, however, be sure #In both 2.6 and 3.0 you may also be able to use the __stdout__ attribute in the sys module, which refers to the original value sys.stdout had at program startup time. You still need to restore sys.stdout to sys.__stdout__ to go back to this original stream value, though. See the sys module documentation for more details. 304 | Chapter 11: Assignments, Expressions, and Prints to give them a file object (or an object that has the same write method as a file object), not a file’s name string. Here is the technique in action: C:\misc> c:\python30\python >>> log = open('log.txt', 'w') >>> print(1, 2, 3, file=log) >>> print(4, 5, 6, file=log) >>> log.close() >>> print(7, 8, 9) 7 8 9 >>> print(open('log.txt').read()) 1 2 3 4 5 6 # 2.6: print >> log, 1, 2, 3 # 2.6: print 7, 8, 9 These extended forms of print are also commonly used to print error messages to the standard error stream, available to your script as the preopened file object sys.stderr. You can either use its file write methods and format the output manually, or print with redirection syntax: >>> import sys >>> sys.stderr.write(('Bad!' * 8) + '\n') Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad! >>> print('Bad!' * 8, file=sys.stderr) Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad! # 2.6: print >> sys.stderr, 'Bad' * 8 Now that you know all about print redirections, the equivalence between printing and file write methods should be fairly obvious. The following interaction prints both ways in 3.0, then redirects the output to an external file to verify that the same text is printed: >>> >>> 1 2 >>> >>> 1 2 4 >>> X = 1; Y = 2 print(X, Y) >>> 4 >>> b'1 >>> b'1 open('temp2', 'w').write(str(X) + ' ' + str(Y) + '\n') # Send to file manually # Print: the easy way import sys sys.stdout.write(str(X) + ' ' + str(Y) + '\n') # Print: the hard way print(X, Y, file=open('temp1', 'w')) # Redirect text to file print(open('temp1', 'rb').read()) 2\r\n' print(open('temp2', 'rb').read()) 2\r\n' # Binary mode for bytes As you can see, unless you happen to enjoy typing, print operations are usually the best option for displaying text. For another example of the equivalence between prints and file writes, watch for a 3.0 print function emulation example in Chapter 18; it uses this code pattern to provide a general 3.0 print function equivalent for use in Python 2.6. Print Operations | 305 Version-Neutral Printing Finally, if you cannot restrict your work to Python 3.0 but still want your prints to be compatible with 3.0, you have some options. For one, you can code 2.6 print statements and let 3.0’s 2to3 conversion script translate them to 3.0 function calls automatically. See the Python 3.0 documentation for more details about this script; it attempts to translate 2.X code to run under 3.0. Alternatively, you can code 3.0 print function calls in your 2.6 code, by enabling the function call variant with a statement like the following: from __future__ import print_function This statement changes 2.6 to support 3.0’s print functions exactly. This way, you can use 3.0 print features and won’t have to change your prints if you later migrate to 3.0. Also keep in mind that simple prints, like those in the first row of Table 11-5, work in either version of Python—because any expression may be enclosed in parentheses, we can always pretend to be calling a 3.0 print function in 2.6 by adding outer parentheses. The only downside to this is that it makes a tuple out of your printed objects if there are more than one—they will print with extra enclosing parentheses. In 3.0, for example, any number of objects may be listed in the call’s parentheses: C:\misc> c:\python30\python >>> print('spam') spam >>> print('spam', 'ham', 'eggs') spam ham eggs # 3.0 print function call syntax # These are mutiple argments The first of these works the same in 2.6, but the second generates a tuple in the output: C:\misc> c:\python26\python >>> print('spam') spam >>> print('spam', 'ham', 'eggs') ('spam', 'ham', 'eggs') # 2.6 print statement, enclosing parens # This is really a tuple object! To be truly portable, you can format the print string as a single object, using the string formatting expression or method call, or other string tools that we studied in Chapter 7: >>> print('%s %s %s' % ('spam', 'ham', 'eggs')) spam ham eggs >>> print('{0} {1} {2}'.format('spam', 'ham', 'eggs')) spam ham eggs Of course, if you can use 3.0 exclusively you can forget such mappings entirely, but many Python programmers will at least encounter, if not write, 2.X code and systems for some time to come. 306 | Chapter 11: Assignments, Expressions, and Prints I use Python 3.0 print function calls throughout this book. I’ll usually warn you that the results may have extra enclosing parentheses in 2.6 because multiple items are a tuple, but I sometimes don’t, so please consider this note a blanket warning—if you see extra parentheses in your printed text in 2.6, either drop the parentheses in your print statements, recode your prints using the version-neutral scheme outlined here, or learn to love superfluous text. Why You Will Care: print and stdout The equivalence between the print operation and writing to sys.stdout is important. It makes it possible to reassign sys.stdout to any user-defined object that provides the same write method as files. Because the print statement just sends text to the sys.stdout.write method, you can capture printed text in your programs by assigning sys.stdout to an object whose write method processes the text in arbitrary ways. For instance, you can send printed text to a GUI window, or tee it off to multiple destinations, by defining an object with a write method that does the required routing. You’ll see an example of this trick when we study classes in Part VI of this book, but abstractly, it looks like this: class FileFaker: def write(self, string): # Do something with printed text in string import sys sys.stdout = FileFaker() print(someObjects) # Sends to class write method This works because print is what we will call in the next part of this book a polymorphic operation—it doesn’t care what sys.stdout is, only that it has a method (i.e., interface) called write. This redirection to objects is made even simpler with the file keyword argument in 3.0 and the >> extended form of print in 2.6, because we don’t need to reset sys.stdout explicitly—normal prints will still be routed to the stdout stream: myobj = FileFaker() # 3.0: Redirect to object for one print print(someObjects, file=myobj) # Does not reset sys.stdout myobj = FileFaker() print >> myobj, someObjects # 2.6: same effect # Does not reset sys.stdout Python’s built-in input function reads from the sys.stdin file, so you can intercept read requests in a similar way, using classes that implement file-like read methods instead. See the input and while loop example in Chapter 10 for more background on this. Notice that because printed text goes to the stdout stream, it’s the way to print HTML in CGI scripts used on the Web. It also enables you to redirect Python script input and output at the operating system’s shell command line, as usual: Print Operations | 307 python script.py < inputfile > outputfile python script.py | filterProgram Python’s print operation redirection tools are essentially pure-Python alternatives to these shell syntax forms. Chapter Summary In this chapter, we began our in-depth look at Python statements by exploring assignments, expressions, and print operations. Although these are generally simple to use, they have some alternative forms that, while optional, are often convenient in practice: augmented assignment statements and the redirection form of print operations, for example, allow us to avoid some manual coding work. Along the way, we also studied the syntax of variable names, stream redirection techniques, and a variety of common mistakes to avoid, such as assigning the result of an append method call back to a variable. In the next chapter, we’ll continue our statement tour by filling in details about the if statement, Python’s main selection tool; there, we’ll also revisit Python’s syntax model in more depth and look at the behavior of Boolean expressions. Before we move on, though, the end-of-chapter quiz will test your knowledge of what you’ve learned here. Test Your Knowledge: Quiz 1. 2. 3. 4. Name three ways that you can assign three variables to the same value. Why might you need to care when assigning three variables to a mutable object? What’s wrong with saying L = L.sort()? How might you use the print operation to send text to an external file? Test Your Knowledge: Answers 1. You can use multiple-target assignments (A = B = C = 0), sequence assignment (A, B, C = 0, 0, 0), or multiple assignment statements on three separate lines (A = 0, B = 0, and C = 0). With the latter technique, as introduced in Chapter 10, you can also string the three separate statements together on the same line by separating them with semicolons (A = 0; B = 0; C = 0). 308 | Chapter 11: Assignments, Expressions, and Prints 2. If you assign them this way: A = B = C = [] all three names reference the same object, so changing it in-place from one (e.g., A.append(99)) will affect the others. This is true only for in-place changes to mu- table objects like lists and dictionaries; for immutable objects such as numbers and strings, this issue is irrelevant. 3. The list sort method is like append in that it makes an in-place change to the subject list—it returns None, not the list it changes. The assignment back to L sets L to None, not to the sorted list. As we’ll see later in this part of the book, a newer builtin function, sorted, sorts any sequence and returns a new list with the sorting result; because this is not an in-place change, its result can be meaningfully assigned to a name. 4. To print to a file for a single print operation, you can use 3.0’s print(X, file=F) call form, use 2.6’s extended print >> file, X statement form, or assign sys.stdout to a manually opened file before the print and restore the original after. You can also redirect all of a program’s printed text to a file with special syntax in the system shell, but this is outside Python’s scope. Test Your Knowledge: Answers | 309 CHAPTER 12 if Tests and Syntax Rules This chapter presents the Python if statement, which is the main statement used for selecting from alternative actions based on test results. Because this is our first in-depth look at compound statements—statements that embed other statements—we will also explore the general concepts behind the Python statement syntax model here in more detail than we did in the introduction in Chapter 10. Because the if statement introduces the notion of tests, this chapter will also deal with Boolean expressions and fill in some details on truth tests in general. if Statements In simple terms, the Python if statement selects actions to perform. It’s the primary selection tool in Python and represents much of the logic a Python program possesses. It’s also our first compound statement. Like all compound Python statements, the if statement may contain other statements, including other ifs. In fact, Python lets you combine statements in a program sequentially (so that they execute one after another), and in an arbitrarily nested fashion (so that they execute only under certain conditions). General Format The Python if statement is typical of if statements in most procedural languages. It takes the form of an if test, followed by one or more optional elif (“else if”) tests and a final optional else block. The tests and the else part each have an associated block of nested statements, indented under a header line. When the if statement runs, Python executes the block of code associated with the first test that evaluates to true, or the else block if all tests prove false. The general form of an if statement looks like this: if <test1>: <statements1> elif <test2>: <statements2> else: <statements3> # if test # Associated block # Optional elifs # Optional else 311 Basic Examples To demonstrate, let’s look at a few simple examples of the if statement at work. All parts are optional, except the initial if test and its associated statements. Thus, in the simplest case, the other parts are omitted: >>> if 1: ... print('true') ... true Notice how the prompt changes to ... for continuation lines when typing interactively in the basic interface used here; in IDLE, you’ll simply drop down to an indented line instead (hit Backspace to back up). A blank line (which you can get by pressing Enter twice) terminates and runs the entire statement. Remember that 1 is Boolean true, so this statement’s test always succeeds. To handle a false result, code the else: >>> if not 1: ... print('true') ... else: ... print('false') ... false Multiway Branching Now here’s an example of a more complex if statement, with all its optional parts present: >>> >>> ... ... ... ... ... ... Run x = 'killer rabbit' if x == 'roger': print("how's jessica?") elif x == 'bugs': print("what's up doc?") else: print('Run away! Run away!') away! Run away! This multiline statement extends from the if line through the else block. When it’s run, Python executes the statements nested under the first test that is true, or the else part if all tests are false (in this example, they are). In practice, both the elif and else parts may be omitted, and there may be more than one statement nested in each section. Note that the words if, elif, and else are associated by the fact that they line up vertically, with the same indentation. If you’ve used languages like C or Pascal, you might be interested to know that there is no switch or case statement in Python that selects an action based on a variable’s value. Instead, multiway branching is coded either as a series of if/elif tests, as in the prior example, or by indexing dictionaries or searching lists. Because dictionaries and lists can be built at runtime, they’re sometimes more flexible than hardcoded if logic: 312 | Chapter 12: if Tests and Syntax Rules >>> choice = 'ham' >>> print({'spam': ... 'ham': ... 'eggs': ... 'bacon': 1.99 1.25, # A dictionary-based 'switch' 1.99, # Use has_key or get for default 0.99, 1.10}[choice]) Although it may take a few moments for this to sink in the first time you see it, this dictionary is a multiway branch—indexing on the key choice branches to one of a set of values, much like a switch in C. An almost equivalent but more verbose Python if statement might look like this: >>> if choice == 'spam': ... print(1.25) ... elif choice == 'ham': ... print(1.99) ... elif choice == 'eggs': ... print(0.99) ... elif choice == 'bacon': ... print(1.10) ... else: ... print('Bad choice') ... 1.99 Notice the else clause on the if here to handle the default case when no key matches. As we saw in Chapter 8, dictionary defaults can be coded with in expressions, get method calls, or exception catching. All of the same techniques can be used here to code a default action in a dictionary-based multiway branch. Here’s the get scheme at work with defaults: >>> branch = {'spam': 1.25, ... 'ham': 1.99, ... 'eggs': 0.99} >>> print(branch.get('spam', 'Bad choice')) 1.25 >>> print(branch.get('bacon', 'Bad choice')) Bad choice An in membership test in an if statement can have the same default effect: >>> >>> ... ... ... ... Bad choice = 'bacon' if choice in branch: print(branch[choice]) else: print('Bad choice') choice Dictionaries are good for associating values with keys, but what about the more complicated actions you can code in the statement blocks associated with if statements? In Part IV, you’ll learn that dictionaries can also contain functions to represent more complex branch actions and implement general jump tables. Such functions appear as if Statements | 313 dictionary values, may be coded as function names or lambdas, and are called by adding parentheses to trigger their actions; stay tuned for more on this topic in Chapter 19. Although dictionary-based multiway branching is useful in programs that deal with more dynamic data, most programmers will probably find that coding an if statement is the most straightforward way to perform multiway branching. As a rule of thumb in coding, when in doubt, err on the side of simplicity and readability; it’s the “Pythonic” way. Python Syntax Rules I introduced Python’s syntax model in Chapter 10. Now that we’re stepping up to larger statements like the if, this section reviews and expands on the syntax ideas introduced earlier. In general, Python has a simple, statement-based syntax. However, there are a few properties you need to know about: • Statements execute one after another, until you say otherwise. Python normally runs statements in a file or nested block in order from first to last, but statements like if (and, as you’ll see, loops) cause the interpreter to jump around in your code. Because Python’s path through a program is called the control flow, statements such as if that affect it are often called control-flow statements. • Block and statement boundaries are detected automatically. As we’ve seen, there are no braces or “begin/end” delimiters around blocks of code in Python; instead, Python uses the indentation of statements under a header to group the statements in a nested block. Similarly, Python statements are not normally terminated with semicolons; rather, the end of a line usually marks the end of the statement coded on that line. • Compound statements = header + “:” + indented statements. All compound statements in Python follow the same pattern: a header line terminated with a colon, followed by one or more nested statements, usually indented under the header. The indented statements are called a block (or sometimes, a suite). In the if statement, the elif and else clauses are part of the if, but they are also header lines with nested blocks of their own. • Blank lines, spaces, and comments are usually ignored. Blank lines are ignored in files (but not at the interactive prompt, when they terminate compound statements). Spaces inside statements and expressions are almost always ignored (except in string literals, and when used for indentation). Comments are always ignored: they start with a # character (not inside a string literal) and extend to the end of the current line. • Docstrings are ignored but are saved and displayed by tools. Python supports an additional comment form called documentation strings (docstrings for short), which, unlike # comments, are retained at runtime for inspection. Docstrings are simply strings that show up at the top of program files and some statements. Python 314 | Chapter 12: if Tests and Syntax Rules ignores their contents, but they are automatically attached to objects at runtime and may be displayed with documentation tools. Docstrings are part of Python’s larger documentation strategy and are covered in the last chapter in this part of the book. As you’ve seen, there are no variable type declarations in Python; this fact alone makes for a much simpler language syntax than what you may be used to. However, for most new users the lack of the braces and semicolons used to mark blocks and statements in many other languages seems to be the most novel syntactic feature of Python, so let’s explore what this means in more detail. Block Delimiters: Indentation Rules Python detects block boundaries automatically, by line indentation—that is, the empty space to the left of your code. All statements indented the same distance to the right belong to the same block of code. In other words, the statements within a block line up vertically, as in a column. The block ends when the end of the file or a lesser-indented line is encountered, and more deeply nested blocks are simply indented further to the right than the statements in the enclosing block. For instance, Figure 12-1 demonstrates the block structure of the following code: x = 1 if x: y = 2 if y: print('block2') print('block1') print('block0') Figure 12-1. Nested blocks of code: a nested block starts with a statement indented further to the right and ends with either a statement that is indented less, or the end of the file. Python Syntax Rules | 315 This code contains three blocks: the first (the top-level code of the file) is not indented at all, the second (within the outer if statement) is indented four spaces, and the third (the print statement under the nested if) is indented eight spaces. In general, top-level (unnested) code must start in column 1. Nested blocks can start in any column; indentation may consist of any number of spaces and tabs, as long as it’s the same for all the statements in a given single block. That is, Python doesn’t care how you indent your code; it only cares that it’s done consistently. Four spaces or one tab per indentation level are common conventions, but there is no absolute standard in the Python world. Indenting code is quite natural in practice. For example, the following (arguably silly) code snippet demonstrates common indentation errors in Python code: x = 'SPAM' if 'rubbery' in 'shrubbery': print(x * 8) x += 'NI' if x.endswith('NI'): x *= 2 print(x) # Error: first line indented # Error: unexpected indentation # Error: inconsistent indentation The properly indented version of this code looks like the following—even for an artificial example like this, proper indentation makes the code’s intent much more apparent: x = 'SPAM' if 'rubbery' in 'shrubbery': print(x * 8) x += 'NI' if x.endswith('NI'): x *= 2 print(x) # Prints "SPAMNISPAMNI" It’s important to know that the only major place in Python where whitespace matters is where it’s used to the left of your code, for indentation; in most other contexts, space can be coded or not. However, indentation is really part of Python syntax, not just a stylistic suggestion: all the statements within any given single block must be indented to the same level, or Python reports a syntax error. This is intentional—because you don’t need to explicitly mark the start and end of a nested block of code, some of the syntactic clutter found in other languages is unnecessary in Python. As described in Chapter 10, making indentation part of the syntax model also enforces consistency, a crucial component of readability in structured programming languages like Python. Python’s syntax is sometimes described as “what you see is what you get”—the indentation of each line of code unambiguously tells readers what it is associated with. This uniform and consistent appearance makes Python code easier to maintain and reuse. 316 | Chapter 12: if Tests and Syntax Rules Indentation is more natural than the details might imply, and it makes your code reflect its logical structure. Consistently indented code always satisfies Python’s rules. Moreover, most text editors (including IDLE) make it easy to follow Python’s indentation model by automatically indenting code as you type it. Avoid mixing tabs and spaces: New error checking in 3.0 One rule of thumb: although you can use spaces or tabs to indent, it’s usually not a good idea to mix the two within a block—use one or the other. Technically, tabs count for enough spaces to move the current column number up to a multiple of 8, and your code will work if you mix tabs and spaces consistently. However, such code can be difficult to change. Worse, mixing tabs and spaces makes your code difficult to read— tabs may look very different in the next programmer’s editor than they do in yours. In fact, Python 3.0 now issues an error, for these very reasons, when a script mixes tabs and spaces for indentation inconsistently within a block (that is, in a way that makes it dependent on a tab’s equivalent in spaces). Python 2.6 allows such scripts to run, but it has a -t command-line flag that will warn you about inconsistent tab usage and a -tt flag that will issue errors for such code (you can use these switches in a command line like python –t main.py in a system shell window). Python 3.0’s error case is equivalent to 2.6’s -tt switch. Statement Delimiters: Lines and Continuations A statement in Python normally ends at the end of the line on which it appears. When a statement is too long to fit on a single line, though, a few special rules may be used to make it span multiple lines: • Statements may span multiple lines if you’re continuing an open syntactic pair. Python lets you continue typing a statement on the next line if you’re coding something enclosed in a (), {}, or [] pair. For instance, expressions in parentheses and dictionary and list literals can span any number of lines; your statement doesn’t end until the Python interpreter reaches the line on which you type the closing part of the pair (a ), }, or ]). Continuation lines (lines 2 and beyond of the statement) can start at any indentation level you like, but you should try to make them align vertically for readability if possible. This open pairs rule also covers set and dictionary comprehensions in Python 3.0. • Statements may span multiple lines if they end in a backslash. This is a somewhat outdated feature, but if a statement needs to span multiple lines, you can also add a backslash (a \ not embedded in a string literal or comment) at the end of the prior line to indicate you’re continuing on the next line. Because you can also continue by adding parentheses around most constructs, backslashes are almost never used. This approach is error-prone: accidentally forgetting a \ usually generates a syntax error and might even cause the next line to be silently mistaken to be a new statement, with unexpected results. Python Syntax Rules | 317 • Special rules for string literals. As we learned in Chapter 7, triple-quoted string blocks are designed to span multiple lines normally. We also learned in Chapter 7 that adjacent string literals are implicitly concatenated; when used in conjunction with the open pairs rule mentioned earlier, wrapping this construct in parentheses allows it to span multiple lines. • Other rules. There are a few other points to mention with regard to statement delimiters. Although uncommon, you can terminate a statement with a semicolon—this convention is sometimes used to squeeze more than one simple (noncompound) statement onto a single line. Also, comments and blank lines can appear anywhere in a file; comments (which begin with a # character) terminate at the end of the line on which they appear. A Few Special Cases Here’s what a continuation line looks like using the open syntactic pairs rule. Delimited constructs, such as lists in square brackets, can span across any number of lines: L = ["Good", "Bad", "Ugly"] # Open pairs may span lines This also works for anything in parentheses (expressions, function arguments, function headers, tuples, and generator expressions), as well as anything in curly braces (dictionaries and, in 3.0, set literals and set and dictionary comprehensions). Some of these are tools we’ll study in later chapters, but this rule naturally covers most constructs that span lines in practice. If you like using backslashes to continue lines, you can, but it’s not common practice in Python: if a == b and c == d and d == e and f == g: print('olde') \ # Backslashes allow continuations... Because any expression can be enclosed in parentheses, you can usually use the open pairs technique instead if you need your code to span multiple lines—simply wrap a part of your statement in parentheses: if (a == b and c == d and d == e and e == f): print('new') # But parentheses usually do too In fact, backslashes are frowned on, because they’re too easy to not notice and too easy to omit altogether. In the following, x is assigned 10 with the backslash, as intended; if the backslash is accidentally omitted, though, x is assigned 6 instead, and no error is reported (the +4 is a valid expression statement by itself). 318 | Chapter 12: if Tests and Syntax Rules In a real program with a more complex assignment, this could be the source of a very nasty bug:* x = 1 + 2 + 3 \ +4 # Omitting the \ makes this very different As another special case, Python allows you to write more than one noncompound statement (i.e., statements without nested statements) on the same line, separated by semicolons. Some coders use this form to save program file real estate, but it usually makes for more readable code if you stick to one statement per line for most of your work: x = 1; y = 2; print(x) # More than one simple statement As we learned in Chapter 7, triple-quoted string literals span lines too. In addition, if two string literals appear next to each other, they are concatenated as if a + had been added between them—when used in conjunction with the open pairs rule, wrapping in parentheses allows this form to span multiple lines. For example, the first of the following inserts newline characters at line breaks and assigns S to '\naaaa\nbbbb \ncccc', and the second implicitly concatenates and assigns S to 'aaaabbbbcccc'; comments are ignored in the second form, but included in the string in the first: S = """ aaaa bbbb cccc""" S = ('aaaa' 'bbbb' 'cccc') # Comments here are ignored Finally, Python lets you move a compound statement’s body up to the header line, provided the body is just a simple (noncompound) statement. You’ll most often see this used for simple if statements with a single test and action: if 1: print('hello') # Simple statement on header line You can combine some of these special cases to write code that is difficult to read, but I don’t recommend it; as a rule of thumb, try to keep each statement on a line of its own, and indent all but the simplest of blocks. Six months down the road, you’ll be happy you did. * Frankly, it’s surprising that this wasn’t removed in Python 3.0, given some of its other changes! (See Table P-2 of the Preface for a list of 3.0 removals; some seem fairly innocuous in comparison with the dangers inherent in backslash continuations.) Then again, this book’s goal is Python instruction, not populist outrage, so the best advice I can give is simply: don’t do this. Python Syntax Rules | 319 Truth Tests The notions of comparison, equality, and truth values were introduced in Chapter 9. Because the if statement is the first statement we’ve looked at that actually uses test results, we’ll expand on some of these ideas here. In particular, Python’s Boolean operators are a bit different from their counterparts in languages like C. In Python: • • • • • Any nonzero number or nonempty object is true. Zero numbers, empty objects, and the special object None are considered false. Comparisons and equality tests are applied recursively to data structures. Comparisons and equality tests return True or False (custom versions of 1 and 0). Boolean and and or operators return a true or false operand object. In short, Boolean operators are used to combine the results of other tests. There are three Boolean expression operators in Python: X and Y Is true if both X and Y are true X or Y Is true if either X or Y is true not X Is true if X is false (the expression returns True or False) Here, X and Y may be any truth value, or any expression that returns a truth value (e.g., an equality test, range comparison, and so on). Boolean operators are typed out as words in Python (instead of C’s &&, ||, and !). Also, Boolean and and or operators return a true or false object in Python, not the values True or False. Let’s look at a few examples to see how this works: >>> 2 < 3, 3 < 2 (True, False) # Less-than: return True or False (1 or 0) Magnitude comparisons such as these return True or False as their truth results, which, as we learned in Chapters 5 and 9, are really just custom versions of the integers 1 and 0 (they print themselves differently but are otherwise the same). On the other hand, the and and or operators always return an object—either the object on the left side of the operator or the object on the right. If we test their results in if or other statements, they will be as expected (remember, every object is inherently true or false), but we won’t get back a simple True or False. 320 | Chapter 12: if Tests and Syntax Rules For or tests, Python evaluates the operand objects from left to right and returns the first one that is true. Moreover, Python stops at the first true operand it finds. This is usually called short-circuit evaluation, as determining a result short-circuits (terminates) the rest of the expression: >>> (2, >>> 3 >>> {} 2 or 3, 3 or 2 3) [] or 3 # Return left operand if true # Else, return right operand (true or false) [] or {} In the first line of the preceding example, both operands (2 and 3) are true (i.e., are nonzero), so Python always stops and returns the one on the left. In the other two tests, the left operand is false (an empty object), so Python simply evaluates and returns the object on the right (which may happen to have either a true or a false value when tested). and operations also stop as soon as the result is known; however, in this case Python evaluates the operands from left to right and stops at the first false object: >>> (3, >>> [] >>> [] 2 and 3, 3 and 2 2) [] and {} # Return left operand if false # Else, return right operand (true or false) 3 and [] Here, both operands are true in the first line, so Python evaluates both sides and returns the object on the right. In the second test, the left operand is false ([]), so Python stops and returns it as the test result. In the last test, the left side is true (3), so Python evaluates and returns the object on the right (which happens to be a false []). The end result of all this is the same as in C and most other languages—you get a value that is logically true or false if tested in an if or while. However, in Python Booleans return either the left or the right object, not a simple integer flag. This behavior of and and or may seem esoteric at first glance, but see this chapter’s sidebar “Why You Will Care: Booleans” on page 323 for examples of how it is sometimes used to advantage in coding by Python programmers. The next section also shows a common way to leverage this behavior, and its replacement in more recent versions of Python. The if/else Ternary Expression One common role for the prior section’s Boolean operators is to code an expression that runs the same as an if statement. Consider the following statement, which sets A to either Y or Z, based on the truth value of X: The if/else Ternary Expression | 321 if X: A = Y else: A = Z Sometimes, though, the items involved in such a statement are so simple that it seems like overkill to spread them across four lines. At other times, we may want to nest such a construct in a larger statement instead of assigning its result to a variable. For these reasons (and, frankly, because the C language has a similar tool†), Python 2.5 introduced a new expression format that allows us to say the same thing in one expression: A = Y if X else Z This expression has the exact same effect as the preceding four-line if statement, but it’s simpler to code. As in the statement equivalent, Python runs expression Y only if X turns out to be true, and runs expression Z only if X turns out to be false. That is, it short-circuits, just like the Boolean operators described in the prior section. Here are some examples of it in action: >>> >>> 't' >>> >>> 'f' A = 't' if 'spam' else 'f' A # Nonempty is true A = 't' if '' else 'f' A Prior to Python 2.5 (and after 2.5, if you insist), the same effect can often be achieved by a careful combination of the and and or operators, because they return either the object on the left side or the object on the right: A = ((X and Y) or Z) This works, but there is a catch—you have to be able to assume that Y will be Boolean true. If that is the case, the effect is the same: the and runs first and returns Y if X is true; if it’s not, the or simply returns Z. In other words, we get “if X then Y else Z.” This and/or combination also seems to require a “moment of great clarity” to understand the first time you see it, and it’s no longer required as of 2.5—use the equivalent and more robust and mnemonic Y if X else Z instead if you need this as an expression, or use a full if statement if the parts are nontrivial. As a side note, using the following expression in Python is similar because the bool function will translate X into the equivalent of integer 1 or 0, which can then be used to pick true and false values from a list: A = [Z, Y][bool(X)] † In fact, Python’s X if Y else Z has a slightly different order than C’s Y ? X : Z. This was reportedly done in response to analysis of common use patterns in Python code. According to rumor, this order was also chosen in part to discourage ex-C programmers from overusing it! Remember, simple is better than complex, in Python and elsewhere. 322 | Chapter 12: if Tests and Syntax Rules For example: >>> ['f', 't'][bool('')] 'f' >>> ['f', 't'][bool('spam')] 't' However, this isn’t exactly the same, because Python will not short-circuit—it will always run both Z and Y, regardless of the value of X. Because of such complexities, you’re better off using the simpler and more easily understood if/else expression as of Python 2.5 and later. Again, though, you should use even that sparingly, and only if its parts are all fairly simple; otherwise, you’re better off coding the full if statement form to make changes easier in the future. Your coworkers will be happy you did. Still, you may see the and/or version in code written prior to 2.5 (and in code written by C programmers who haven’t quite let go of their dark coding pasts...). Why You Will Care: Booleans One common way to use the somewhat unusual behavior of Python Boolean operators is to select from a set of objects with an or. A statement such as this: X = A or B or C or None sets X to the first nonempty (that is, true) object among A, B, and C, or to None if all of them are empty. This works because the or operator returns one of its two objects, and it turns out to be a fairly common coding paradigm in Python: to select a nonempty object from among a fixed-size set, simply string them together in an or expression. In simpler form, this is also commonly used to designate a default—the following sets X to A if A is true (or nonempty), and to default otherwise: X = A or default It’s also important to understand short-circuit evaluation because expressions on the right of a Boolean operator might call functions that perform substantial or important work, or have side effects that won’t happen if the short-circuit rule takes effect: if f1() or f2(): ... Here, if f1 returns a true (or nonempty) value, Python will never run f2. To guarantee that both functions will be run, call them before the or: tmp1, tmp2 = f1(), f2() if tmp1 or tmp2: ... You’ve already seen another application of this behavior in this chapter: because of the way Booleans work, the expression ((A and B) or C) can be used to emulate an if/ else statement—almost (see this chapter’s discussion of this form for details). We met additional Boolean use cases in prior chapters. As we saw in Chapter 9, because all objects are inherently true or false, it’s common and easier in Python to test an object directly ( if X:) than to compare it to an empty value (if X != '':). For a string, the two tests are equivalent. As we also saw in Chapter 5, the preset Booleans values True and False are the same as the integers 1 and 0 and are useful for initializing variables The if/else Ternary Expression | 323 (X = False), for loop tests (while True:), and for displaying results at the interactive prompt. Also watch for the discussion of operator overloading in Part VI: when we define new object types with classes, we can specify their Boolean nature with either the __bool__ or __len__ methods (__bool__ is named __nonzero__ in 2.6). The latter of these is tried if the former is absent and designates false by returning a length of zero—an empty object is considered false. Chapter Summary In this chapter, we studied the Python if statement. Additionally, because this was our first compound and logical statement, we reviewed Python’s general syntax rules and explored the operation of truth tests in more depth than we were able to previously. Along the way, we also looked at how to code multiway branching in Python and learned about the if/else expression introduced in Python 2.5. The next chapter continues our look at procedural statements by expanding on the while and for loops. There, we’ll learn about alternative ways to code loops in Python, some of which may be better than others. Before that, though, here is the usual chapter quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. How might you code a multiway branch in Python? How can you code an if/else statement as an expression in Python? How can you make a single statement span many lines? What do the words True and False mean? Test Your Knowledge: Answers 1. An if statement with multiple elif clauses is often the most straightforward way to code a multiway branch, though not necessarily the most concise. Dictionary indexing can often achieve the same result, especially if the dictionary contains callable functions coded with def statements or lambda expressions. 2. In Python 2.5 and later, the expression form Y if X else Z returns Y if X is true, or Z otherwise; it’s the same as a four-line if statement. The and/or combination (((X and Y) or Z)) can work the same way, but it’s more obscure and requires that the Y part be true. 324 | Chapter 12: if Tests and Syntax Rules 3. Wrap up the statement in an open syntactic pair ((), [], or {}), and it can span as many lines as you like; the statement ends when Python sees the closing (right) half of the pair, and lines 2 and beyond of the statement can begin at any indentation level. 4. True and False are just custom versions of the integers 1 and 0, respectively: they always stand for Boolean true and false values in Python. They’re available for use in truth tests and variable initialization and are printed for expression results at the interactive prompt. Test Your Knowledge: Answers | 325 CHAPTER 13 while and for Loops This chapter concludes our tour of Python procedural statements by presenting the language’s two main looping constructs—statements that repeat an action over and over. The first of these, the while statement, provides a way to code general loops. The second, the for statement, is designed for stepping through the items in a sequence object and running a block of code for each. We’ve seen both of these informally already, but we’ll fill in additional usage details here. While we’re at it, we’ll also study a few less prominent statements used within loops, such as break and continue, and cover some built-ins commonly used with loops, such as range, zip, and map. Although the while and for statements covered here are the primary syntax provided for coding repeated actions, there are additional looping operations and concepts in Python. Because of that, the iteration story is continued in the next chapter, where we’ll explore the related ideas of Python’s iteration protocol (used by the for loop) and list comprehensions (a close cousin to the for loop). Later chapters explore even more exotic iteration tools such as generators, filter, and reduce. For now, though, let’s keep things simple. while Loops Python’s while statement is the most general iteration construct in the language. In simple terms, it repeatedly executes a block of (normally indented) statements as long as a test at the top keeps evaluating to a true value. It is called a “loop” because control keeps looping back to the start of the statement until the test becomes false. When the test becomes false, control passes to the statement that follows the while block. The net effect is that the loop’s body is executed repeatedly while the test at the top is true; if the test is false to begin with, the body never runs. 327 General Format In its most complex form, the while statement consists of a header line with a test expression, a body of one or more indented statements, and an optional else part that is executed if control exits the loop without a break statement being encountered. Python keeps evaluating the test at the top and executing the statements nested in the loop body until the test returns a false value: while <test>: <statements1> else: <statements2> # Loop test # Loop body # Optional else # Run if didn't exit loop with break Examples To illustrate, let’s look at a few simple while loops in action. The first, which consists of a print statement nested in a while loop, just prints a message forever. Recall that True is just a custom version of the integer 1 and always stands for a Boolean true value; because the test is always true, Python keeps executing the body forever, or until you stop its execution. This sort of behavior is usually called an infinite loop: >>> while True: ... print('Type Ctrl-C to stop me!') The next example keeps slicing off the first character of a string until the string is empty and hence false. It’s typical to test an object directly like this instead of using the more verbose equivalent (while x != '':). Later in this chapter, we’ll see other ways to step more directly through the items in a string with a for loop. >>> x = 'spam' >>> while x: ... print(x, end=' ') ... x = x[1:] ... spam pam am m # While x is not empty # Strip first character off x Note the end=' ' keyword argument used here to place all outputs on the same line separated by a space; see Chapter 11 if you’ve forgotten why this works as it does. The following code counts from the value of a up to, but not including, b. We’ll see an easier way to do this with a Python for loop and the built-in range function later: >>> a=0; b=10 >>> while a < b: ... print(a, end=' ') ... a += 1 ... 0 1 2 3 4 5 6 7 8 9 # One way to code counter loops # Or, a = a + 1 Finally, notice that Python doesn’t have what some languages call a “do until” loop statement. However, we can simulate one with a test and break at the bottom of the loop body: 328 | Chapter 13: while and for Loops while True: ...loop body... if exitTest(): break To fully understand how this structure works, we need to move on to the next section and learn more about the break statement. break, continue, pass, and the Loop else Now that we’ve seen a few Python loops in action, it’s time to take a look at two simple statements that have a purpose only when nested inside loops—the break and continue statements. While we’re looking at oddballs, we will also study the loop else clause here, because it is intertwined with break, and Python’s empty placeholder statement, the pass (which is not tied to loops per se, but falls into the general category of simple one-word statements). In Python: break Jumps out of the closest enclosing loop (past the entire loop statement) continue Jumps to the top of the closest enclosing loop (to the loop’s header line) pass Does nothing at all: it’s an empty statement placeholder Loop else block Runs if and only if the loop is exited normally (i.e., without hitting a break) General Loop Format Factoring in break and continue statements, the general format of the while loop looks like this: while <test1>: <statements1> if <test2>: break if <test3>: continue else: <statements2> # Exit loop now, skip else # Go to top of loop now, to test1 # Run if we didn't hit a 'break' break and continue statements can appear anywhere inside the while (or for) loop’s body, but they are usually coded further nested in an if test to take action in response to some condition. Let’s turn to a few simple examples to see how these statements come together in practice. break, continue, pass, and the Loop else | 329 pass Simple things first: the pass statement is a no-operation placeholder that is used when the syntax requires a statement, but you have nothing useful to say. It is often used to code an empty body for a compound statement. For instance, if you want to code an infinite loop that does nothing each time through, do it with a pass: while True: pass # Type Ctrl-C to stop me! Because the body is just an empty statement, Python gets stuck in this loop. pass is roughly to statements as None is to objects—an explicit nothing. Notice that here the while loop’s body is on the same line as the header, after the colon; as with if statements, this only works if the body isn’t a compound statement. This example does nothing forever. It probably isn’t the most useful Python program ever written (unless you want to warm up your laptop computer on a cold winter’s day!); frankly, though, I couldn’t think of a better pass example at this point in the book. We’ll see other places where pass makes more sense later—for instance, to ignore exceptions caught by try statements, and to define empty class objects with attributes that behave like “structs” and “records” in other languages. A pass is also sometime coded to mean “to be filled in later,” to stub out the bodies of functions temporarily: def func1(): pass # Add real code here later def func2(): pass We can’t leave the body empty without getting a syntax error, so we say pass instead. Version skew note: Python 3.0 (but not 2.6) allows ellipses coded as ... (literally, three consecutive dots) to appear any place an expression can. Because ellipses do nothing by themselves, this can serve as an alternative to the pass statement, especially for code to be filled in later—a sort of Python “TBD”: def func1(): ... # Alternative to pass def func2(): ... func1() # Does nothing if called Ellipses can also appear on the same line as a statement header and may be used to initialize variable names if no specific type is required: def func1(): ... def func2(): ... # Works on same line too >>> X = ... # Alternative to None 330 | Chapter 13: while and for Loops >>> X Ellipsis This notation is new in Python 3.0 (and goes well beyond the original intent of ... in slicing extensions), so time will tell if it becomes widespread enough to challenge pass and None in these roles. continue The continue statement causes an immediate jump to the top of a loop. It also sometimes lets you avoid statement nesting. The next example uses continue to skip odd numbers. This code prints all even numbers less than 10 and greater than or equal to 0. Remember, 0 means false and % is the remainder of division operator, so this loop counts down to 0, skipping numbers that aren’t multiples of 2 (it prints 8 6 4 2 0): x = 10 while x: x = x−1 if x % 2 != 0: continue print(x, end=' ') # Or, x -= 1 # Odd? -- skip print Because continue jumps to the top of the loop, you don’t need to nest the print statement inside an if test; the print is only reached if the continue is not run. If this sounds similar to a “goto” in other languages, it should. Python has no “goto” statement, but because continue lets you jump about in a program, many of the warnings about readability and maintainability you may have heard about goto apply. continue should probably be used sparingly, especially when you’re first getting started with Python. For instance, the last example might be clearer if the print were nested under the if: x = 10 while x: x = x−1 if x % 2 == 0: print(x, end=' ') # Even? -- print break The break statement causes an immediate exit from a loop. Because the code that follows it in the loop is not executed if the break is reached, you can also sometimes avoid nesting by including a break. For example, here is a simple interactive loop (a variant of a larger example we studied in Chapter 10) that inputs data with input (known as raw_input in Python 2.6) and exits when the user enters “stop” for the name request: >>> while True: ... name = input('Enter name:') ... if name == 'stop': break ... age = input('Enter age: ') ... print('Hello', name, '=>', int(age) ** 2) ... Enter name:mel Enter age: 40 break, continue, pass, and the Loop else | 331 Hello Enter Enter Hello Enter mel => 1600 name:bob age: 30 bob => 900 name:stop Notice how this code converts the age input to an integer with int before raising it to the second power; as you’ll recall, this is necessary because input returns user input as a string. In Chapter 35, you’ll see that input also raises an exception at end-of-file (e.g., if the user types Ctrl-Z or Ctrl-D); if this matters, wrap input in try statements. Loop else When combined with the loop else clause, the break statement can often eliminate the need for the search status flags used in other languages. For instance, the following piece of code determines whether a positive integer y is prime by searching for factors greater than 1: x = y // 2 while x > 1: if y % x == 0: print(y, 'has factor', x) break x -= 1 else: print(y, 'is prime') # For some y > 1 # Remainder # Skip else # Normal exit Rather than setting a flag to be tested when the loop is exited, it inserts a break where a factor is found. This way, the loop else clause can assume that it will be executed only if no factor is found; if you don’t hit the break, the number is prime. The loop else clause is also run if the body of the loop is never executed, as you don’t run a break in that event either; in a while loop, this happens if the test in the header is false to begin with. Thus, in the preceding example you still get the “is prime” message if x is initially less than or equal to 1 (for instance, if y is 2). This example determines primes, but only informally so. Numbers less than 2 are not considered prime by the strict mathematical definition. To be really picky, this code also fails for negative numbers and succeeds for floating-point numbers with no decimal digits. Also note that its code must use // instead of / in Python 3.0 because of the migration of / to “true division,” as described in Chapter 5 (we need the initial division to truncate remainders, not retain them!). If you want to experiment with this code, be sure to see the exercise at the end of Part IV, which wraps it in a function for reuse. 332 | Chapter 13: while and for Loops More on the loop else Because the loop else clause is unique to Python, it tends to perplex some newcomers. In general terms, the loop else provides explicit syntax for a common coding scenario— it is a coding structure that lets us catch the “other” way out of a loop, without setting and checking flags or conditions. Suppose, for instance, that we are writing a loop to search a list for a value, and we need to know whether the value was found after we exit the loop. We might code such a task this way: found = False while x and not found: if match(x[0]): print('Ni') found = True else: x = x[1:] if not found: print('not found') # Value at front? # Slice off front and repeat Here, we initialize, set, and later test a flag to determine whether the search succeeded or not. This is valid Python code, and it does work; however, this is exactly the sort of structure that the loop else clause is there to handle. Here’s an else equivalent: while x: if match(x[0]): print('Ni') break x = x[1:] else: print('Not found') # Exit when x empty # Exit, go around else # Only here if exhausted x This version is more concise. The flag is gone, and we’ve replaced the if test at the loop end with an else (lined up vertically with the word while). Because the break inside the main part of the while exits the loop and goes around the else, this serves as a more structured way to catch the search-failure case. Some readers might have noticed that the prior example’s else clause could be replaced with a test for an empty x after the loop (e.g., if not x:). Although that’s true in this example, the else provides explicit syntax for this coding pattern (it’s more obviously a search-failure clause here), and such an explicit empty test may not apply in some cases. The loop else becomes even more useful when used in conjunction with the for loop—the topic of the next section—because sequence iteration is not under your control. break, continue, pass, and the Loop else | 333 Why You Will Care: Emulating C while Loops The section on expression statements in Chapter 11 stated that Python doesn’t allow statements such as assignments to appear in places where it expects an expression. That means this common C language coding pattern won’t work in Python: while ((x = next()) != NULL) {...process x...} C assignments return the value assigned, but Python assignments are just statements, not expressions. This eliminates a notorious class of C errors (you can’t accidentally type = in Python when you mean ==). If you need similar behavior, though, there are at least three ways to get the same effect in Python while loops without embedding assignments in loop tests. You can move the assignment into the loop body with a break: while True: x = next() if not x: break ...process x... or move the assignment into the loop with tests: x = True while x: x = next() if x: ...process x... or move the first assignment outside the loop: x = next() while x: ...process x... x = next() Of these three coding patterns, the first may be considered by some to be the least structured, but it also seems to be the simplest and is the most commonly used. A simple Python for loop may replace some C loops as well. for Loops The for loop is a generic sequence iterator in Python: it can step through the items in any ordered sequence object. The for statement works on strings, lists, tuples, other built-in iterables, and new objects that we’ll see how to create later with classes. We met it in brief when studying sequence object types; let’s expand on its usage more formally here. General Format The Python for loop begins with a header line that specifies an assignment target (or targets), along with the object you want to step through. The header is followed by a block of (normally indented) statements that you want to repeat: 334 | Chapter 13: while and for Loops for <target> in <object>: <statements> else: <statements> # Assign object items to target # Repeated loop body: use target # If we didn't hit a 'break' When Python runs a for loop, it assigns the items in the sequence object to the target one by one and executes the loop body for each. The loop body typically uses the assignment target to refer to the current item in the sequence as though it were a cursor stepping through the sequence. The name used as the assignment target in a for header line is usually a (possibly new) variable in the scope where the for statement is coded. There’s not much special about it; it can even be changed inside the loop’s body, but it will automatically be set to the next item in the sequence when control returns to the top of the loop again. After the loop this variable normally still refers to the last item visited, which is the last item in the sequence unless the loop exits with a break statement. The for statement also supports an optional else block, which works exactly as it does in a while loop—it’s executed if the loop exits without running into a break statement (i.e., if all items in the sequence have been visited). The break and continue statements introduced earlier also work the same in a for loop as they do in a while. The for loop’s complete format can be described this way: for <target> in <object>: <statements> if <test>: break if <test>: continue else: <statements> # Assign object items to target # Exit loop now, skip else # Go to top of loop now # If we didn't hit a 'break' Examples Let’s type a few for loops interactively now, so you can see how they are used in practice. Basic usage As mentioned earlier, a for loop can step across any kind of sequence object. In our first example, for instance, we’ll assign the name x to each of the three items in a list in turn, from left to right, and the print statement will be executed for each. Inside the print statement (the loop body), the name x refers to the current item in the list: >>> for x in ["spam", "eggs", "ham"]: ... print(x, end=' ') ... spam eggs ham The next two examples compute the sum and product of all the items in a list. Later in this chapter and later in the book we’ll meet tools that apply operations such as + and * to items in a list automatically, but it’s usually just as easy to use a for: for Loops | 335 >>> >>> ... ... >>> 10 >>> >>> ... >>> 24 sum = 0 for x in [1, 2, 3, 4]: sum = sum + x sum prod = 1 for item in [1, 2, 3, 4]: prod *= item prod Other data types Any sequence works in a for, as it’s a generic tool. For example, for loops work on strings and tuples: >>> S = "lumberjack" >>> T = ("and", "I'm", "okay") >>> for x in S: print(x, end=' ') ... l u m b e r j a c k # Iterate over a string >>> for x in T: print(x, end=' ') ... and I'm okay # Iterate over a tuple In fact, as we’ll in the next chapter when we explore the notion of “iterables,” for loops can even work on some objects that are not sequences—files and dictionaries work, too! Tuple assignment in for loops If you’re iterating through a sequence of tuples, the loop target itself can actually be a tuple of targets. This is just another case of the tuple-unpacking assignment we studied in Chapter 11 at work. Remember, the for loop assigns items in the sequence object to the target, and assignment works the same everywhere: >>> T = [(1, 2), (3, 4), (5, 6)] >>> for (a, b) in T: ... print(a, b) ... 1 2 3 4 5 6 # Tuple assignment at work Here, the first time through the loop is like writing (a,b) = (1,2), the second time is like writing (a,b) = (3,4), and so on. The net effect is to automatically unpack the current tuple on each iteration. This form is commonly used in conjunction with the zip call we’ll meet later in this chapter to implement parallel traversals. It also makes regular appearances in conjunction with SQL databases in Python, where query result tables are returned as sequences 336 | Chapter 13: while and for Loops of sequences like the list used here—the outer list is the database table, the nested tuples are the rows within the table, and tuple assignment extracts columns. Tuples in for loops also come in handy to iterate through both keys and values in dictionaries using the items method, rather than looping through the keys and indexing to fetch the values manually: >>> D = {'a': 1, 'b': 2, 'c': 3} >>> for key in D: ... print(key, '=>', D[key]) ... a => 1 c => 3 b => 2 # Use dict keys iterator and index >>> list(D.items()) [('a', 1), ('c', 3), ('b', 2)] >>> for (key, value) in D.items(): ... print(key, '=>', value) ... a => 1 c => 3 b => 2 # Iterate over both keys and values It’s important to note that tuple assignment in for loops isn’t a special case; any assignment target works syntactically after the word for. Although we can always assign manually within the loop to unpack: >>> T [(1, 2), (3, 4), (5, 6)] >>> for both in T: ... a, b = both ... print(a, b) ... 1 2 3 4 5 6 # Manual assignment equivalent Tuples in the loop header save us an extra step when iterating through sequences of sequences. As suggested in Chapter 11, even nested structures may be automatically unpacked this way in a for: >>> ((a, b), c) = ((1, 2), 3) >>> a, b, c (1, 2, 3) # Nested sequences work too >>> for ((a, b), c) in [((1, 2), 3), ((4, 5), 6)]: print(a, b, c) ... 1 2 3 4 5 6 for Loops | 337 But this is no special case—the for loop simply runs the sort of assignment we ran just before it, on each iteration. Any nested sequence structure may be unpacked this way, just because sequence assignment is so generic: >>> for ((a, b), c) in [([1, 2], 3), ['XY', 6]]: print(a, b, c) ... 1 2 3 X Y 6 Python 3.0 extended sequence assignment in for loops In fact, because the loop variable in a for loop can really be any assignment target, we can also use Python 3.0’s extended sequence-unpacking assignment syntax here to extract items and sections of sequences within sequences. Really, this isn’t a special case either, but simply a new assignment form in 3.0 (as discussed in Chapter 11); because it works in assignment statements, it automatically works in for loops. Consider the tuple assignment form introduced in the prior section. A tuple of values is assigned to a tuple of names on each iteration, exactly like a simple assignment statement: >>> a, b, c = (1, 2, 3) >>> a, b, c (1, 2, 3) # Tuple assignment >>> for (a, b, c) in [(1, 2, 3), (4, 5, 6)]: ... print(a, b, c) ... 1 2 3 4 5 6 # Used in for loop In Python 3.0, because a sequence can be assigned to a more general set of names with a starred name to collect multiple items, we can use the same syntax to extract parts of nested sequences in the for loop: >>> a, *b, c = (1, 2, 3, 4) >>> a, b, c (1, [2, 3], 4) # Extended seq assignment >>> for (a, *b, c) in [(1, 2, 3, 4), (5, 6, 7, 8)]: ... print(a, b, c) ... 1 [2, 3] 4 5 [6, 7] 8 In practice, this approach might be used to pick out multiple columns from rows of data represented as nested sequences. In Python 2.X starred names aren’t allowed, but you can achieve similar effects by slicing. The only difference is that slicing returns a type-specific result, whereas starred names always are assigned lists: >>> for all in [(1, 2, 3, 4), (5, 6, 7, 8)]: ... a, b, c = all[0], all[1:3], all[3] ... print(a, b, c) 338 | Chapter 13: while and for Loops # Manual slicing in 2.6 ... 1 (2, 3) 4 5 (6, 7) 8 See Chapter 11 for more on this assignment form. Nested for loops Now let’s look at a for loop that’s a bit more sophisticated than those we’ve seen so far. The next example illustrates statement nesting and the loop else clause in a for. Given a list of objects (items) and a list of keys (tests), this code searches for each key in the objects list and reports on the search’s outcome: >>> items = ["aaa", 111, (4, 5), 2.01] >>> tests = [(4, 5), 3.14] >>> >>> for key in tests: ... for item in items: ... if item == key: ... print(key, "was found") ... break ... else: ... print(key, "not found!") ... (4, 5) was found 3.14 not found! # A set of objects # Keys to search for # For all keys # For all items # Check for match Because the nested if runs a break when a match is found, the loop else clause can assume that if it is reached, the search has failed. Notice the nesting here. When this code runs, there are two loops going at the same time: the outer loop scans the keys list, and the inner loop scans the items list for each key. The nesting of the loop else clause is critical; it’s indented to the same level as the header line of the inner for loop, so it’s associated with the inner loop, not the if or the outer for. Note that this example is easier to code if we employ the in operator to test membership. Because in implicitly scans an object looking for a match (at least logically), it replaces the inner loop: >>> for key in tests: ... if key in items: ... print(key, "was found") ... else: ... print(key, "not found!") ... (4, 5) was found 3.14 not found! # For all keys # Let Python check for a match In general, it’s a good idea to let Python do as much of the work as possible (as in this solution) for the sake of brevity and performance. The next example performs a typical data-structure task with a for—collecting common items in two sequences (strings). It’s roughly a simple set intersection routine; after the loop runs, res refers to a list that contains all the items found in seq1 and seq2: for Loops | 339 >>> seq1 = "spam" >>> seq2 = "scam" >>> >>> res = [] >>> for x in seq1: ... if x in seq2: ... res.append(x) ... >>> res ['s', 'a', 'm'] # Start empty # Scan first sequence # Common item? # Add to result end Unfortunately, this code is equipped to work only on two specific variables: seq1 and seq2. It would be nice if this loop could somehow be generalized into a tool you could use more than once. As you’ll see, that simple idea leads us to functions, the topic of the next part of the book. Why You Will Care: File Scanners In general, loops come in handy anywhere you need to repeat an operation or process something more than once. Because files contain multiple characters and lines, they are one of the more typical use cases for loops. To load a file’s contents into a string all at once, you simply call the file object’s read method: file = open('test.txt', 'r') print(file.read()) # Read contents into a string But to load a file in smaller pieces, it’s common to code either a while loop with breaks on end-of-file, or a for loop. To read by characters, either of the following codings will suffice: file = open('test.txt') while True: char = file.read(1) if not char: break print(char) # Read by character for char in open('test.txt').read(): print(char) The for loop here also processes each character, but it loads the file into memory all at once (and assumes it fits!). To read by lines or blocks instead, you can use while loop code like this: file = open('test.txt') while True: line = file.readline() if not line: break print(line, end='') # Read line by line file = open('test.txt', 'rb') while True: chunk = file.read(10) if not chunk: break print(chunk) # Read byte chunks: up to 10 bytes 340 | Chapter 13: while and for Loops # Line already has a \n You typically read binary data in blocks. To read text files line by line, though, the for loop tends to be easiest to code and the quickest to run: for line in open('test.txt').readlines(): print(line, end='') for line in open('test.txt'): print(line, end='') # Use iterators: best text input mode The file readlines method loads a file all at once into a line-string list, and the last example here relies on file iterators to automatically read one line on each loop iteration (iterators are covered in detail in Chapter 14). See the library manual for more on the calls used here. The last example here is generally the best option for text files—besides its simplicity, it works for arbitrarily large files and doesn’t load the entire file into memory all at once. The iterator version may be the quickest, but I/O performance is less clear-cut in Python 3.0. In some 2.X Python code, you may also see the name open replaced with file and the file object’s older xreadlines method used to achieve the same effect as the file’s automatic line iterator (it’s like readlines but doesn’t load the file into memory all at once). Both file and xreadlines are removed in Python 3.0, because they are redundant; you shouldn’t use them in 2.6 either, but they may pop up in older code and resources. Watch for more on reading files in Chapter 36; as we’ll see there, text and binary files have slightly different semantics in 3.0. Loop Coding Techniques The for loop subsumes most counter-style loops. It’s generally simpler to code and quicker to run than a while, so it’s the first tool you should reach for whenever you need to step through a sequence. But there are also situations where you will need to iterate in more specialized ways. For example, what if you need to visit every second or third item in a list, or change the list along the way? How about traversing more than one sequence in parallel, in the same for loop? You can always code such unique iterations with a while loop and manual indexing, but Python provides two built-ins that allow you to specialize the iteration in a for: • The built-in range function produces a series of successively higher integers, which can be used as indexes in a for. • The built-in zip function returns a series of parallel-item tuples, which can be used to traverse multiple sequences in a for. Because for loops typically run quicker than while-based counter loops, it’s to your advantage to use tools like these that allow you to use for when possible. Let’s look at each of these built-ins in turn. Loop Coding Techniques | 341 Counter Loops: while and range The range function is really a general tool that can be used in a variety of contexts. Although it’s used most often to generate indexes in a for, you can use it anywhere you need a list of integers. In Python 3.0, range is an iterator that generates items on demand, so we need to wrap it in a list call to display its results all at once (more on iterators in Chapter 14): >>> list(range(5)), list(range(2, 5)), list(range(0, 10, 2)) ([0, 1, 2, 3, 4], [2, 3, 4], [0, 2, 4, 6, 8]) With one argument, range generates a list of integers from zero up to but not including the argument’s value. If you pass in two arguments, the first is taken as the lower bound. An optional third argument can give a step; if it is used, Python adds the step to each successive integer in the result (the step defaults to 1). Ranges can also be nonpositive and nonascending, if you want them to be: >>> list(range(−5, 5)) [−5, −4, −3, −2, −1, 0, 1, 2, 3, 4] >>> list(range(5, −5, −1)) [5, 4, 3, 2, 1, 0, −1, −2, −3, −4] Although such range results may be useful all by themselves, they tend to come in most handy within for loops. For one thing, they provide a simple way to repeat an action a specific number of times. To print three lines, for example, use a range to generate the appropriate number of integers; for loops force results from range automatically in 3.0, so we don’t need list here: >>> for i in range(3): ... print(i, 'Pythons') ... 0 Pythons 1 Pythons 2 Pythons range is also commonly used to iterate over a sequence indirectly. The easiest and fastest way to step through a sequence exhaustively is always with a simple for, as Python handles most of the details for you: >>> X = 'spam' >>> for item in X: print(item, end=' ') ... s p a m # Simple iteration Internally, the for loop handles the details of the iteration automatically when used this way. If you really need to take over the indexing logic explicitly, you can do it with a while loop: >>> i = 0 >>> while i < len(X): ... print(X[i], end=' ') ... i += 1 342 | Chapter 13: while and for Loops # while loop iteration ... s p a m You can also do manual indexing with a for, though, if you use range to generate a list of indexes to iterate through. It’s a multistep process, but it’s sufficient to generate offsets, rather than the items at those offsets: >>> X 'spam' >>> len(X) # Length of string 4 >>> list(range(len(X))) # All legal offsets into X [0, 1, 2, 3] >>> >>> for i in range(len(X)): print(X[i], end=' ') # Manual for indexing ... s p a m Note that because this example is stepping over a list of offsets into X, not the actual items of X, we need to index back into X within the loop to fetch each item. Nonexhaustive Traversals: range and Slices The last example in the prior section works, but it’s not the fastest option. It’s also more work than we need to do. Unless you have a special indexing requirement, you’re always better off using the simple for loop form in Python—as a general rule, use for instead of while whenever possible, and don’t use range calls in for loops except as a last resort. This simpler solution is better: >>> for item in X: print(item) ... # Simple iteration However, the coding pattern used in the prior example does allow us to do more specialized sorts of traversals. For instance, we can skip items as we go: >>> S = 'abcdefghijk' >>> list(range(0, len(S), 2)) [0, 2, 4, 6, 8, 10] >>> for i in range(0, len(S), 2): print(S[i], end=' ') ... a c e g i k Here, we visit every second item in the string S by stepping over the generated range list. To visit every third item, change the third range argument to be 3, and so on. In effect, using range this way lets you skip items in loops while still retaining the simplicity of the for loop construct. Still, this is probably not the ideal best-practice technique in Python today. If you really want to skip items in a sequence, the extended three-limit form of the slice expression, presented in Chapter 7, provides a simpler route to the same goal. To visit every second character in S, for example, slice with a stride of 2: Loop Coding Techniques | 343 >>> S = 'abcdefghijk' >>> for c in S[::2]: print(c, end=' ') ... a c e g i k The result is the same, but substantially easier for you to write and for others to read. The only real advantage to using range here instead is that it does not copy the string and does not create a list in 3.0; for very large strings, it may save memory. Changing Lists: range Another common place where you may use the range and for combination is in loops that change a list as it is being traversed. Suppose, for example, that you need to add 1 to every item in a list. You can try this with a simple for loop, but the result probably won’t be exactly what you want: >>> L = [1, 2, 3, 4, 5] >>> ... ... >>> [1, >>> 6 for x in L: x += 1 L 2, 3, 4, 5] x This doesn’t quite work—it changes the loop variable x, not the list L. The reason is somewhat subtle. Each time through the loop, x refers to the next integer already pulled out of the list. In the first iteration, for example, x is integer 1. In the next iteration, the loop body sets x to a different object, integer 2, but it does not update the list where 1 originally came from. To really change the list as we march across it, we need to use indexes so we can assign an updated value to each position as we go. The range/len combination can produce the required indexes for us: >>> L = [1, 2, 3, 4, 5] >>> for i in range(len(L)): ... L[i] += 1 ... >>> L [2, 3, 4, 5, 6] # Add one to each item in L # Or L[i] = L[i] + 1 When coded this way, the list is changed as we proceed through the loop. There is no way to do the same with a simple for x in L:-style loop, because such a loop iterates through actual items, not list positions. But what about the equivalent while loop? Such a loop requires a bit more work on our part, and likely runs more slowly: >>> i = 0 >>> while i < len(L): ... L[i] += 1 344 | Chapter 13: while and for Loops ... i += 1 ... >>> L [3, 4, 5, 6, 7] Here again, though, the range solution may not be ideal either. A list comprehension expression of the form: [x+1 for x in L] would do similar work, albeit without changing the original list in-place (we could assign the expression’s new list object result back to L, but this would not update any other references to the original list). Because this is such a central looping concept, we’ll save a complete exploration of list comprehensions for the next chapter. Parallel Traversals: zip and map As we’ve seen, the range built-in allows us to traverse sequences with for in a nonexhaustive fashion. In the same spirit, the built-in zip function allows us to use for loops to visit multiple sequences in parallel. In basic operation, zip takes one or more sequences as arguments and returns a series of tuples that pair up parallel items taken from those sequences. For example, suppose we’re working with two lists: >>> L1 = [1,2,3,4] >>> L2 = [5,6,7,8] To combine the items in these lists, we can use zip to create a list of tuple pairs (like range, zip is an iterable object in 3.0, so we must wrap it in a list call to display all its results at once—more on iterators in the next chapter): >>> zip(L1, L2) <zip object at 0x026523C8> >>> list(zip(L1, L2)) [(1, 5), (2, 6), (3, 7), (4, 8)] # list() required in 3.0, not 2.6 Such a result may be useful in other contexts as well, but when wedded with the for loop, it supports parallel iterations: >>> ... ... 1 5 2 6 3 7 4 8 for (x, y) in zip(L1, L2): print(x, y, '--', x+y) ----- 6 8 10 12 Here, we step over the result of the zip call—that is, the pairs of items pulled from the two lists. Notice that this for loop again uses the tuple assignment form we met earlier to unpack each tuple in the zip result. The first time through, it’s as though we ran the assignment statement (x, y) = (1, 5). Loop Coding Techniques | 345 The net effect is that we scan both L1 and L2 in our loop. We could achieve a similar effect with a while loop that handles indexing manually, but it would require more typing and would likely run more slowly than the for/zip approach. Strictly speaking, the zip function is more general than this example suggests. For instance, it accepts any type of sequence (really, any iterable object, including files), and it accepts more than two arguments. With three arguments, as in the following example, it builds a list of three-item tuples with items from each sequence, essentially projecting by columns (technically, we get an N-ary tuple for N arguments): >>> T1, T2, T3 = (1,2,3), (4,5,6), (7,8,9) >>> T3 (7, 8, 9) >>> list(zip(T1, T2, T3)) [(1, 4, 7), (2, 5, 8), (3, 6, 9)] Moreover, zip truncates result tuples at the length of the shortest sequence when the argument lengths differ. In the following, we zip together two strings to pick out characters in parallel, but the result has only as many tuples as the length of the shortest sequence: >>> S1 = 'abc' >>> S2 = 'xyz123' >>> >>> list(zip(S1, S2)) [('a', 'x'), ('b', 'y'), ('c', 'z')] map equivalence in Python 2.6 In Python 2.X, the related built-in map function pairs items from sequences in a similar fashion, but it pads shorter sequences with None if the argument lengths differ instead of truncating to the shortest length: >>> S1 = 'abc' >>> S2 = 'xyz123' >>> map(None, S1, S2) # 2.X only [('a', 'x'), ('b', 'y'), ('c', 'z'), (None, '1'), (None, '2'), (None,'3')] This example is using a degenerate form of the map built-in, which is no longer supported in 3.0. Normally, map takes a function and one or more sequence arguments and collects the results of calling the function with parallel items taken from the sequence(s). We’ll study map in detail in Chapters 19 and 20, but as a brief example, the following maps the built-in ord function across each item in a string and collects the results (like zip, map is a value generator in 3.0 and so must be passed to list to collect all its results at once): >>> list(map(ord, 'spam')) [115, 112, 97, 109] 346 | Chapter 13: while and for Loops This works the same as the following loop statement, but is often quicker: >>> res = [] >>> for c in 'spam': res.append(ord(c)) >>> res [115, 112, 97, 109] Version skew note: The degenerate form of map using a function argument of None is no longer supported in Python 3.0, because it largely overlaps with zip (and was, frankly, a bit at odds with map’s functionapplication purpose). In 3.0, either use zip or write loop code to pad results yourself. We’ll see how to do this in Chapter 20, after we’ve had a chance to study some additional iteration concepts. Dictionary construction with zip In Chapter 8, I suggested that the zip call used here can also be handy for generating dictionaries when the sets of keys and values must be computed at runtime. Now that we’re becoming proficient with zip, I’ll explain how it relates to dictionary construction. As you’ve learned, you can always create a dictionary by coding a dictionary literal, or by assigning to keys over time: >>> D1 = {'spam':1, 'eggs':3, 'toast':5} >>> D1 {'toast': 5, 'eggs': 3, 'spam': 1} >>> >>> >>> >>> D1 = {} D1['spam'] = 1 D1['eggs'] = 3 D1['toast'] = 5 What to do, though, if your program obtains dictionary keys and values in lists at runtime, after you’ve coded your script? For example, say you had the following keys and values lists: >>> keys = ['spam', 'eggs', 'toast'] >>> vals = [1, 3, 5] One solution for turning those lists into a dictionary would be to zip the lists and step through them in parallel with a for loop: >>> list(zip(keys, vals)) [('spam', 1), ('eggs', 3), ('toast', 5)] >>> D2 = {} >>> for (k, v) in zip(keys, vals): D2[k] = v ... >>> D2 {'toast': 5, 'eggs': 3, 'spam': 1} Loop Coding Techniques | 347 It turns out, though, that in Python 2.2 and later you can skip the for loop altogether and simply pass the zipped keys/values lists to the built-in dict constructor call: >>> keys = ['spam', 'eggs', 'toast'] >>> vals = [1, 3, 5] >>> D3 = dict(zip(keys, vals)) >>> D3 {'toast': 5, 'eggs': 3, 'spam': 1} The built-in name dict is really a type name in Python (you’ll learn more about type names, and subclassing them, in Chapter 31). Calling it achieves something like a listto-dictionary conversion, but it’s really an object construction request. In the next chapter we’ll explore a related but richer concept, the list comprehension, which builds lists in a single expression; we’ll also revisit 3.0 dictionary comprehensions an alternative to the dict cal for zipped key/value pairs. Generating Both Offsets and Items: enumerate Earlier, we discussed using range to generate the offsets of items in a string, rather than the items at those offsets. In some programs, though, we need both: the item to use, plus an offset as we go. Traditionally, this was coded with a simple for loop that also kept a counter of the current offset: >>> S = 'spam' >>> offset = 0 >>> for item in S: ... print(item, ... offset += 1 ... s appears at offset p appears at offset a appears at offset m appears at offset 'appears at offset', offset) 0 1 2 3 This works, but in recent Python releases a new built-in named enumerate does the job for us: >>> S = 'spam' >>> for (offset, item) in enumerate(S): ... print(item, 'appears at offset', offset) ... s appears at offset 0 p appears at offset 1 a appears at offset 2 m appears at offset 3 The enumerate function returns a generator object—a kind of object that supports the iteration protocol that we will study in the next chapter and will discuss in more detail in the next part of the book. In short, it has a __next__ method called by the next builtin function, which returns an (index, value) tuple each time through the loop. We can unpack these tuples with tuple assignment in the for loop (much like using zip): 348 | Chapter 13: while and for Loops >>> E = enumerate(S) >>> E <enumerate object at 0x02765AA8> >>> next(E) (0, 's') >>> next(E) (1, 'p') >>> next(E) (2, 'a') As usual, we don’t normally see this machinery because iteration contexts— including list comprehensions, the subject of Chapter 14—run the iteration protocol automatically: >>> [c * i for (i, c) in enumerate(S)] ['', 'p', 'aa', 'mmm'] To fully understand iteration concepts like enumerate, zip, and list comprehensions, we need to move on to the next chapter for a more formal dissection. Chapter Summary In this chapter, we explored Python’s looping statements as well as some concepts related to looping in Python. We looked at the while and for loop statements in depth, and we learned about their associated else clauses. We also studied the break and continue statements, which have meaning only inside loops, and met several built-in tools commonly used in for loops, including range, zip, map, and enumerate (although their roles as iterators in Python 3.0 won’t be fully uncovered until the next chapter). In the next chapter, we continue the iteration story by discussing list comprehensions and the iteration protocol in Python—concepts strongly related to for loops. There, we’ll also explain some of the subtleties of iterable tools we met here, such as range and zip. As always, though, before moving on let’s exercise what you’ve picked up here with a quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. What are the main functional differences between a while and a for? What’s the difference between break and continue? When is a loop’s else clause executed? How can you code a counter-based loop in Python? What can a range be used for in a for loop? Test Your Knowledge: Quiz | 349 Test Your Knowledge: Answers 1. The while loop is a general looping statement, but the for is designed to iterate across items in a sequence (really, iterable). Although the while can imitate the for with counter loops, it takes more code and might run slower. 2. The break statement exits a loop immediately (you wind up below the entire while or for loop statement), and continue jumps back to the top of the loop (you wind up positioned just before the test in while or the next item fetch in for). 3. The else clause in a while or for loop will be run once as the loop is exiting, if the loop exits normally (without running into a break statement). A break exits the loop immediately, skipping the else part on the way out (if there is one). 4. Counter loops can be coded with a while statement that keeps track of the index manually, or with a for loop that uses the range built-in function to generate successive integer offsets. Neither is the preferred way to work in Python, if you need to simply step across all the items in a sequence. Instead, use a simple for loop instead, without range or counters, whenever possible; it will be easier to code and usually quicker to run. 5. The range built-in can be used in a for to implement a fixed number of repetitions, to scan by offsets instead of items at offsets, to skip successive items as you go, and to change a list while stepping across it. None of these roles requires range, and most have alternatives—scanning actual items, three-limit slices, and list comprehensions are often better solutions today (despite the natural inclinations of ex-C programmers to want to count things!). 350 | Chapter 13: while and for Loops CHAPTER 14 Iterations and Comprehensions, Part 1 In the prior chapter we met Python’s two looping statements, while and for. Although they can handle most repetitive tasks programs need to perform, the need to iterate over sequences is so common and pervasive that Python provides additional tools to make it simpler and more efficient. This chapter begins our exploration of these tools. Specifically, it presents the related concepts of Python’s iteration protocol—a methodcall model used by the for loop—and fills in some details on list comprehensions—a close cousin to the for loop that applies an expression to items in an iterable. Because both of these tools are related to both the for loop and functions, we’ll take a two-pass approach to covering them in this book: this chapter introduces the basics in the context of looping tools, serving as something of continuation of the prior chapter, and a later chapter (Chapter 20) revisits them in the context of function-based tools. In this chapter, we’ll also sample additional iteration tools in Python and touch on the new iterators available in Python 3.0. One note up front: some of the concepts presented in these chapters may seem advanced at first glance. With practice, though, you’ll find that these tools are useful and powerful. Although never strictly required, because they’ve become commonplace in Python code, a basic understanding can also help if you must read programs written by others. Iterators: A First Look In the preceding chapter, I mentioned that the for loop can work on any sequence type in Python, including lists, tuples, and strings, like this: >>> for x in [1, 2, 3, 4]: print(x ** 2, end=' ') ... 1 4 9 16 >>> for x in (1, 2, 3, 4): print(x ** 3, end=' ') ... 1 8 27 64 351 >>> for x in 'spam': print(x * 2, end=' ') ... ss pp aa mm Actually, the for loop turns out to be even more generic than this—it works on any iterable object. In fact, this is true of all iteration tools that scan objects from left to right in Python, including for loops, the list comprehensions we’ll study in this chapter, in membership tests, the map built-in function, and more. The concept of “iterable objects” is relatively recent in Python, but it has come to permeate the language’s design. It’s essentially a generalization of the notion of sequences—an object is considered iterable if it is either a physically stored sequence or an object that produces one result at a time in the context of an iteration tool like a for loop. In a sense, iterable objects include both physical sequences and virtual sequences computed on demand.* The Iteration Protocol: File Iterators One of the easiest ways to understand what this means is to look at how it works with a built-in type such as the file. Recall from Chapter 9 that open file objects have a method called readline, which reads one line of text from a file at a time—each time we call the readline method, we advance to the next line. At the end of the file, an empty string is returned, which we can detect to break out of the loop: >>> f = open('script1.py') >>> f.readline() 'import sys\n' >>> f.readline() 'print(sys.path)\n' >>> f.readline() 'x = 2\n' >>> f.readline() 'print(2 ** 33)\n' >>> f.readline() '' # Read a 4-line script file in this directory # readline loads one line on each call # Returns empty string at end-of-file However, files also have a method named __next__ that has a nearly identical effect— it returns the next line from a file each time it is called. The only noticeable difference is that __next__ raises a built-in StopIteration exception at end-of-file instead of returning an empty string: >>> f = open('script1.py') >>> f.__next__() 'import sys\n' >>> f.__next__() 'print(sys.path)\n' # __next__ loads one line on each call too # But raises an exception at end-of-file * Terminology in this topic tends to be a bit loose. This text uses the terms “iterable” and “iterator” interchangeably to refer to an object that supports iteration in general. Sometimes the term “iterable” refers to an object that supports iter and “iterator” refers to an object return by iter that supports next(I), but that convention is not universal in either the Python world or this book. 352 | Chapter 14: Iterations and Comprehensions, Part 1 >>> f.__next__() 'x = 2\n' >>> f.__next__() 'print(2 ** 33)\n' >>> f.__next__() Traceback (most recent call last): ...more exception text omitted... StopIteration This interface is exactly what we call the iteration protocol in Python. Any object with a __next__ method to advance to a next result, which raises StopIteration at the end of the series of results, is considered iterable in Python. Any such object may also be stepped through with a for loop or other iteration tool, because all iteration tools normally work internally by calling __next__ on each iteration and catching the StopIteration exception to determine when to exit. The net effect of this magic is that, as mentioned in Chapter 9, the best way to read a text file line by line today is to not read it at all—instead, allow the for loop to automatically call __next__ to advance to the next line on each iteration. The file object’s iterator will do the work of automatically loading lines as you go. The following, for example, reads a file line by line, printing the uppercase version of each line along the way, without ever explicitly reading from the file at all: >>> for line in open('script1.py'): ... print(line.upper(), end='') ... IMPORT SYS PRINT(SYS.PATH) X = 2 PRINT(2 ** 33) # Use file iterators to read by lines # Calls __next__, catches StopIteration Notice that the print uses end='' here to suppress adding a \n, because line strings already have one (without this, our output would be double-spaced). This is considered the best way to read text files line by line today, for three reasons: it’s the simplest to code, might be the quickest to run, and is the best in terms of memory usage. The older, original way to achieve the same effect with a for loop is to call the file readlines method to load the file’s content into memory as a list of line strings: >>> for line in open('script1.py').readlines(): ... print(line.upper(), end='') ... IMPORT SYS PRINT(SYS.PATH) X = 2 PRINT(2 ** 33) This readlines technique still works, but it is not considered the best practice today and performs poorly in terms of memory usage. In fact, because this version really does load the entire file into memory all at once, it will not even work for files too big to fit into the memory space available on your computer. By contrast, because it reads one line at a time, the iterator-based version is immune to such memory-explosion issues. Iterators: A First Look | 353 The iterator version might run quicker too, though this can vary per release (Python 3.0 made this advantage less clear-cut by rewriting I/O libraries to support Unicode text and be less system-dependent). As mentioned in the prior chapter’s sidebar, “Why You Will Care: File Scanners” on page 340, it’s also possible to read a file line by line with a while loop: >>> f = open('script1.py') >>> while True: ... line = f.readline() ... if not line: break ... print(line.upper(), end='') ... ...same output... However, this may run slower than the iterator-based for loop version, because iterators run at C language speed inside Python, whereas the while loop version runs Python byte code through the Python virtual machine. Any time we trade Python code for C code, speed tends to increase. This is not an absolute truth, though, especially in Python 3.0; we’ll see timing techniques later in this book for measuring the relative speed of alternatives like these. Manual Iteration: iter and next To support manual iteration code (with less typing), Python 3.0 also provides a builtin function, next, that automatically calls an object’s __next__ method. Given an iterable object X, the call next(X) is the same as X.__next__(), but noticeably simpler. With files, for instance, either form may be used: >>> f = open('script1.py') >>> f.__next__() 'import sys\n' >>> f.__next__() 'print(sys.path)\n' >>> f = open('script1.py') >>> next(f) 'import sys\n' >>> next(f) 'print(sys.path)\n' # Call iteration method directly # next built-in calls __next__ Technically, there is one more piece to the iteration protocol. When the for loop begins, it obtains an iterator from the iterable object by passing it to the iter built-in function; the object returned by iter has the required next method. This becomes obvious if we look at how for loops internally process built-in sequence types such as lists: >>> >>> >>> 1 >>> 2 L = [1, 2, 3] I = iter(L) I.next() # Obtain an iterator object # Call next to advance to next item I.next() 354 | Chapter 14: Iterations and Comprehensions, Part 1 >>> I.next() 3 >>> I.next() Traceback (most recent call last): ...more omitted... StopIteration This initial step is not required for files, because a file object is its own iterator. That is, files have their own __next__ method and so do not need to return a different object that does: >>> f = open('script1.py') >>> iter(f) is f True >>> f.__next__() 'import sys\n' Lists, and many other built-in objects, are not their own iterators because they support multiple open iterations. For such objects, we must call iter to start iterating: >>> L = [1, 2, 3] >>> iter(L) is L False >>> L.__next__() AttributeError: 'list' object has no attribute '__next__' >>> I = iter(L) >>> I.__next__() 1 >>> next(I) 2 # Same as I.__next__() Although Python iteration tools call these functions automatically, we can use them to apply the iteration protocol manually, too. The following interaction demonstrates the equivalence between automatic and manual iteration:† >>> L = [1, 2, 3] >>> >>> for X in L: ... print(X ** 2, end=' ') ... 1 4 9 # Automatic iteration # Obtains iter, calls __next__, catches exceptions >>> I = iter(L) # Manual iteration: what for loops usually do † Technically speaking, the for loop calls the internal equivalent of I.__next__, instead of the next(I) used here. There is rarely any difference between the two, but as we’ll see in the next section, there are some builtin objects in 3.0 (such as os.popen results) that support the former and not the latter, but may be still be iterated across in for loops. Your manual iterations can generally use either call scheme. If you care for the full story, in 3.0 os.popen results have been reimplemented with the subprocess module and a wrapper class, whose __getattr__ method is no longer called in 3.0 for implicit __next__ fetches made by the next built-in, but is called for explicit fetches by name—a 3.0 change issue we’ll confront in Chapters 37 and 38, which apparently burns some standard library code too! Also in 3.0, the related 2.6 calls os.popen2/3/4 are no longer available; use subprocess.Popen with appropriate arguments instead (see the Python 3.0 library manual for the new required code). Iterators: A First Look | 355 >>> while True: ... try: ... X = next(I) ... except StopIteration: ... break ... print(X ** 2, end=' ') ... 1 4 9 # try statement catches exceptions # Or call I.__next__ To understand this code, you need to know that try statements run an action and catch exceptions that occur while the action runs (we’ll explore exceptions in depth in Part VII). I should also note that for loops and other iteration contexts can sometimes work differently for user-defined classes, repeatedly indexing an object instead of running the iteration protocol. We’ll defer that story until we study class operator overloading in Chapter 29. Version skew note: In Python 2.6, the iteration method is named X.next() instead of X.__next__(). For portability, the next(X) built-in function is available in Python 2.6 too (but not earlier), and calls 2.6’s X.next() instead of 3.0’s X.__next__(). Iteration works the same in 2.6 in all other ways, though; simply use X.next() or next(X) for manual iterations, instead of 3.0’s X.__next__(). Prior to 2.6, use manual X.next() calls instead of next(X). Other Built-in Type Iterators Besides files and physical sequences like lists, other types have useful iterators as well. The classic way to step through the keys of a dictionary, for example, is to request its keys list explicitly: >>> D = {'a':1, 'b':2, 'c':3} >>> for key in D.keys(): ... print(key, D[key]) ... a 1 c 3 b 2 In recent versions of Python, though, dictionaries have an iterator that automatically returns one key at a time in an iteration context: >>> I = iter(D) >>> next(I) 'a' >>> next(I) 'c' >>> next(I) 'b' >>> next(I) Traceback (most recent call last): 356 | Chapter 14: Iterations and Comprehensions, Part 1 ...more omitted... StopIteration The net effect is that we no longer need to call the keys method to step through dictionary keys—the for loop will use the iteration protocol to grab one key each time through: >>> for key in D: ... print(key, D[key]) ... a 1 c 3 b 2 We can’t delve into their details here, but other Python object types also support the iterator protocol and thus may be used in for loops too. For instance, shelves (an accessby-key filesystem for Python objects) and the results from os.popen (a tool for reading the output of shell commands) are iterable as well: >>> import os >>> P = os.popen('dir') >>> P.__next__() ' Volume in drive C is SQ004828V03\n' >>> P.__next__() ' Volume Serial Number is 08BE-3CD4\n' >>> next(P) TypeError: _wrap_close object is not an iterator Notice that popen objects support a P.next() method in Python 2.6. In 3.0, they support the P.__next__() method, but not the next(P) built-in; since the latter is defined to call the former, it’s not clear if this behavior will endure in future releases (as described in an earlier footnote, this appears to be an implementation issue). This is only an issue for manual iteration, though; if you iterate over these objects automatically with for loops and other iteration contexts (described in the next sections), they return successive lines in either Python version. The iteration protocol also is the reason that we’ve had to wrap some results in a list call to see their values all at once. Objects that are iterable return results one at a time, not in a physical list: >>> R = range(5) >>> R range(0, 5) >>> I = iter(R) >>> next(I) 0 >>> next(I) 1 >>> list(range(5)) [0, 1, 2, 3, 4] # Ranges are iterables in 3.0 # Use iteration protocol to produce results # Or use list to collect all results at once Iterators: A First Look | 357 Now that you have a better understanding of this protocol, you should be able to see how it explains why the enumerate tool introduced in the prior chapter works the way it does: >>> E = enumerate('spam') # enumerate is an iterable too >>> E <enumerate object at 0x0253F508> >>> I = iter(E) >>> next(I) # Generate results with iteration protocol (0, 's') >>> next(I) # Or use list to force generation to run (1, 'p') >>> list(enumerate('spam')) [(0, 's'), (1, 'p'), (2, 'a'), (3, 'm')] We don’t normally see this machinery because for loops run it for us automatically to step through results. In fact, everything that scans left-to-right in Python employs the iteration protocol in the same way—including the topic of the next section. List Comprehensions: A First Look Now that we’ve seen how the iteration protocol works, let’s turn to a very common use case. Together with for loops, list comprehensions are one of the most prominent contexts in which the iteration protocol is applied. In the previous chapter, we learned how to use range to change a list as we step across it: >>> L = [1, 2, 3, 4, 5] >>> for i in range(len(L)): ... L[i] += 10 ... >>> L [11, 12, 13, 14, 15] This works, but as I mentioned there, it may not be the optimal “best-practice” approach in Python. Today, the list comprehension expression makes many such prior use cases obsolete. Here, for example, we can replace the loop with a single expression that produces the desired result list: >>> L = [x + 10 for x in L] >>> L [21, 22, 23, 24, 25] The net result is the same, but it requires less coding on our part and is likely to run substantially faster. The list comprehension isn’t exactly the same as the for loop statement version because it makes a new list object (which might matter if there are multiple references to the original list), but it’s close enough for most applications and is a common and convenient enough approach to merit a closer look here. 358 | Chapter 14: Iterations and Comprehensions, Part 1 List Comprehension Basics We met the list comprehension briefly in Chapter 4. Syntactically, its syntax is derived from a construct in set theory notation that applies an operation to each item in a set, but you don’t have to know set theory to use this tool. In Python, most people find that a list comprehension simply looks like a backward for loop. To get a handle on the syntax, let’s dissect the prior section’s example in more detail: >>> L = [x + 10 for x in L] List comprehensions are written in square brackets because they are ultimately a way to construct a new list. They begin with an arbitrary expression that we make up, which uses a loop variable that we make up (x + 10). That is followed by what you should now recognize as the header of a for loop, which names the loop variable, and an iterable object (for x in L). To run the expression, Python executes an iteration across L inside the interpreter, assigning x to each item in turn, and collects the results of running the items through the expression on the left side. The result list we get back is exactly what the list comprehension says—a new list containing x + 10, for every x in L. Technically speaking, list comprehensions are never really required because we can always build up a list of expression results manually with for loops that append results as we go: >>> res = [] >>> for x in L: ... res.append(x + 10) ... >>> res [21, 22, 23, 24, 25] In fact, this is exactly what the list comprehension does internally. However, list comprehensions are more concise to write, and because this code pattern of building up result lists is so common in Python work, they turn out to be very handy in many contexts. Moreover, list comprehensions can run much faster than manual for loop statements (often roughly twice as fast) because their iterations are performed at C language speed inside the interpreter, rather than with manual Python code; especially for larger data sets, there is a major performance advantage to using them. Using List Comprehensions on Files Let’s work through another common use case for list comprehensions to explore them in more detail. Recall that the file object has a readlines method that loads the file into a list of line strings all at once: >>> f = open('script1.py') >>> lines = f.readlines() List Comprehensions: A First Look | 359 >>> lines ['import sys\n', 'print(sys.path)\n', 'x = 2\n', 'print(2 ** 33)\n'] This works, but the lines in the result all include the newline character (\n) at the end. For many programs, the newline character gets in the way—we have to be careful to avoid double-spacing when printing, and so on. It would be nice if we could get rid of these newlines all at once, wouldn’t it? Any time we start thinking about performing an operation on each item in a sequence, we’re in the realm of list comprehensions. For example, assuming the variable lines is as it was in the prior interaction, the following code does the job by running each line in the list through the string rstrip method to remove whitespace on the right side (a line[:−1] slice would work, too, but only if we can be sure all lines are properly terminated): >>> lines = [line.rstrip() for line in lines] >>> lines ['import sys', 'print(sys.path)', 'x = 2', 'print(2 ** 33)'] This works as planned. Because list comprehensions are an iteration context just like for loop statements, though, we don’t even have to open the file ahead of time. If we open it inside the expression, the list comprehension will automatically use the iteration protocol we met earlier in this chapter. That is, it will read one line from the file at a time by calling the file’s next method, run the line through the rstrip expression, and add it to the result list. Again, we get what we ask for—the rstrip result of a line, for every line in the file: >>> lines = [line.rstrip() for line in open('script1.py')] >>> lines ['import sys', 'print(sys.path)', 'x = 2', 'print(2 ** 33)'] This expression does a lot implicitly, but we’re getting a lot of work for free here— Python scans the file and builds a list of operation results automatically. It’s also an efficient way to code this operation: because most of this work is done inside the Python interpreter, it is likely much faster than an equivalent for statement. Again, especially for large files, the speed advantages of list comprehensions can be significant. Besides their efficiency, list comprehensions are also remarkably expressive. In our example, we can run any string operation on a file’s lines as we iterate. Here’s the list comprehension equivalent to the file iterator uppercase example we met earlier, along with a few others (the method chaining in the second of these examples works because string methods return a new string, to which we can apply another string method): >>> [line.upper() for line in open('script1.py')] ['IMPORT SYS\n', 'PRINT(SYS.PATH)\n', 'X = 2\n', 'PRINT(2 ** 33)\n'] >>> [line.rstrip().upper() for line in open('script1.py')] ['IMPORT SYS', 'PRINT(SYS.PATH)', 'X = 2', 'PRINT(2 ** 33)'] >>> [line.split() for line in open('script1.py')] [['import', 'sys'], ['print(sys.path)'], ['x', '=', '2'], ['print(2', '**','33)']] 360 | Chapter 14: Iterations and Comprehensions, Part 1 >>> [line.replace(' ', '!') for line in open('script1.py')] ['import!sys\n', 'print(sys.path)\n', 'x!=!2\n', 'print(2!**!33)\n'] >>> [('sys' in line, line[0]) for line in open('script1.py')] [(True, 'i'), (True, 'p'), (False, 'x'), (False, 'p')] Extended List Comprehension Syntax In fact, list comprehensions can be even more advanced in practice. As one particularly useful extension, the for loop nested in the expression can have an associated if clause to filter out of the result items for which the test is not true. For example, suppose we want to repeat the prior section’s file-scanning example, but we need to collect only lines that begin with the letter p (perhaps the first character on each line is an action code of some sort). Adding an if filter clause to our expression does the trick: >>> lines = [line.rstrip() for line in open('script1.py') if line[0] == 'p'] >>> lines ['print(sys.path)', 'print(2 ** 33)'] Here, the if clause checks each line read from the file to see whether its first character is p; if not, the line is omitted from the result list. This is a fairly big expression, but it’s easy to understand if we translate it to its simple for loop statement equivalent. In general, we can always translate a list comprehension to a for statement by appending as we go and further indenting each successive part: >>> res = [] >>> for line in open('script1.py'): ... if line[0] == 'p': ... res.append(line.rstrip()) ... >>> res ['print(sys.path)', 'print(2 ** 33)'] This for statement equivalent works, but it takes up four lines instead of one and probably runs substantially slower. List comprehensions can become even more complex if we need them to—for instance, they may contain nested loops, coded as a series of for clauses. In fact, their full syntax allows for any number of for clauses, each of which can have an optional associated if clause (we’ll be more formal about their syntax in Chapter 20). For example, the following builds a list of the concatenation of x + y for every x in one string and every y in another. It effectively collects the permutation of the characters in two strings: >>> [x + y for x in 'abc' for y in 'lmn'] ['al', 'am', 'an', 'bl', 'bm', 'bn', 'cl', 'cm', 'cn'] List Comprehensions: A First Look | 361 Again, one way to understand this expression is to convert it to statement form by indenting its parts. The following is an equivalent, but likely slower, alternative way to achieve the same effect: >>> res = [] >>> for x in 'abc': ... for y in 'lmn': ... res.append(x + y) ... >>> res ['al', 'am', 'an', 'bl', 'bm', 'bn', 'cl', 'cm', 'cn'] Beyond this complexity level, though, list comprehension expressions can often become too compact for their own good. In general, they are intended for simple types of iterations; for more involved work, a simpler for statement structure will probably be easier to understand and modify in the future. As usual in programming, if something is difficult for you to understand, it’s probably not a good idea. We’ll revisit list comprehensions in Chapter 20, in the context of functional programming tools; as we’ll see, they turn out to be just as related to functions as they are to looping statements. Other Iteration Contexts Later in the book, we’ll see that user-defined classes can implement the iteration protocol too. Because of this, it’s sometimes important to know which built-in tools make use of it—any tool that employs the iteration protocol will automatically work on any built-in type or user-defined class that provides it. So far, I’ve been demonstrating iterators in the context of the for loop statement, because this part of the book is focused on statements. Keep in mind, though, that every tool that scans from left to right across objects uses the iteration protocol. This includes the for loops we’ve seen: >>> for line in open('script1.py'): ... print(line.upper(), end='') ... IMPORT SYS PRINT(SYS.PATH) X = 2 PRINT(2 ** 33) # Use file iterators However, list comprehensions, the in membership test, the map built-in function, and other built-ins such as the sorted and zip calls also leverage the iteration protocol. When applied to a file, all of these use the file object’s iterator automatically to scan line by line: >>> uppers = [line.upper() for line in open('script1.py')] >>> uppers ['IMPORT SYS\n', 'PRINT(SYS.PATH)\n', 'X = 2\n', 'PRINT(2 ** 33)\n'] 362 | Chapter 14: Iterations and Comprehensions, Part 1 >>> map(str.upper, open('script1.py')) <map object at 0x02660710> # map is an iterable in 3.0 >>> list( map(str.upper, open('script1.py')) ) ['IMPORT SYS\n', 'PRINT(SYS.PATH)\n', 'X = 2\n', 'PRINT(2 ** 33)\n'] >>> 'y = 2\n' in open('script1.py') False >>> 'x = 2\n' in open('script1.py') True We introduced the map call used here in the preceding chapter; it’s a built-in that applies a function call to each item in the passed-in iterable object. map is similar to a list comprehension but is more limited because it requires a function instead of an arbitrary expression. It also returns an iterable object itself in Python 3.0, so we must wrap it in a list call to force it to give us all its values at once; more on this change later in this chapter. Because map, like the list comprehension, is related to both for loops and functions, we’ll also explore both again in Chapters 19 and 20. Python includes various additional built-ins that process iterables, too: sorted sorts items in an iterable, zip combines items from iterables, enumerate pairs items in an iterable with relative positions, filter selects items for which a function is true, and reduce runs pairs of items in an iterable through a function. All of these accept iterables, and zip, enumerate, and filter also return an iterable in Python 3.0, like map. Here they are in action running the file’s iterator automatically to scan line by line: >>> sorted(open('script1.py')) ['import sys\n', 'print(2 ** 33)\n', 'print(sys.path)\n', 'x = 2\n'] >>> list(zip(open('script1.py'), open('script1.py'))) [('import sys\n', 'import sys\n'), ('print(sys.path)\n', 'print(sys.path)\n'), ('x = 2\n', 'x = 2\n'), ('print(2 ** 33)\n', 'print(2 ** 33)\n')] >>> list(enumerate(open('script1.py'))) [(0, 'import sys\n'), (1, 'print(sys.path)\n'), (2, 'x = 2\n'), (3, 'print(2 ** 33)\n')] >>> list(filter(bool, open('script1.py'))) ['import sys\n', 'print(sys.path)\n', 'x = 2\n', 'print(2 ** 33)\n'] >>> import functools, operator >>> functools.reduce(operator.add, open('script1.py')) 'import sys\nprint(sys.path)\nx = 2\nprint(2 ** 33)\n' All of these are iteration tools, but they have unique roles. We met zip and enumerate in the prior chapter; filter and reduce are in Chapter 19’s functional programming domain, so we’ll defer details for now. We first saw the sorted function used here at work in Chapter 4, and we used it for dictionaries in Chapter 8. sorted is a built-in that employs the iteration protocol—it’s like the original list sort method, but it returns the new sorted list as a result and runs Other Iteration Contexts | 363 on any iterable object. Notice that, unlike map and others, sorted returns an actual list in Python 3.0 instead of an iterable. Other built-in functions support the iteration protocol as well (but frankly, are harder to cast in interesting examples related to files). For example, the sum call computes the sum of all the numbers in any iterable; the any and all built-ins return True if any or all items in an iterable are True, respectively; and max and min return the largest and smallest item in an iterable, respectively. Like reduce, all of the tools in the following examples accept any iterable as an argument and use the iteration protocol to scan it, but return a single result: >>> sum([3, 2, 4, 1, 15 >>> any(['spam', '', True >>> all(['spam', '', False >>> max([3, 2, 5, 1, 5 >>> min([3, 2, 5, 1, 1 5, 0]) # sum expects numbers only 'ni']) 'ni']) 4]) 4]) Strictly speaking, the max and min functions can be applied to files as well—they automatically use the iteration protocol to scan the file and pick out the lines with the highest and lowest string values, respectively (though I’ll leave valid use cases to your imagination): >>> max(open('script1.py')) 'x = 2\n' >>> min(open('script1.py')) 'import sys\n' # Line with max/min string value Interestingly, the iteration protocol is even more pervasive in Python today than the examples so far have demonstrated—everything in Python’s built-in toolset that scans an object from left to right is defined to use the iteration protocol on the subject object. This even includes more esoteric tools such as the list and tuple built-in functions (which build new objects from iterables), the string join method (which puts a substring between strings contained in an iterable), and even sequence assignments. Consequently, all of these will also work on an open file and automatically read one line at a time: >>> list(open('script1.py')) ['import sys\n', 'print(sys.path)\n', 'x = 2\n', 'print(2 ** 33)\n'] >>> tuple(open('script1.py')) ('import sys\n', 'print(sys.path)\n', 'x = 2\n', 'print(2 ** 33)\n') >>> '&&'.join(open('script1.py')) 'import sys\n&&print(sys.path)\n&&x = 2\n&&print(2 ** 33)\n' >>> a, b, c, d = open('script1.py') >>> a, d 364 | Chapter 14: Iterations and Comprehensions, Part 1 ('import sys\n', 'print(2 ** 33)\n') >>> a, *b = open('script1.py') # 3.0 extended form >>> a, b ('import sys\n', ['print(sys.path)\n', 'x = 2\n', 'print(2 ** 33)\n']) Earlier, we saw that the built-in dict call accepts an iterable zip result, too. For that matter, so does the set call, as well as the new set and dictionary comprehension expressions in Python 3.0, which we met in Chapters 4, 5, and 8: >>> set(open('script1.py')) {'print(sys.path)\n', 'x = 2\n', 'print(2 ** 33)\n', 'import sys\n'} >>> {line for line in open('script1.py')} {'print(sys.path)\n', 'x = 2\n', 'print(2 ** 33)\n', 'import sys\n'} >>> {ix: line for ix, line in enumerate(open('script1.py'))} {0: 'import sys\n', 1: 'print(sys.path)\n', 2: 'x = 2\n', 3: 'print(2 ** 33)\n'} In fact, both set and dictionary comprehensions support the extended syntax of list comprehensions we met earlier in this chapter, including if tests: >>> {line for line in open('script1.py') if line[0] == 'p'} {'print(sys.path)\n', 'print(2 ** 33)\n'} >>> {ix: line for (ix, line) in enumerate(open('script1.py')) if line[0] == 'p'} {1: 'print(sys.path)\n', 3: 'print(2 ** 33)\n'} Like the list comprehension, both of these scan the file line by line and pick out lines that begin with the letter “p.” They also happen to build sets and dictionaries in the end, but we get a lot of work “for free” by combining file iteration and comprehension syntax. There’s one last iteration context that’s worth mentioning, although it’s a bit of a preview: in Chapter 18, we’ll learn that a special *arg form can be used in function calls to unpack a collection of values into individual arguments. As you can probably predict by now, this accepts any iterable, too, including files (see Chapter 18 for more details on the call syntax): >>> def f(a, b, c, d): print(a, b, c, d, sep='&') ... >>> f(1, 2, 3, 4) 1&2&3&4 >>> f(*[1, 2, 3, 4]) # Unpacks into arguments 1&2&3&4 >>> f(*open('script1.py')) import sys &print(sys.path) &x = 2 &print(2 ** 33) # Iterates by lines too! In fact, because this argument-unpacking syntax in calls accepts iterables, it’s also possible to use the zip built-in to unzip zipped tuples, by making prior or nested zip results Other Iteration Contexts | 365 arguments for another zip call (warning: you probably shouldn’t read the following example if you plan to operate heavy machinery anytime soon!): >>> X = (1, 2) >>> Y = (3, 4) >>> >>> list(zip(X, Y)) [(1, 3), (2, 4)] >>> >>> A, B = zip(*zip(X, Y)) >>> A (1, 2) >>> B (3, 4) # Zip tuples: returns an iterable # Unzip a zip! Still other tools in Python, such as the range built-in and dictionary view objects, return iterables instead of processing them. To see how these have been absorbed into the iteration protocol in Python 3.0 as well, we need to move on to the next section. New Iterables in Python 3.0 One of the fundamental changes in Python 3.0 is that it has a stronger emphasis on iterators than 2.X. In addition to the iterators associated with built-in types such as files and dictionaries, the dictionary methods keys, values, and items return iterable objects in Python 3.0, as do the built-in functions range, map, zip, and filter. As shown in the prior section, the last three of these functions both return iterators and process them. All of these tools produce results on demand in Python 3.0, instead of constructing result lists as they do in 2.6. Although this saves memory space, it can impact your coding styles in some contexts. In various places in this book so far, for example, we’ve had to wrap up various function and method call results in a list(...) call in order to force them to produce all their results at once: >>> zip('abc', 'xyz') <zip object at 0x02E66710> # An iterable in Python 3.0 (a list in 2.6) >>> list(zip('abc', 'xyz')) [('a', 'x'), ('b', 'y'), ('c', 'z')] # Force list of results in 3.0 to display This isn’t required in 2.6, because functions like zip return lists of results. In 3.0, though, they return iterable objects, producing results on demand. This means extra typing is required to display the results at the interactive prompt (and possibly in some other contexts), but it’s an asset in larger programs—delayed evaluation like this conserves memory and avoids pauses while large result lists are computed. Let’s take a quick look at some of the new 3.0 iterables in action. 366 | Chapter 14: Iterations and Comprehensions, Part 1 The range Iterator We studied the range built-in’s basic behavior in the prior chapter. In 3.0, it returns an iterator that generates numbers in the range on demand, instead of building the result list in memory. This subsumes the older 2.X xrange (see the upcoming version skew note), and you must use list(range(...)) to force an actual range list if one is needed (e.g., to display results): C:\\misc> c:\python30\python >>> R = range(10) >>> R range(0, 10) >>> >>> 0 >>> 1 >>> 2 I = iter(R) next(I) # range returns an iterator, not a list # Make an iterator from the range # Advance to next result # What happens in for loops, comprehensions, etc. next(I) next(I) >>> list(range(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] # To force a list if required Unlike the list returned by this call in 2.X, range objects in 3.0 support only iteration, indexing, and the len function. They do not support any other sequence operations (use list(...) if you require more list tools): >>> len(R) 10 >>> R[0] 0 >>> R[-1] 9 # range also does len and indexing, but no others >>> next(I) 3 >>> I.__next__() 4 # Continue taking from iterator, where left off # .next() becomes .__next__(), but use new next() Version skew note: Python 2.X also has a built-in called xrange, which is like range but produces items on demand instead of building a list of results in memory all at once. Since this is exactly what the new iteratorbased range does in Python 3.0, xrange is no longer available in 3.0—it has been subsumed. You may still see it in 2.X code, though, especially since range builds result lists there and so is not as efficient in its memory usage. As noted in a sidebar in the prior chapter, the file.xread lines() method used to minimize memory use in 2.X has been dropped in Python 3.0 for similar reasons, in favor of file iterators. New Iterables in Python 3.0 | 367 The map, zip, and filter Iterators Like range, the map, zip, and filter built-ins also become iterators in 3.0 to conserve space, rather than producing a result list all at once in memory. All three not only process iterables, as in 2.X, but also return iterable results in 3.0. Unlike range, though, they are their own iterators—after you step through their results once, they are exhausted. In other words, you can’t have multiple iterators on their results that maintain different positions in those results. Here is the case for the map built-in we met in the prior chapter. As with other iterators, you can force a list with list(...) if you really need one, but the default behavior can save substantial space in memory for large result sets: >>> M = map(abs, (-1, 0, 1)) >>> M <map object at 0x0276B890> >>> next(M) 1 >>> next(M) 0 >>> next(M) 1 >>> next(M) StopIteration # map returns an iterator, not a list >>> for x in M: print(x) ... # map iterator is now empty: one pass only >>> >>> ... 1 0 1 >>> [1, M = map(abs, (-1, 0, 1)) for x in M: print(x) # Make a new iterator to scan again # Iteration contexts auto call next() list(map(abs, (-1, 0, 1))) 0, 1] # Can force a real list if needed # Use iterator manually: exhausts results # These do not support len() or indexing The zip built-in, introduced in the prior chapter, returns iterators that work the same way: >>> Z = zip((1, 2, 3), (10, 20, 30)) >>> Z <zip object at 0x02770EE0> # zip is the same: a one-pass iterator >>> list(Z) [(1, 10), (2, 20), (3, 30)] >>> for pair in Z: print(pair) ... >>> Z = zip((1, 2, 3), (10, 20, 30)) >>> for pair in Z: print(pair) ... (1, 10) 368 | Chapter 14: Iterations and Comprehensions, Part 1 # Exhausted after one pass # Iterator used automatically or manually (2, 20) (3, 30) >>> >>> (1, >>> (2, Z = zip((1, 2, 3), (10, 20, 30)) next(Z) 10) next(Z) 20) The filter built-in, which we’ll study in the next part of this book, is also analogous. It returns items in an iterable for which a passed-in function returns True (as we’ve learned, in Python True includes nonempty objects): >>> filter(bool, ['spam', '', 'ni']) <filter object at 0x0269C6D0> >>> list(filter(bool, ['spam', '', 'ni'])) ['spam', 'ni'] Like most of the tools discussed in this section, filter both accepts an iterable to process and returns an iterable to generate results in 3.0. Multiple Versus Single Iterators It’s interesting to see how the range object differs from the built-ins described in this section—it supports len and indexing, it is not its own iterator (you make one with iter when iterating manually), and it supports multiple iterators over its result that remember their positions independently: >>> R = range(3) # range allows multiple iterators >>> next(R) TypeError: range object is not an iterator >>> >>> 0 >>> 1 >>> >>> 0 >>> 2 I1 = iter(R) next(I1) next(I1) I2 = iter(R) next(I2) # Two iterators on one range next(I1) # I1 is at a different spot than I2 By contrast, zip, map, and filter do not support multiple active iterators on the same result: >>> >>> >>> >>> (1, >>> (2, >>> Z = zip((1, 2, 3), (10, 11, 12)) I1 = iter(Z) I2 = iter(Z) next(I1) 10) next(I1) 11) next(I2) # Two iterators on one zip # I2 is at same spot as I1! New Iterables in Python 3.0 | 369 (3, 12) >>> M = map(abs, (-1, 0, 1)) >>> I1 = iter(M); I2 = iter(M) >>> print(next(I1), next(I1), next(I1)) 1 0 1 >>> next(I2) StopIteration # Ditto for map (and filter) >>> R = range(3) >>> I1, I2 = iter(R), iter(R) >>> [next(I1), next(I1), next(I1)] [0 1 2] >>> next(I2) 0 # But range allows many iterators When we code our own iterable objects with classes later in the book (Chapter 29), we’ll see that multiple iterators are usually supported by returning new objects for the iter call; a single iterator generally means an object returns itself. In Chapter 20, we’ll also find that generator functions and expressions behave like map and zip instead of range in this regard, supporting a single active iteration. In that chapter, we’ll see some subtle implications of one-shot iterators in loops that attempt to scan multiple times. Dictionary View Iterators As we saw briefly in Chapter 8, in Python 3.0 the dictionary keys, values, and items methods return iterable view objects that generate result items one at a time, instead of producing result lists all at once in memory. View items maintain the same physical ordering as that of the dictionary and reflect changes made to the underlying dictionary. Now that we know more about iterators, here’s the rest of the story: >>> D = dict(a=1, b=2, c=3) >>> D {'a': 1, 'c': 3, 'b': 2} >>> K = D.keys() >>> K <dict_keys object at 0x026D83C0> # A view object in 3.0, not a list >>> next(K) # Views are not iterators themselves TypeError: dict_keys object is not an iterator >>> I = iter(K) >>> next(I) 'a' >>> next(I) 'c' # Views have an iterator, # which can be used manually # but does not support len(), index >>> for k in D.keys(): print(k, end=' ') ... a c b # All iteration contexts use auto 370 | Chapter 14: Iterations and Comprehensions, Part 1 As for all iterators, you can always force a 3.0 dictionary view to build a real list by passing it to the list built-in. However, this usually isn’t required except to display results interactively or to apply list operations like indexing: >>> K = D.keys() >>> list(K) ['a', 'c', 'b'] >>> V = D.values() >>> V <dict_values object at 0x026D8260> >>> list(V) [1, 3, 2] # Can still force a real list if needed # Ditto for values() and items() views >>> list(D.items()) [('a', 1), ('c', 3), ('b', 2)] >>> for (k, v) in D.items(): print(k, v, end=' ') ... a 1 c 3 b 2 In addition, 3.0 dictionaries still have iterators themselves, which return successive keys. Thus, it’s not often necessary to call keys directly in this context: >>> D {'a': 1, 'c': 3, 'b': 2} >>> I = iter(D) >>> next(I) 'a' >>> next(I) 'c' # Dictionaries still have own iterator # Returns next key on each iteration >>> for key in D: print(key, end=' ') ... a c b # Still no need to call keys() to iterate # But keys is an iterator in 3.0 too! Finally, remember again that because keys no longer returns a list, the traditional coding pattern for scanning a dictionary by sorted keys won’t work in 3.0. Instead, convert keys views first with a list call, or use the sorted call on either a keys view or the dictionary itself, as follows: >>> D {'a': 1, 'c': 3, 'b': 2} >>> for k in sorted(D.keys())): print(k, D[k], end=' ') ... a 1 b 2 c 3 >>> D {'a': 1, 'c': 3, 'b': 2} >>> for k in sorted(D): print(k, D[k], end=' ') ... a 1 b 2 c 3 # Best practice key sorting New Iterables in Python 3.0 | 371 Other Iterator Topics We’ll learn more about both list comprehensions and iterators in Chapter 20, in conjunction with functions, and again in Chapter 29 when we study classes. As you’ll see later: • User-defined functions can be turned into iterable generator functions, with yield statements. • List comprehensions morph into iterable generator expressions when coded in parentheses. • User-defined classes are made iterable with __iter__ or __getitem__ operator overloading. In particular, user-defined iterators defined with classes allow arbitrary objects and operations to be used in any of the iteration contexts we’ve met here. Chapter Summary In this chapter, we explored concepts related to looping in Python. We took our first substantial look at the iteration protocol in Python—a way for nonsequence objects to take part in iteration loops—and at list comprehensions. As we saw, a list comprehension is an expression similar to a for loop that applies another expression to all the items in any iterable object. Along the way, we also saw other built-in iteration tools at work and studied recent iteration additions in Python 3.0. This wraps up our tour of specific procedural statements and related tools. The next chapter closes out this part of the book by discussing documentation options for Python code; documentation is also part of the general syntax model, and it’s an important component of well-written programs. In the next chapter, we’ll also dig into a set of exercises for this part of the book before we turn our attention to larger structures such as functions. As usual, though, let’s first exercise what we’ve learned here with a quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. How are for loops and iterators related? How are for loops and list comprehensions related? Name four iteration contexts in the Python language. What is the best way to read line by line from a text file today? What sort of weapons would you expect to see employed by the Spanish Inquisition? 372 | Chapter 14: Iterations and Comprehensions, Part 1 Test Your Knowledge: Answers 1. The for loop uses the iteration protocol to step through items in the object across which it is iterating. It calls the object’s __next__ method (run by the next built-in) on each iteration and catches the StopIteration exception to determine when to stop looping. Any object that supports this model works in a for loop and in other iteration contexts. 2. Both are iteration tools. List comprehensions are a concise and efficient way to perform a common for loop task: collecting the results of applying an expression to all items in an iterable object. It’s always possible to translate a list comprehension to a for loop, and part of the list comprehension expression looks like the header of a for loop syntactically. 3. Iteration contexts in Python include the for loop; list comprehensions; the map built-in function; the in membership test expression; and the built-in functions sorted, sum, any, and all. This category also includes the list and tuple built-ins, string join methods, and sequence assignments, all of which use the iteration protocol (the __next__ method) to step across iterable objects one item at a time. 4. The best way to read lines from a text file today is to not read it explicitly at all: instead, open the file within an iteration context such as a for loop or list comprehension, and let the iteration tool automatically scan one line at a time by running the file’s next method on each iteration. This approach is generally best in terms of coding simplicity, execution speed, and memory space requirements. 5. I’ll accept any of the following as correct answers: fear, intimidation, nice red uniforms, a comfy chair, and soft pillows. Test Your Knowledge: Answers | 373 CHAPTER 15 The Documentation Interlude This part of the book concludes with a look at techniques and tools used for documenting Python code. Although Python code is designed to be readable, a few well-placed human-readable comments can do much to help others understand the workings of your programs. Python includes syntax and tools to make documentation easier. Although this is something of a tools-related concept, the topic is presented here partly because it involves Python’s syntax model, and partly as a resource for readers struggling to understand Python’s toolset. For the latter purpose, I’ll expand here on documentation pointers first given in Chapter 4. As usual, in addition to the chapter quiz this concluding chapter ends with some warnings about common pitfalls and a set of exercises for this part of the text. Python Documentation Sources By this point in the book, you’re probably starting to realize that Python comes with an amazing amount of prebuilt functionality—built-in functions and exceptions, predefined object attributes and methods, standard library modules, and more. And we’ve really only scratched the surface of each of these categories. One of the first questions that bewildered beginners often ask is: how do I find information on all the built-in tools? This section provides hints on the various documentation sources available in Python. It also presents documentation strings (docstrings) and the PyDoc system that makes use of them. These topics are somewhat peripheral to the core language itself, but they become essential knowledge as soon as your code reaches the level of the examples and exercises in this part of the book. As summarized in Table 15-1, there are a variety of places to look for information on Python, with generally increasing verbosity. Because documentation is such a crucial tool in practical programming, we’ll explore each of these categories in the sections that follow. 375 Table 15-1. Python documentation sources Form Role # comments In-file documentation The dir function Lists of attributes available in objects Docstrings: __doc__ In-file documentation attached to objects PyDoc: The help function Interactive help for objects PyDoc: HTML reports Module documentation in a browser The standard manual set Official language and library descriptions Web resources Online tutorials, examples, and so on Published books Commercially available reference texts # Comments Hash-mark comments are the most basic way to document your code. Python simply ignores all the text following a # (as long as it’s not inside a string literal), so you can follow this character with words and descriptions meaningful to programmers. Such comments are accessible only in your source files, though; to code comments that are more widely available, you’ll need to use docstrings. In fact, current best practice generally dictates that docstrings are best for larger functional documentation (e.g., “my file does this”), and # comments are best limited to smaller code documentation (e.g., “this strange expression does that”). More on docstrings in a moment. The dir Function The built-in dir function is an easy way to grab a list of all the attributes available inside an object (i.e., its methods and simpler data items). It can be called on any object that has attributes. For example, to find out what’s available in the standard library’s sys module, import it and pass it to dir (these results are from Python 3.0; they might vary slightly on 2.6): >>> import sys >>> dir(sys) ['__displayhook__', '__doc__', '__excepthook__', '__name__', '__package__', '__stderr__', '__stdin__', '__stdout__', '_clear_type_cache', '_current_frames', '_getframe', 'api_version', 'argv', 'builtin_module_names', 'byteorder', 'call_tracing', 'callstats', 'copyright', 'displayhook', 'dllhandle', 'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix', 'executable', 'exit', 'flags', 'float_info', 'getcheckinterval', 'getdefaultencoding', ...more names omitted...] Only some of the many names are displayed here; run these statements on your machine to see the full list. 376 | Chapter 15: The Documentation Interlude To find out what attributes are provided in built-in object types, run dir on a literal (or existing instance) of the desired type. For example, to see list and string attributes, you can pass empty objects: >>> dir([]) ['__add__', '__class__', '__contains__', ...more... 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort'] >>> dir('') ['__add__', '__class__', '__contains__', ...more... 'capitalize', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', ' maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', ...more names omitted...] dir results for any built-in type include a set of attributes that are related to the imple- mentation of that type (technically, operator overloading methods); they all begin and end with double underscores to make them distinct, and you can safely ignore them at this point in the book. Incidentally, you can achieve the same effect by passing a type name to dir instead of a literal: >>> dir(str) == dir('') True >>> dir(list) == dir([]) True # Same result as prior example This works because names like str and list that were once type converter functions are actually names of types in Python today; calling one of these invokes its constructor to generate an instance of that type. I’ll have more to say about constructors and operator overloading methods when we discuss classes in Part VI. The dir function serves as a sort of memory-jogger—it provides a list of attribute names, but it does not tell you anything about what those names mean. For such extra information, we need to move on to the next documentation source. Docstrings: __doc__ Besides # comments, Python supports documentation that is automatically attached to objects and retained at runtime for inspection. Syntactically, such comments are coded as strings at the tops of module files and function and class statements, before any other executable code (# comments are OK before them). Python automatically stuffs the strings, known as docstrings, into the __doc__ attributes of the corresponding objects. Python Documentation Sources | 377 User-defined docstrings For example, consider the following file, docstrings.py. Its docstrings appear at the beginning of the file and at the start of a function and a class within it. Here, I’ve used triple-quoted block strings for multiline comments in the file and the function, but any sort of string will work. We haven’t studied the def or class statements in detail yet, so ignore everything about them except the strings at their tops: """ Module documentation Words Go Here """ spam = 40 def square(x): """ function documentation can we have your liver then? """ return x ** 2 # square class Employee: "class documentation" pass print(square(4)) print(square.__doc__) The whole point of this documentation protocol is that your comments are retained for inspection in __doc__ attributes after the file is imported. Thus, to display the docstrings associated with the module and its objects, we simply import the file and print their __doc__ attributes, where Python has saved the text: >>> import docstrings 16 function documentation can we have your liver then? >>> print(docstrings.__doc__) Module documentation Words Go Here >>> print(docstrings.square.__doc__) function documentation can we have your liver then? >>> print(docstrings.Employee.__doc__) class documentation 378 | Chapter 15: The Documentation Interlude Note that you will generally want to use print to print docstrings; otherwise, you’ll get a single string with embedded newline characters. You can also attach docstrings to methods of classes (covered in Part VI), but because these are just def statements nested in class statements, they’re not a special case. To fetch the docstring of a method function inside a class within a module, you would simply extend the path to go through the class: module.class.method.__doc__ (we’ll see an example of method docstrings in Chapter 28). Docstring standards There is no broad standard about what should go into the text of a docstring (although some companies have internal standards). There have been various markup language and template proposals (e.g., HTML or XML), but they don’t seem to have caught on in the Python world. And frankly, convincing Python programmers to document their code using handcoded HTML is probably not going to happen in our lifetimes! Documentation tends to have a low priority amongst programmers in general. Usually, if you get any comments in a file at all, you count yourself lucky. I strongly encourage you to document your code liberally, though—it really is an important part of wellwritten programs. The point here is that there is presently no standard on the structure of docstrings; if you want to use them, anything goes today. Built-in docstrings As it turns out, built-in modules and objects in Python use similar techniques to attach documentation above and beyond the attribute lists returned by dir. For example, to see an actual human-readable description of a built-in module, import it and print its __doc__ string: >>> import sys >>> print(sys.__doc__) This module provides access to some objects used or maintained by the interpreter and to functions that interact strongly with the interpreter. Dynamic objects: argv -path -modules ...more command line arguments; argv[0] is the script pathname if known module search path; path[0] is the script directory, else '' -- dictionary of loaded modules text omitted... Functions, classes, and methods within built-in modules have attached descriptions in their __doc__ attributes as well: >>> print(sys.getrefcount.__doc__) getrefcount(object) -> integer Return the reference count of object. The count returned is generally one higher than you might expect, because it includes the (temporary) ...more text omitted... Python Documentation Sources | 379 You can also read about built-in functions via their docstrings: >>> print(int.__doc__) int(x[, base]) -> integer Convert a string or number to an integer, if possible. A floating point argument will be truncated towards zero (this does not include a ...more text omitted... >>> print(map.__doc__) map(func, *iterables) --> map object Make an iterator that computes the function using arguments from each of the iterables. Stops when the shortest iterable is exhausted. You can get a wealth of information about built-in tools by inspecting their docstrings this way, but you don’t have to—the help function, the topic of the next section, does this automatically for you. PyDoc: The help Function The docstring technique proved to be so useful that Python now ships with a tool that makes docstrings even easier to display. The standard PyDoc tool is Python code that knows how to extract docstrings and associated structural information and format them into nicely arranged reports of various types. Additional tools for extracting and formatting docstrings are available in the open source domain (including tools that may support structured text—search the Web for pointers), but Python ships with PyDoc in its standard library. There are a variety of ways to launch PyDoc, including command-line script options (see the Python library manual for details). Perhaps the two most prominent PyDoc interfaces are the built-in help function and the PyDoc GUI/HTML interface. The help function invokes PyDoc to generate a simple textual report (which looks much like a “manpage” on Unix-like systems): >>> import sys >>> help(sys.getrefcount) Help on built-in function getrefcount in module sys: getrefcount(...) getrefcount(object) -> integer Return the reference count of object. The count returned is generally one higher than you might expect, because it includes the (temporary) ...more omitted... Note that you do not have to import sys in order to call help, but you do have to import sys to get help on sys; it expects an object reference to be passed in. For larger objects such as modules and classes, the help display is broken down into multiple sections, a few of which are shown here. Run this interactively to see the full report: 380 | Chapter 15: The Documentation Interlude >>> help(sys) Help on built-in module sys: NAME sys FILE (built-in) MODULE DOCS http://docs.python.org/library/sys DESCRIPTION This module provides access to some objects used or maintained by the interpreter and to functions that interact strongly with the interpreter. ...more omitted... FUNCTIONS __displayhook__ = displayhook(...) displayhook(object) -> None Print an object to sys.stdout and also save it in builtins. ...more omitted... DATA __stderr__ = <io.TextIOWrapper object at 0x0236E950> __stdin__ = <io.TextIOWrapper object at 0x02366550> __stdout__ = <io.TextIOWrapper object at 0x02366E30> ...more omitted... Some of the information in this report is docstrings, and some of it (e.g., function call patterns) is structural information that PyDoc gleans automatically by inspecting objects’ internals, when available. You can also use help on built-in functions, methods, and types. To get help for a built-in type, use the type name (e.g., dict for dictionary, str for string, list for list). You’ll get a large display that describes all the methods available for that type: >>> help(dict) Help on class dict in module builtins: class dict(object) | dict() -> new empty dictionary. | dict(mapping) -> new dictionary initialized from a mapping object's ...more omitted... >>> help(str.replace) Help on method_descriptor: replace(...) S.replace (old, new[, count]) -> str Return a copy of S with all occurrences of substring ...more omitted... >>> help(ord) Python Documentation Sources | 381 Help on built-in function ord in module builtins: ord(...) ord(c) -> integer Return the integer ordinal of a one-character string. Finally, the help function works just as well on your modules as it does on built-ins. Here it is reporting on the docstrings.py file we coded earlier. Again, some of this is docstrings, and some is information automatically extracted by inspecting objects’ structures: >>> import docstrings >>> help(docstrings.square) Help on function square in module docstrings: square(x) function documentation can we have your liver then? >>> help(docstrings.Employee) Help on class Employee in module docstrings: class Employee(builtins.object) | class documentation | | Data descriptors defined here: ...more omitted... >>> help(docstrings) Help on module docstrings: NAME docstrings FILE c:\misc\docstrings.py DESCRIPTION Module documentation Words Go Here CLASSES builtins.object Employee class Employee(builtins.object) | class documentation | | Data descriptors defined here: ...more omitted... FUNCTIONS square(x) function documentation 382 | Chapter 15: The Documentation Interlude can we have your liver then? DATA spam = 40 PyDoc: HTML Reports The help function is nice for grabbing documentation when working interactively. For a more grandiose display, however, PyDoc also provides a GUI interface (a simple but portable Python/tkinter script) and can render its report in HTML page format, viewable in any web browser. In this mode, PyDoc can run locally or as a remote server in client/server mode; reports contain automatically created hyperlinks that allow you to click your way through the documentation of related components in your application. To start PyDoc in this mode, you generally first launch the search engine GUI captured in Figure 15-1. You can start this either by selecting the “Module Docs” item in Python’s Start button menu on Windows, or by launching the pydoc.py script in Python’s standard library directory: Lib on Windows (run pydoc.py with a -g command-line argument). Enter the name of a module you’re interested in, and press the Enter key; PyDoc will march down your module import search path (sys.path) looking for references to the requested module. Figure 15-1. The Pydoc top-level search engine GUI: type the name of a module you want documentation for, press Enter, select the module, and then press “go to selected” (or omit the module name and press “open browser” to see all available modules). Once you’ve found a promising entry, select it and click “go to selected.” PyDoc will spawn a web browser on your machine to display the report rendered in HTML format. Figure 15-2 shows the information PyDoc displays for the built-in glob module. Notice the hyperlinks in the Modules section of this page—you can click these to jump to the PyDoc pages for related (imported) modules. For larger pages, PyDoc also generates hyperlinks to sections within the page. Python Documentation Sources | 383 Figure 15-2. When you find a module in the Figure 15-1 GUI (such as this built-in standard library module) and press “go to selected,” the module’s documentation is rendered in HTML and displayed in a web browser window like this one. Like the help function interface, the GUI interface works on user-defined modules as well as built-ins. Figure 15-3 shows the page generated for our docstrings.py module file. PyDoc can be customized and launched in various ways we won’t cover here; see its entry in Python’s standard library manual for more details. The main thing to take away from this section is that PyDoc essentially gives you implementation reports “for free”—if you are good about using docstrings in your files, PyDoc does all the work of collecting and formatting them for display. PyDoc only helps for objects like functions and modules, but it provides an easy way to access a middle level of documentation for such tools—its reports are more useful than raw attribute lists, and less exhaustive than the standard manuals. Cool PyDoc trick of the day: If you leave the module name empty in the top input field of the window in Figure 15-1 and press the “open browser” button, PyDoc will produce a web page containing a hyperlink to every module you can possibly import on your computer. This includes Python standard library modules, modules of third-party 384 | Chapter 15: The Documentation Interlude Figure 15-3. PyDoc can serve up documentation pages for both built-in and user-coded modules. Here is the page for a user-defined module, showing all its documentation strings (docstrings) extracted from the source file. extensions you may have installed, user-defined modules on your import search path, and even statically or dynamically linked-in C-coded modules. Such information is hard to come by otherwise without writing code that inspects a set of module sources. PyDoc can also be run to save the HTML documentation for a module in a file for later viewing or printing; see its documentation for pointers. Also, note that PyDoc might not work well if run on scripts that read from standard input—PyDoc imports the target module to inspect its contents, and there may be no connection for standard input text when it is run in GUI mode. Modules that can be imported without immediate input requirements will always work under PyDoc, though. Python Documentation Sources | 385 The Standard Manual Set For the complete and most up-to-date description of the language and its toolset, Python’s standard manuals stand ready to serve. Python’s manuals ship in HTML and other formats, and they are installed with the Python system on Windows—they are available in your Start button’s menu for Python, and they can also be opened from the Help menu within IDLE. You can also fetch the manual set separately from http://www .python.org in a variety of formats, or read them online at that site (follow the Documentation link). On Windows, the manuals are a compiled help file to support searches, and the online versions at the Python website include a web-based search page. When opened, the Windows format of the manuals displays a root page like that in Figure 15-4. The two most important entries here are most likely the Library Reference (which documents built-in types, functions, exceptions, and standard library modules) and the Language Reference (which provides a formal description of language-level details). The tutorial listed on this page also provides a brief introduction for newcomers. Figure 15-4. Python’s standard manual set, available online at http://www.python.org, from IDLE’s Help menu, and in the Windows Start button menu. It’s a searchable help file on Windows, and there is a search engine for the online version. Of these, the Library Reference is the one you’ll want to use most of the time. 386 | Chapter 15: The Documentation Interlude Web Resources At the official Python website (http://www.python.org), you’ll find links to various Python resources, some of which cover special topics or domains. Click the Documentation link to access an online tutorial and the Beginners Guide to Python. The site also lists non-English Python resources. You will find numerous Python wikis, blogs, websites, and a host of other resources on the Web today. To sample the online community, try searching for a term like “Python programming” in Google. Published Books As a final resource, you can choose from a large collection of reference books for Python. Bear in mind that books tend to lag behind the cutting edge of Python changes, partly because of the work involved in writing, and partly because of the natural delays built into the publishing cycle. Usually, by the time a book comes out, it’s three or more months behind the current Python state. Unlike standard manuals, books are also generally not free. Still, for many, the convenience and quality of a professionally published text is worth the cost. Moreover, Python changes so slowly that books are usually still relevant years after they are published, especially if their authors post updates on the Web. See the Preface for pointers to other Python books. Common Coding Gotchas Before the programming exercises for this part of the book, let’s run through some of the most common mistakes beginners make when coding Python statements and programs. Many of these are warnings I’ve thrown out earlier in this part of the book, collected here for ease of reference. You’ll learn to avoid these pitfalls once you’ve gained a bit of Python coding experience, but a few words now might help you avoid falling into some of these traps initially: • Don’t forget the colons. Always remember to type a : at the end of compound statement headers (the first line of an if, while, for, etc.). You’ll probably forget at first (I did, and so have most of my 3,000 Python students over the years), but you can take some comfort from the fact that it will soon become an unconscious habit. • Start in column 1. Be sure to start top-level (unnested) code in column 1. That includes unnested code typed into module files, as well as unnested code typed at the interactive prompt. Common Coding Gotchas | 387 • Blank lines matter at the interactive prompt. Blank lines in compound statements are always ignored in module files, but when you’re typing code at the interactive prompt, they end the statement. In other words, blank lines tell the interactive command line that you’ve finished a compound statement; if you want to continue, don’t hit the Enter key at the ... prompt (or in IDLE) until you’re really done. • Indent consistently. Avoid mixing tabs and spaces in the indentation of a block, unless you know what your text editor does with tabs. Otherwise, what you see in your editor may not be what Python sees when it counts tabs as a number of spaces. This is true in any block-structured language, not just Python—if the next programmer has her tabs set differently, she will not understand the structure of your code. It’s safer to use all tabs or all spaces for each block. • Don’t code C in Python. A reminder for C/C++ programmers: you don’t need to type parentheses around tests in if and while headers (e.g., if (X==1):). You can, if you like (any expression can be enclosed in parentheses), but they are fully superfluous in this context. Also, do not terminate all your statements with semicolons; it’s technically legal to do this in Python as well, but it’s totally useless unless you’re placing more than one statement on a single line (the end of a line normally terminates a statement). And remember, don’t embed assignment statements in while loop tests, and don’t use {} around blocks (indent your nested code blocks consistently instead). • Use simple for loops instead of while or range. Another reminder: a simple for loop (e.g., for x in seq:) is almost always simpler to code and quicker to run than a while- or range-based counter loop. Because Python handles indexing internally for a simple for, it can sometimes be twice as fast as the equivalent while. Avoid the temptation to count things in Python! • Beware of mutables in assignments. I mentioned this in Chapter 11: you need to be careful about using mutables in a multiple-target assignment (a = b = []), as well as in an augmented assignment (a += [1, 2]). In both cases, in-place changes may impact other variables. See Chapter 11 for details. • Don’t expect results from functions that change objects in-place. We encountered this one earlier, too: in-place change operations like the list.append and list.sort methods introduced in Chapter 8 do not return values (other than None), so you should call them without assigning the result. It’s not uncommon for beginners to say something like mylist = mylist.append(X) to try to get the result of an append, but what this actually does is assign mylist to None, not to the modified list (in fact, you’ll lose your reference to the list altogether). A more devious example of this pops up in Python 2.X code when trying to step through dictionary items in a sorted fashion. It’s fairly common to see code like for k in D.keys().sort():. This almost works—the keys method builds a keys list, and the sort method orders it—but because the sort method returns None, the loop fails because it is ultimately a loop over None (a nonsequence). This fails even 388 | Chapter 15: The Documentation Interlude sooner in Python 3.0, because dictionary keys are views, not lists! To code this correctly, either use the newer sorted built-in function, which returns the sorted list, or split the method calls out to statements: Ks = list(D.keys()), then Ks.sort(), and finally, for k in Ks:. This, by the way, is one case where you’ll still want to call the keys method explicitly for looping, instead of relying on the dictionary iterators—iterators do not sort. • Always use parentheses to call a function. You must add parentheses after a function name to call it, whether it takes arguments or not (e.g., use function(), not function). In Part IV, we’ll see that functions are simply objects that have a special operation—a call that you trigger with the parentheses. In classes, this problem seems to occur most often with files; it’s common to see beginners type file.close to close a file, rather than file.close(). Because it’s legal to reference a function without calling it, the first version with no parentheses succeeds silently, but it does not close the file! • Don’t use extensions or paths in imports and reloads. Omit directory paths and file suffixes in import statements (e.g., say import mod, not import mod.py). (We discussed module basics in Chapter 3 and will continue studying modules in Part V.) Because modules may have other suffixes besides .py (.pyc, for instance), hardcoding a particular suffix is not only illegal syntax, but doesn’t make sense. Any platform-specific directory path syntax comes from module search path settings, not the import statement. Chapter Summary This chapter took us on a tour of program documentation—both documentation we write ourselves for our own programs, and documentation available for built-in tools. We met docstrings, explored the online and manual resources for Python reference, and learned how PyDoc’s help function and web page interface provide extra sources of documentation. Because this is the last chapter in this part of the book, we also reviewed common coding mistakes to help you avoid them. In the next part of this book, we’ll start applying what we already know to larger program constructs: functions. Before moving on, however, be sure to work through the set of lab exercises for this part of the book that appear at the end of this chapter. And even before that, let’s run through this chapter’s quiz. Test Your Knowledge: Quiz 1. When should you use documentation strings instead of hash-mark comments? 2. Name three ways you can view documentation strings. Test Your Knowledge: Quiz | 389 3. How can you obtain a list of the available attributes in an object? 4. How can you get a list of all available modules on your computer? 5. Which Python book should you purchase after this one? Test Your Knowledge: Answers 1. Documentation strings (docstrings) are considered best for larger, functional documentation, describing the use of modules, functions, classes, and methods in your code. Hash-mark comments are today best limited to micro-documentation about arcane expressions or statements. This is partly because docstrings are easier to find in a source file, but also because they can be extracted and displayed by the PyDoc system. 2. You can see docstrings by printing an object’s __doc__ attribute, by passing it to PyDoc’s help function, and by selecting modules in PyDoc’s GUI search engine in client/server mode. Additionally, PyDoc can be run to save a module’s documentation in an HTML file for later viewing or printing. 3. The built-in dir(X) function returns a list of all the attributes attached to any object. 4. Run the PyDoc GUI interface, leave the module name blank, and select “open browser”; this opens a web page containing a link to every module available to your programs. 5. Mine, of course. (Seriously, the Preface lists a few recommended follow-up books, both for reference and for application tutorials.) Test Your Knowledge: Part III Exercises Now that you know how to code basic program logic, the following exercises will ask you to implement some simple tasks with statements. Most of the work is in exercise 4, which lets you explore coding alternatives. There are always many ways to arrange statements, and part of learning Python is learning which arrangements work better than others. See Part III in Appendix B for the solutions. 1. Coding basic loops. a. Write a for loop that prints the ASCII code of each character in a string named S. Use the built-in function ord(character) to convert each character to an ASCII integer. (Test it interactively to see how it works.) b. Next, change your loop to compute the sum of the ASCII codes of all the characters in a string. 390 | Chapter 15: The Documentation Interlude c. Finally, modify your code again to return a new list that contains the ASCII codes of each character in the string. Does the expression map(ord, S) have a similar effect? (Hint: see Chapter 14.) 2. Backslash characters. What happens on your machine when you type the following code interactively? for i in range(50): print('hello %d\n\a' % i) Beware that if it’s run outside of the IDLE interface this example may beep at you, so you may not want to run it in a crowded lab. IDLE prints odd characters instead of beeping (see the backslash escape characters in Table 7-2). 3. Sorting dictionaries. In Chapter 8, we saw that dictionaries are unordered collections. Write a for loop that prints a dictionary’s items in sorted (ascending) order. (Hint: use the dictionary keys and list sort methods, or the newer sorted built-in function.) 4. Program logic alternatives. Consider the following code, which uses a while loop and found flag to search a list of powers of 2 for the value of 2 raised to the fifth power (32). It’s stored in a module file called power.py. L = [1, 2, 4, 8, 16, 32, 64] X = 5 found = False i = 0 while not found and i < len(L): if 2 ** X == L[i]: found = True else: i = i+1 if found: print('at index', i) else: print(X, 'not found') C:\book\tests> python power.py at index 5 As is, the example doesn’t follow normal Python coding techniques. Follow the steps outlined here to improve it (for all the transformations, you may either type your code interactively or store it in a script file run from the system command line—using a file makes this exercise much easier): a. First, rewrite this code with a while loop else clause to eliminate the found flag and final if statement. b. Next, rewrite the example to use a for loop with an else clause, to eliminate the explicit list-indexing logic. (Hint: to get the index of an item, use the list index method—L.index(X) returns the offset of the first X in list L.) Test Your Knowledge: Part III Exercises | 391 c. Next, remove the loop completely by rewriting the example with a simple in operator membership expression. (See Chapter 8 for more details, or type this to test: 2 in [1,2,3].) d. Finally, use a for loop and the list append method to generate the powers-of-2 list (L) instead of hardcoding a list literal. Deeper thoughts: e. Do you think it would improve performance to move the 2 ** X expression outside the loops? How would you code that? f. As we saw in exercise 1, Python includes a map(function, list) tool that can generate a powers-of-2 list, too: map(lambda x: 2 ** x, range(7)). Try typing this code interactively; we’ll meet lambda more formally in Chapter 19. 392 | Chapter 15: The Documentation Interlude PART IV Functions CHAPTER 16 Function Basics In Part III, we looked at basic procedural statements in Python. Here, we’ll move on to explore a set of additional statements that we can use to create functions of our own. In simple terms, a function is a device that groups a set of statements so they can be run more than once in a program. Functions also can compute a result value and let us specify parameters that serve as function inputs, which may differ each time the code is run. Coding an operation as a function makes it a generally useful tool, which we can use in a variety of contexts. More fundamentally, functions are the alternative to programming by cutting and pasting—rather than having multiple redundant copies of an operation’s code, we can factor it into a single function. In so doing, we reduce our future work radically: if the operation must be changed later, we only have one copy to update, not many. Functions are the most basic program structure Python provides for maximizing code reuse and minimizing code redundancy. As we’ll see, functions are also a design tool that lets us split complex systems into manageable parts. Table 16-1 summarizes the primary function-related tools we’ll study in this part of the book. Table 16-1. Function-related statements and expressions Statement Examples Calls myfunc('spam', 'eggs', meat=ham) def, return def adder(a, b=1, *c): return a + b + c[0] global def changer(): global x; x = 'new' nonlocal def changer(): nonlocal x; x = 'new' yield def squares(x): for i in range(x): yield i ** 2 lambda funcs = [lambda x: x**2, lambda x: x*3] 395 Why Use Functions? Before we get into the details, let’s establish a clear picture of what functions are all about. Functions are a nearly universal program-structuring device. You may have come across them before in other languages, where they may have been called subroutines or procedures. As a brief introduction, functions serve two primary development roles: Maximizing code reuse and minimizing redundancy As in most programming languages, Python functions are the simplest way to package logic you may wish to use in more than one place and more than one time. Up until now, all the code we’ve been writing has run immediately. Functions allow us to group and generalize code to be used arbitrarily many times later. Because they allow us to code an operation in a single place and use it in many places, Python functions are the most basic factoring tool in the language: they allow us to reduce code redundancy in our programs, and thereby reduce maintenance effort. Procedural decomposition Functions also provide a tool for splitting systems into pieces that have well-defined roles. For instance, to make a pizza from scratch, you would start by mixing the dough, rolling it out, adding toppings, baking it, and so on. If you were programming a pizza-making robot, functions would help you divide the overall “make pizza” task into chunks—one function for each subtask in the process. It’s easier to implement the smaller tasks in isolation than it is to implement the entire process at once. In general, functions are about procedure—how to do something, rather than what you’re doing it to. We’ll see why this distinction matters in Part VI, when we start making new object with classes. In this part of the book, we’ll explore the tools used to code functions in Python: function basics, scope rules, and argument passing, along with a few related concepts such as generators and functional tools. Because its importance begins to become more apparent at this level of coding, we’ll also revisit the notion of polymorphism introduced earlier in the book. As you’ll see, functions don’t imply much new syntax, but they do lead us to some bigger programming ideas. Coding Functions Although it wasn’t made very formal, we’ve already used some functions in earlier chapters. For instance, to make a file object, we called the built-in open function; similarly, we used the len built-in function to ask for the number of items in a collection object. In this chapter, we will explore how to write new functions in Python. Functions we write behave the same way as the built-ins we’ve already seen: they are called in 396 | Chapter 16: Function Basics expressions, are passed values, and return results. But writing new functions requires the application of a few additional ideas that haven’t yet been introduced. Moreover, functions behave very differently in Python than they do in compiled languages like C. Here is a brief introduction to the main concepts behind Python functions, all of which we will study in this part of the book: • def is executable code. Python functions are written with a new statement, the def. Unlike functions in compiled languages such as C, def is an executable statement—your function does not exist until Python reaches and runs the def. In fact, it’s legal (and even occasionally useful) to nest def statements inside if statements, while loops, and even other defs. In typical operation, def statements are coded in module files and are naturally run to generate functions when a module file is first imported. • def creates an object and assigns it to a name. When Python reaches and runs a def statement, it generates a new function object and assigns it to the function’s name. As with all assignments, the function name becomes a reference to the function object. There’s nothing magic about the name of a function—as you’ll see, the function object can be assigned to other names, stored in a list, and so on. Function objects may also have arbitrary user-defined attributes attached to them to record data. • lambda creates an object but returns it as a result. Functions may also be created with the lambda expression, a feature that allows us to in-line function definitions in places where a def statement won’t work syntactically (this is a more advanced concept that we’ll defer until Chapter 19). • return sends a result object back to the caller. When a function is called, the caller stops until the function finishes its work and returns control to the caller. Functions that compute a value send it back to the caller with a return statement; the returned value becomes the result of the function call. • yield sends a result object back to the caller, but remembers where it left off. Functions known as generators may also use the yield statement to send back a value and suspend their state such that they may be resumed later, to produce a series of results over time. This is another advanced topic covered later in this part of the book. • global declares module-level variables that are to be assigned. By default, all names assigned in a function are local to that function and exist only while the function runs. To assign a name in the enclosing module, functions need to list it in a global statement. More generally, names are always looked up in scopes— places where variables are stored—and assignments bind names to scopes. • nonlocal declares enclosing function variables that are to be assigned. Similarly, the nonlocal statement added in Python 3.0 allows a function to assign a name that exists in the scope of a syntactically enclosing def statement. This allows Coding Functions | 397 enclosing functions to serve as a place to retain state—information remembered when a function is called—without using shared global names. • Arguments are passed by assignment (object reference). In Python, arguments are passed to functions by assignment (which, as we’ve learned, means by object reference). As you’ll see, in Python’s model the caller and function share objects by references, but there is no name aliasing. Changing an argument name within a function does not also change the corresponding name in the caller, but changing passed-in mutable objects can change objects shared by the caller. • Arguments, return values, and variables are not declared. As with everything in Python, there are no type constraints on functions. In fact, nothing about a function needs to be declared ahead of time: you can pass in arguments of any type, return any kind of object, and so on. As one consequence, a single function can often be applied to a variety of object types—any objects that sport a compatible interface (methods and expressions) will do, regardless of their specific types. If some of the preceding words didn’t sink in, don’t worry—we’ll explore all of these concepts with real code in this part of the book. Let’s get started by expanding on some of these ideas and looking at a few examples. def Statements The def statement creates a function object and assigns it to a name. Its general format is as follows: def <name>(arg1, arg2,... argN): <statements> As with all compound Python statements, def consists of a header line followed by a block of statements, usually indented (or a simple statement after the colon). The statement block becomes the function’s body—that is, the code Python executes each time the function is called. The def header line specifies a function name that is assigned the function object, along with a list of zero or more arguments (sometimes called parameters) in parentheses. The argument names in the header are assigned to the objects passed in parentheses at the point of call. Function bodies often contain a return statement: def <name>(arg1, arg2,... argN): ... return <value> The Python return statement can show up anywhere in a function body; it ends the function call and sends a result back to the caller. The return statement consists of an object expression that gives the function’s result. The return statement is optional; if it’s not present, the function exits when the control flow falls off the end of the function 398 | Chapter 16: Function Basics body. Technically, a function without a return statement returns the None object automatically, but this return value is usually ignored. Functions may also contain yield statements, which are designed to produce a series of values over time, but we’ll defer discussion of these until we survey generator topics in Chapter 20. def Executes at Runtime The Python def is a true executable statement: when it runs, it creates a new function object and assigns it to a name. (Remember, all we have in Python is runtime; there is no such thing as a separate compile time.) Because it’s a statement, a def can appear anywhere a statement can—even nested in other statements. For instance, although defs normally are run when the module enclosing them is imported, it’s also completely legal to nest a function def inside an if statement to select between alternative definitions: if test: def func(): ... else: def func(): ... ... func() # Define func this way # Or else this way # Call the version selected and built One way to understand this code is to realize that the def is much like an = statement: it simply assigns a name at runtime. Unlike in compiled languages such as C, Python functions do not need to be fully defined before the program runs. More generally, defs are not evaluated until they are reached and run, and the code inside defs is not evaluated until the functions are later called. Because function definition happens at runtime, there’s nothing special about the function name. What’s important is the object to which it refers: othername = func othername() # Assign function object # Call func again Here, the function was assigned to a different name and called through the new name. Like everything else in Python, functions are just objects; they are recorded explicitly in memory at program execution time. In fact, besides calls, functions allow arbitrary attributes to be attached to record information for later use: def func(): ... func() func.attr = value # Create function object # Call object # Attach attributes Coding Functions | 399 A First Example: Definitions and Calls Apart from such runtime concepts (which tend to seem most unique to programmers with backgrounds in traditional compiled languages), Python functions are straightforward to use. Let’s code a first real example to demonstrate the basics. As you’ll see, there are two sides to the function picture: a definition (the def that creates a function) and a call (an expression that tells Python to run the function’s body). Definition Here’s a definition typed interactively that defines a function called times, which returns the product of its two arguments: >>> def times(x, y): ... return x * y ... # Create and assign function # Body executed when called When Python reaches and runs this def, it creates a new function object that packages the function’s code and assigns the object to the name times. Typically, such a statement is coded in a module file and runs when the enclosing file is imported; for something this small, though, the interactive prompt suffices. Calls After the def has run, you can call (run) the function in your program by adding parentheses after the function’s name. The parentheses may optionally contain one or more object arguments, to be passed (assigned) to the names in the function’s header: >>> times(2, 4) 8 # Arguments in parentheses This expression passes two arguments to times. As mentioned previously, arguments are passed by assignment, so in this case the name x in the function header is assigned the value 2, y is assigned the value 4, and the function’s body is run. For this function, the body is just a return statement that sends back the result as the value of the call expression. The returned object was printed here interactively (as in most languages, 2 * 4 is 8 in Python), but if we needed to use it later we could instead assign it to a variable. For example: >>> x = times(3.14, 4) >>> x 12.56 # Save the result object Now, watch what happens when the function is called a third time, with very different kinds of objects passed in: >>> times('Ni', 4) 'NiNiNiNi' 400 | Chapter 16: Function Basics # Functions are "typeless" This time, our function means something completely different (Monty Python reference again intended). In this third call, a string and an integer are passed to x and y, instead of two numbers. Recall that * works on both numbers and sequences; because we never declare the types of variables, arguments, or return values in Python, we can use times to either multiply numbers or repeat sequences. In other words, what our times function means and does depends on what we pass into it. This is a core idea in Python (and perhaps the key to using the language well), which we’ll explore in the next section. Polymorphism in Python As we just saw, the very meaning of the expression x * y in our simple times function depends completely upon the kinds of objects that x and y are—thus, the same function can perform multiplication in one instance and repetition in another. Python leaves it up to the objects to do something reasonable for the syntax. Really, * is just a dispatch mechanism that routes control to the objects being processed. This sort of type-dependent behavior is known as polymorphism, a term we first met in Chapter 4 that essentially means that the meaning of an operation depends on the objects being operated upon. Because it’s a dynamically typed language, polymorphism runs rampant in Python. In fact, every operation is a polymorphic operation in Python: printing, indexing, the * operator, and much more. This is deliberate, and it accounts for much of the language’s conciseness and flexibility. A single function, for instance, can generally be applied to a whole category of object types automatically. As long as those objects support the expected interface (a.k.a. protocol), the function can process them. That is, if the objects passed into a function have the expected methods and expression operators, they are plug-and-play compatible with the function’s logic. Even in our simple times function, this means that any two objects that support a * will work, no matter what they may be, and no matter when they are coded. This function will work on two numbers (performing multiplication), or a string and a number (performing repetition), or any other combination of objects supporting the expected interface—even class-based objects we have not even coded yet. Moreover, if the objects passed in do not support this expected interface, Python will detect the error when the * expression is run and raise an exception automatically. It’s therefore pointless to code error checking ourselves. In fact, doing so would limit our function’s utility, as it would be restricted to work only on objects whose types we test for. This turns out to be a crucial philosophical difference between Python and statically typed languages like C++ and Java: in Python, your code is not supposed to care about specific data types. If it does, it will be limited to working on just the types you anticipated when you wrote it, and it will not support other compatible object types that A First Example: Definitions and Calls | 401 may be coded in the future. Although it is possible to test for types with tools like the type built-in function, doing so breaks your code’s flexibility. By and large, we code to object interfaces in Python, not data types. Of course, this polymorphic model of programming means we have to test our code to detect errors, rather than providing type declarations a compiler can use to detect some types of errors for us ahead of time. In exchange for an initial bit of testing, though, we radically reduce the amount of code we have to write and radically increase our code’s flexibility. As you’ll learn, it’s a net win in practice. A Second Example: Intersecting Sequences Let’s look at a second function example that does something a bit more useful than multiplying arguments and further illustrates function basics. In Chapter 13, we coded a for loop that collected items held in common in two strings. We noted there that the code wasn’t as useful as it could be because it was set up to work only on specific variables and could not be rerun later. Of course, we could copy the code and paste it into each place where it needs to be run, but this solution is neither good nor general—we’d still have to edit each copy to support different sequence names, and changing the algorithm would then require changing multiple copies. Definition By now, you can probably guess that the solution to this dilemma is to package the for loop inside a function. Doing so offers a number of advantages: • Putting the code in a function makes it a tool that you can run as many times as you like. • Because callers can pass in arbitrary arguments, functions are general enough to work on any two sequences (or other iterables) you wish to intersect. • When the logic is packaged in a function, you only have to change code in one place if you ever need to change the way the intersection works. • Coding the function in a module file means it can be imported and reused by any program run on your machine. In effect, wrapping the code in a function makes it a general intersection utility: def intersect(seq1, seq2): res = [] for x in seq1: if x in seq2: res.append(x) return res # Start empty # Scan seq1 # Common item? # Add to end The transformation from the simple code of Chapter 13 to this function is straightforward; we’ve just nested the original logic under a def header and made the objects on 402 | Chapter 16: Function Basics which it operates passed-in parameter names. Because this function computes a result, we’ve also added a return statement to send a result object back to the caller. Calls Before you can call a function, you have to make it. To do this, run its def statement, either by typing it interactively or by coding it in a module file and importing the file. Once you’ve run the def, you can call the function by passing any two sequence objects in parentheses: >>> s1 = "SPAM" >>> s2 = "SCAM" >>> intersect(s1, s2) ['S', 'A', 'M'] # Strings Here, we’ve passed in two strings, and we get back a list containing the characters in common. The algorithm the function uses is simple: “for every item in the first argument, if that item is also in the second argument, append the item to the result.” It’s a little shorter to say that in Python than in English, but it works out the same. To be fair, our intersect function is fairly slow (it executes nested loops), isn’t really mathematical intersection (there may be duplicates in the result), and isn’t required at all (as we’ve seen, Python’s set data type provides a built-in intersection operation). Indeed, the function could be replaced with a single list comprehension expression, as it exhibits the classic loop collector code pattern: >>> [x for x in s1 if x in s2] ['S', 'A', 'M'] As a function basics example, though, it does the job—this single piece of code can apply to an entire range of object types, as the next section explains. Polymorphism Revisited Like all functions in Python, intersect is polymorphic. That is, it works on arbitrary types, as long as they support the expected object interface: >>> x = intersect([1, 2, 3], (1, 4)) >>> x [1] # Mixed types # Saved result object This time, we passed in different types of objects to our function—a list and a tuple (mixed types)—and it still picked out the common items. Because you don’t have to specify the types of arguments ahead of time, the intersect function happily iterates through any kind of sequence objects you send it, as long as they support the expected interfaces. For intersect, this means that the first argument has to support the for loop, and the second has to support the in membership test. Any two such objects will work, regardless of their specific types—that includes physically stored sequences like strings A Second Example: Intersecting Sequences | 403 and lists; all the iterable objects we met in Chapter 14, including files and dictionaries; and even any class-based objects we code that apply operator overloading techniques (we’ll discuss these later in the book).* Here again, if we pass in objects that do not support these interfaces (e.g., numbers), Python will automatically detect the mismatch and raise an exception for us—which is exactly what we want, and the best we could do on our own if we coded explicit type tests. By not coding type tests and allowing Python to detect the mismatches for us, we both reduce the amount of code we need to write and increase our code’s flexibility. Local Variables Probably the most interesting part of this example is its names. It turns out that the variable res inside intersect is what in Python is called a local variable—a name that is visible only to code inside the function def and that exists only while the function runs. In fact, because all names assigned in any way inside a function are classified as local variables by default, nearly all the names in intersect are local variables: • res is obviously assigned, so it is a local variable. • Arguments are passed by assignment, so seq1 and seq2 are, too. • The for loop assigns items to a variable, so the name x is also local. All these local variables appear when the function is called and disappear when the function exits—the return statement at the end of intersect sends back the result object, but the name res goes away. To fully explore the notion of locals, though, we need to move on to Chapter 17. Chapter Summary This chapter introduced the core ideas behind function definition—the syntax and operation of the def and return statements, the behavior of function call expressions, and the notion and benefits of polymorphism in Python functions. As we saw, a def statement is executable code that creates a function object at runtime; when the function is later called, objects are passed into it by assignment (recall that assignment means object reference in Python, which, as we learned in Chapter 6, really means pointer internally), and computed values are sent back by return. We also began * This code will always work if we intersect files’ contents obtained with file.readlines(). It may not work to intersect lines in open input files directly, though, depending on the file object’s implementation of the in operator or general iteration. Files must generally be rewound (e.g., with a file.seek(0) or another open) after they have been read to end-of-file once. As we’ll see in Chapter 29 when we study operator overloading, classes implement the in operator either by providing the specific __contains__ method or by supporting the general iteration protocol with the __iter__ or older __getitem__ methods; if coded, classes can define what iteration means for their data. 404 | Chapter 16: Function Basics exploring the concepts of local variables and scopes in this chapter, but we’ll save all the details on those topics for Chapter 17. First, though, a quick quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. What is the point of coding functions? At what time does Python create a function? What does a function return if it has no return statement in it? When does the code nested inside the function definition statement run? What’s wrong with checking the types of objects passed into a function? Test Your Knowledge: Answers 1. Functions are the most basic way of avoiding code redundancy in Python—factoring code into functions means that we have only one copy of an operation’s code to update in the future. Functions are also the basic unit of code reuse in Python— wrapping code in functions makes it a reusable tool, callable in a variety of programs. Finally, functions allow us to divide a complex system into manageable parts, each of which may be developed individually. 2. A function is created when Python reaches and runs the def statement; this statement creates a function object and assigns it the function’s name. This normally happens when the enclosing module file is imported by another module (recall that imports run the code in a file from top to bottom, including any defs), but it can also occur when a def is typed interactively or nested in other statements, such as ifs. 3. A function returns the None object by default if the control flow falls off the end of the function body without running into a return statement. Such functions are usually called with expression statements, as assigning their None results to variables is generally pointless. 4. The function body (the code nested inside the function definition statement) is run when the function is later called with a call expression. The body runs anew each time the function is called. 5. Checking the types of objects passed into a function effectively breaks the function’s flexibility, constraining the function to work on specific types only. Without such checks, the function would likely be able to process an entire range of object types—any objects that support the interface expected by the function will work. (The term interface means the set of methods and expression operators the function’s code runs.) Test Your Knowledge: Answers | 405 CHAPTER 17 Scopes Chapter 16 introduced basic function definitions and calls. As we saw, Python’s basic function model is simple to use, but even simple function examples quickly led us to questions about the meaning of variables in our code. This chapter moves on to present the details behind Python’s scopes—the places where variables are defined and looked up. As we’ll see, the place where a name is assigned in our code is crucial to determining what the name means. We’ll also find that scope usage can have a major impact on program maintenance effort; overuse of globals, for example, is a generally bad thing. Python Scope Basics Now that you’re ready to start writing your own functions, we need to get more formal about what names mean in Python. When you use a name in a program, Python creates, changes, or looks up the name in what is known as a namespace—a place where names live. When we talk about the search for a name’s value in relation to code, the term scope refers to a namespace: that is, the location of a name’s assignment in your code determines the scope of the name’s visibility to your code. Just about everything related to names, including scope classification, happens at assignment time in Python. As we’ve seen, names in Python spring into existence when they are first assigned values, and they must be assigned before they are used. Because names are not declared ahead of time, Python uses the location of the assignment of a name to associate it with (i.e., bind it to) a particular namespace. In other words, the place where you assign a name in your source code determines the namespace it will live in, and hence its scope of visibility. Besides packaging code, functions add an extra namespace layer to your programs— by default, all names assigned inside a function are associated with that function’s namespace, and no other. This means that: • Names defined inside a def can only be seen by the code within that def. You cannot even refer to such names from outside the function. 407 • Names defined inside a def do not clash with variables outside the def, even if the same names are used elsewhere. A name X assigned outside a given def (i.e., in a different def or at the top level of a module file) is a completely different variable from a name X assigned inside that def. In all cases, the scope of a variable (where it can be used) is always determined by where it is assigned in your source code and has nothing to do with which functions call which. In fact, as we’ll learn in this chapter, variables may be assigned in three different places, corresponding to three different scopes: • If a variable is assigned inside a def, it is local to that function. • If a variable is assigned in an enclosing def, it is nonlocal to nested functions. • If a variable is assigned outside all defs, it is global to the entire file. We call this lexical scoping because variable scopes are determined entirely by the locations of the variables in the source code of your program files, not by function calls. For example, in the following module file, the X = 99 assignment creates a global variable named X (visible everywhere in this file), but the X = 88 assignment creates a local variable X (visible only within the def statement): X = 99 def func(): X = 88 Even though both variables are named X, their scopes make them different. The net effect is that function scopes help to avoid name clashes in your programs and help to make functions more self-contained program units. Scope Rules Before we started writing functions, all the code we wrote was at the top level of a module (i.e., not nested in a def), so the names we used either lived in the module itself or were built-ins predefined by Python (e.g., open). Functions provide nested namespaces (scopes) that localize the names they use, such that names inside a function won’t clash with those outside it (in a module or another function). Again, functions define a local scope, and modules define a global scope. The two scopes are related as follows: • The enclosing module is a global scope. Each module is a global scope—that is, a namespace in which variables created (assigned) at the top level of the module file live. Global variables become attributes of a module object to the outside world but can be used as simple variables within a module file. • The global scope spans a single file only. Don’t be fooled by the word “global” here—names at the top level of a file are only global to code within that single file. There is really no notion of a single, all-encompassing global file-based scope in 408 | Chapter 17: Scopes Python. Instead, names are partitioned into modules, and you must always import a module explicitly if you want to be able to use the names its file defines. When you hear “global” in Python, think “module.” • Each call to a function creates a new local scope. Every time you call a function, you create a new local scope—that is, a namespace in which the names created inside that function will usually live. You can think of each def statement (and lambda expression) as defining a new local scope, but because Python allows functions to call themselves to loop (an advanced technique known as recursion), the local scope in fact technically corresponds to a function call—in other words, each call creates a new local namespace. Recursion is useful when processing structures whose shapes can’t be predicted ahead of time. • Assigned names are local unless declared global or nonlocal. By default, all the names assigned inside a function definition are put in the local scope (the namespace associated with the function call). If you need to assign a name that lives at the top level of the module enclosing the function, you can do so by declaring it in a global statement inside the function. If you need to assign a name that lives in an enclosing def, as of Python 3.0 you can do so by declaring it in a nonlocal statement. • All other names are enclosing function locals, globals, or built-ins. Names not assigned a value in the function definition are assumed to be enclosing scope locals (in an enclosing def), globals (in the enclosing module’s namespace), or builtins (in the predefined __builtin__ module Python provides). There are a few subtleties to note here. First, keep in mind that code typed at the interactive command prompt follows these same rules. You may not know it yet, but code run interactively is really entered into a built-in module called __main__; this module works just like a module file, but results are echoed as you go. Because of this, interactively created names live in a module, too, and thus follow the normal scope rules: they are global to the interactive session. You’ll learn more about modules in the next part of this book. Also note that any type of assignment within a function classifies a name as local. This includes = statements, module names in import, function names in def, function argument names, and so on. If you assign a name in any way within a def, it will become a local to that function. Conversely, in-place changes to objects do not classify names as locals; only actual name assignments do. For instance, if the name L is assigned to a list at the top level of a module, a statement L = X within a function will classify L as a local, but L.append(X) will not. In the latter case, we are changing the list object that L references, not L itself— L is found in the global scope as usual, and Python happily modifies it without requiring a global (or nonlocal) declaration. As usual, it helps to keep the distinction between names and objects clear: changing an object is not an assignment to a name. Python Scope Basics | 409 Name Resolution: The LEGB Rule If the prior section sounds confusing, it really boils down to three simple rules. With a def statement: • Name references search at most four scopes: local, then enclosing functions (if any), then global, then built-in. • Name assignments create or change local names by default. • global and nonlocal declarations map assigned names to enclosing module and function scopes. In other words, all names assigned inside a function def statement (or a lambda, an expression we’ll meet later) are locals by default. Functions can freely use names assigned in syntactically enclosing functions and the global scope, but they must declare such nonlocals and globals in order to change them. Python’s name-resolution scheme is sometimes called the LEGB rule, after the scope names: • When you use an unqualified name inside a function, Python searches up to four scopes—the local (L) scope, then the local scopes of any enclosing (E) defs and lambdas, then the global (G) scope, and then the built-in (B) scope—and stops at the first place the name is found. If the name is not found during this search, Python reports an error. As we learned in Chapter 6, names must be assigned before they can be used. • When you assign a name in a function (instead of just referring to it in an expression), Python always creates or changes the name in the local scope, unless it’s declared to be global or nonlocal in that function. • When you assign a name outside any function (i.e., at the top level of a module file, or at the interactive prompt), the local scope is the same as the global scope— the module’s namespace. Figure 17-1 illustrates Python’s four scopes. Note that the second scope lookup layer, E—the scopes of enclosing defs or lambdas—can technically correspond to more than one lookup layer. This case only comes into play when you nest functions within functions, and it is addressed by the nonlocal statement.* Also keep in mind that these rules apply only to simple variable names (e.g., spam). In Parts V and VI, we’ll see that qualified attribute names (e.g., object.spam) live in particular objects and follow a completely different set of lookup rules than those * The scope lookup rule was called the “LGB rule” in the first edition of this book. The enclosing def “E” layer was added later in Python to obviate the task of passing in enclosing scope names explicitly with default arguments—a topic usually of marginal interest to Python beginners that we’ll defer until later in this chapter. Since this scope is addressed by the nonlocal statement in Python 3.0, I suppose the lookup rule might now be better named “LNGB,” but backward compatibility matters in books, too! 410 | Chapter 17: Scopes Figure 17-1. The LEGB scope lookup rule. When a variable is referenced, Python searches for it in this order: in the local scope, in any enclosing functions’ local scopes, in the global scope, and finally in the built-in scope. The first occurrence wins. The place in your code where a variable is assigned usually determines its scope. In Python 3, nonlocal declarations can also force names to be mapped to enclosing function scopes, whether assigned or not. covered here. References to attribute names following periods (.) search one or more objects, not scopes, and may invoke something called “inheritance”; more on this in Part VI of this book. Scope Example Let’s look at a larger example that demonstrates scope ideas. Suppose we wrote the following code in a module file: # Global scope X = 99 # X and func assigned in module: global def func(Y): # Local scope Z = X + Y return Z # Y and Z assigned in function: locals func(1) # func in module: result=100 # X is a global This module and the function it contains use a number of names to do their business. Using Python’s scope rules, we can classify the names as follows: Global names: X, func X is global because it’s assigned at the top level of the module file; it can be referenced inside the function without being declared global. func is global for the same reason; the def statement assigns a function object to the name func at the top level of the module. Python Scope Basics | 411 Local names: Y, Z Y and Z are local to the function (and exist only while the function runs) because they are both assigned values in the function definition: Z by virtue of the = statement, and Y because arguments are always passed by assignment. The whole point behind this name-segregation scheme is that local variables serve as temporary names that you need only while a function is running. For instance, in the preceding example, the argument Y and the addition result Z exist only inside the function; these names don’t interfere with the enclosing module’s namespace (or any other function, for that matter). The local/global distinction also makes functions easier to understand, as most of the names a function uses appear in the function itself, not at some arbitrary place in a module. Also, because you can be sure that local names will not be changed by some remote function in your program, they tend to make programs easier to debug and modify. The Built-in Scope We’ve been talking about the built-in scope in the abstract, but it’s a bit simpler than you may think. Really, the built-in scope is just a built-in module called builtins, but you have to import builtins to query built-ins because the name builtins is not itself built-in.... No, I’m serious! The built-in scope is implemented as a standard library module named builtins, but that name itself is not placed in the built-in scope, so you have to import it in order to inspect it. Once you do, you can run a dir call to see which names are predefined. In Python 3.0: >>> import builtins >>> dir(builtins) ['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BufferError', 'BytesWarning', 'DeprecationWarning', 'EOFError', 'Ellipsis', ...many more names omitted... 'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip'] The names in this list constitute the built-in scope in Python; roughly the first half are built-in exceptions, and the second half are built-in functions. Also in this list are the special names None, True, and False, though they are treated as reserved words. Because Python automatically searches this module last in its LEGB lookup, you get all the names in this list “for free;” that is, you can use them without importing any modules. Thus, there are really two ways to refer to a built-in function—by taking advantage of the LEGB rule, or by manually importing the builtins module: >>> zip <class 'zip'> 412 | Chapter 17: Scopes # The normal way >>> import builtins >>> builtins.zip <class 'zip'> # The hard way The second of these approaches is sometimes useful in advanced work. The careful reader might also notice that because the LEGB lookup procedure takes the first occurrence of a name that it finds, names in the local scope may override variables of the same name in both the global and built-in scopes, and global names may override builtins. A function can, for instance, create a local variable called open by assigning to it: def hider(): open = 'spam' ... open('data.txt') # Local variable, hides built-in # This won't open a file now in this scope! However, this will hide the built-in function called open that lives in the built-in (outer) scope. It’s also usually a bug, and a nasty one at that, because Python will not issue a warning message about it (there are times in advanced programming where you may really want to replace a built-in name by redefining it in your code). Functions can similarly hide global variables of the same name with locals: X = 88 # Global X def func(): X = 99 # Local X: hides global func() print(X) # Prints 88: unchanged Here, the assignment within the function creates a local X that is a completely different variable from the global X in the module outside the function. Because of this, there is no way to change a name outside a function without adding a global (or nonlocal) declaration to the def, as described in the next section. Version skew note: Actually, the tongue twisting gets a bit worse. The Python 3.0 builtins module used here is named __builtin__ in Python 2.6. And just for fun, the name __builtins__ (with the “s”) is preset in most global scopes, including the interactive session, to reference the module known as builtins (a.k.a. __builtin__ in 2.6). That is, after importing builtins, __builtins__ is builtins is True in 3.0, and __builtins__ is __builtin__ is True in 2.6. The net effect is that we can inspect the built-in scope by simply running dir(__builtins__) with no import in both 3.0 and 2.6, but we are advised to use builtins for real work in 3.0. Who said documenting this stuff was easy? Python Scope Basics | 413 Breaking the Universe in Python 2.6 Here’s another thing you can do in Python that you probably shouldn’t—because the names True and False in 2.6 are just variables in the built-in scope and are not reserved, it’s possible to reassign them with a statement like True = False. Don’t worry, you won’t actually break the logical consistency of the universe in so doing! This statement merely redefines the word True for the single scope in which it appears. All other scopes still find the originals in the built-in scope. For more fun, though, in Python 2.6 you could say __builtin__.True = False, to reset True to False for the entire Python process. Alas, this type of assignment has been disallowed in Python 3.0, because True and False are treated as actual reserved words, just like None. In 2.6, though, it sends IDLE into a strange panic state that resets the user code process. This technique can be useful, however, both to illustrate the underlying namespace model and for tool writers who must change built-ins such as open to customized functions. Also, note that third-party tools such as PyChecker will warn about common programming mistakes, including accidental assignment to built-in names (this is known as “shadowing” a built-in in PyChecker). The global Statement The global statement and its nonlocal cousin are the only things that are remotely like declaration statements in Python. They are not type or size declarations, though; they are namespace declarations. The global statement tells Python that a function plans to change one or more global names—i.e., names that live in the enclosing module’s scope (namespace). We’ve talked about global in passing already. Here’s a summary: • Global names are variables assigned at the top level of the enclosing module file. • Global names must be declared only if they are assigned within a function. • Global names may be referenced within a function without being declared. In other words, global allows us to change names that live outside a def at the top level of a module file. As we’ll see later, the nonlocal statement is almost identical but applies to names in the enclosing def’s local scope, rather than names in the enclosing module. The global statement consists of the keyword global, followed by one or more names separated by commas. All the listed names will be mapped to the enclosing module’s scope when assigned or referenced within the function body. For instance: X = 88 # Global X def func(): global X X = 99 # Global X: outside def 414 | Chapter 17: Scopes func() print(X) # Prints 99 We’ve added a global declaration to the example here, such that the X inside the def now refers to the X outside the def; they are the same variable this time. Here is a slightly more involved example of global at work: y, z = 1, 2 def all_global(): global x x = y + z # Global variables in module # Declare globals assigned # No need to declare y, z: LEGB rule Here, x, y, and z are all globals inside the function all_global. y and z are global because they aren’t assigned in the function; x is global because it was listed in a global statement to map it to the module’s scope explicitly. Without the global here, x would be considered local by virtue of the assignment. Notice that y and z are not declared global; Python’s LEGB lookup rule finds them in the module automatically. Also, notice that x might not exist in the enclosing module before the function runs; in this case, the assignment in the function creates x in the module. Minimize Global Variables By default, names assigned in functions are locals, so if you want to change names outside functions you have to write extra code (e.g., global statements). This is by design—as is common in Python, you have to say more to do the potentially “wrong” thing. Although there are times when globals are useful, variables assigned in a def are local by default because that is normally the best policy. Changing globals can lead to well-known software engineering problems: because the variables’ values are dependent on the order of calls to arbitrarily distant functions, programs can become difficult to debug. Consider this module file, for example: X = 99 def func1(): global X X = 88 def func2(): global X X = 77 Now, imagine that it is your job to modify or reuse this module file. What will the value of X be here? Really, that question has no meaning unless it’s qualified with a point of reference in time—the value of X is timing-dependent, as it depends on which function was called last (something we can’t tell from this file alone). The global Statement | 415 The net effect is that to understand this code, you have to trace the flow of control through the entire program. And, if you need to reuse or modify the code, you have to keep the entire program in your head all at once. In this case, you can’t really use one of these functions without bringing along the other. They are dependent on (that is, coupled with) the global variable. This is the problem with globals—they generally make code more difficult to understand and use than code consisting of self-contained functions that rely on locals. On the other hand, short of using object-oriented programming and classes, global variables are probably the most straightforward way to retain shared state information (information that a function needs to remember for use the next time it is called) in Python—local variables disappear when the function returns, but globals do not. Other techniques, such as default mutable arguments and enclosing function scopes, can achieve this, too, but they are more complex than pushing values out to the global scope for retention. Some programs designate a single module to collect globals; as long as this is expected, it is not as harmful. In addition, programs that use multithreading to do parallel processing in Python commonly depend on global variables—they become shared memory between functions running in parallel threads, and so act as a communication device.† For now, though, especially if you are relatively new to programming, avoid the temptation to use globals whenever you can—try to communicate with passed-in arguments and return values instead. Six months from now, both you and your coworkers will be happy you did. Minimize Cross-File Changes Here’s another scope-related issue: although we can change variables in another file directly, we usually shouldn’t. Module files were introduced in Chapter 3 and are covered in more depth in the next part of this book. To illustrate their relationship to scopes, consider these two module files: # first.py X = 99 # This code doesn't know about second.py # second.py import first print(first.X) first.X = 88 # Okay: references a name in another file # But changing it can be too subtle and implicit † Multithreading runs function calls in parallel with the rest of the program and is supported by Python’s standard library modules _thread, threading, and queue (thread, threading, and Queue in Python 2.6). Because all threaded functions run in the same process, global scopes often serve as shared memory between them. Threading is commonly used for long-running tasks in GUIs, to implement nonblocking operations in general and to leverage CPU capacity. It is also beyond this book’s scope; see the Python library manual, as well as the follow-up texts listed in the Preface (such as O’Reilly’s Programming Python), for more details. 416 | Chapter 17: Scopes The first defines a variable X, which the second prints and then changes by assignment. Notice that we must import the first module into the second file to get to its variable at all—as we’ve learned, each module is a self-contained namespace (package of variables), and we must import one module to see inside it from another. That’s the main point about modules: by segregating variables on a per-file basis, they avoid name collisions across files. Really, though, in terms of this chapter’s topic, the global scope of a module file becomes the attribute namespace of the module object once it is imported—importers automatically have access to all of the file’s global variables, because a file’s global scope morphs into an object’s attribute namespace when it is imported. After importing the first module, the second module prints its variable and then assigns it a new value. Referencing the module’s variable to print it is fine—this is how modules are linked together into a larger system normally. The problem with the assignment, however, is that it is far too implicit: whoever’s charged with maintaining or reusing the first module probably has no clue that some arbitrarily far-removed module on the import chain can change X out from under him at runtime. In fact, the second module may be in a completely different directory, and so difficult to notice at all. Although such cross-file variable changes are always possible in Python, they are usually much more subtle than you will want. Again, this sets up too strong a coupling between the two files—because they are both dependent on the value of the variable X, it’s difficult to understand or reuse one file without the other. Such implicit cross-file dependencies can lead to inflexible code at best, and outright bugs at worst. Here again, the best prescription is generally to not do this—the best way to communicate across file boundaries is to call functions, passing in arguments and getting back return values. In this specific case, we would probably be better off coding an accessor function to manage the change: # first.py X = 99 def setX(new): global X X = new # second.py import first first.setX(88) This requires more code and may seem like a trivial change, but it makes a huge difference in terms of readability and maintainability—when a person reading the first module by itself sees a function, that person will know that it is a point of interface and will expect the change to the X. In other words, it removes the element of surprise that is rarely a good thing in software projects. Although we cannot prevent cross-file changes from happening, common sense dictates that they should be minimized unless widely accepted across the program. The global Statement | 417 Other Ways to Access Globals Interestingly, because global-scope variables morph into the attributes of a loaded module object, we can emulate the global statement by importing the enclosing module and assigning to its attributes, as in the following example module file. Code in this file imports the enclosing module, first by name, and then by indexing the sys.modules loaded modules table (more on this table in Chapter 21): # thismod.py var = 99 # Global variable == module attribute def local(): var = 0 # Change local var def glob1(): global var var += 1 # Declare global (normal) # Change global var def glob2(): var = 0 import thismod thismod.var += 1 # Change local var # Import myself # Change global var def glob3(): var = 0 import sys glob = sys.modules['thismod'] glob.var += 1 # Change local var # Import system table # Get module object (or use __name__) # Change global var def test(): print(var) local(); glob1(); glob2(); glob3() print(var) When run, this adds 3 to the global variable (only the first function does not impact it): >>> import thismod >>> thismod.test() 99 102 >>> thismod.var 102 This works, and it illustrates the equivalence of globals to module attributes, but it’s much more work than using the global statement to make your intentions explicit. As we’ve seen, global allows us to change names in a module outside a function. It has a cousin named nonlocal that can be used to change names in enclosing functions, too, but to understand how that can be useful, we first need to explore enclosing functions in general. 418 | Chapter 17: Scopes Scopes and Nested Functions So far, I’ve omitted one part of Python’s scope rules on purpose, because it’s relatively rare to encounter it in practice. However, it’s time to take a deeper look at the letter E in the LEGB lookup rule. The E layer is fairly new (it was added in Python 2.2); it takes the form of the local scopes of any and all enclosing function defs. Enclosing scopes are sometimes also called statically nested scopes. Really, the nesting is a lexical one—nested scopes correspond to physically and syntactically nested code structures in your program’s source code. Nested Scope Details With the addition of nested function scopes, variable lookup rules become slightly more complex. Within a function: • A reference (X) looks for the name X first in the current local scope (function); then in the local scopes of any lexically enclosing functions in your source code, from inner to outer; then in the current global scope (the module file); and finally in the built-in scope (the module builtins). global declarations make the search begin in the global (module file) scope instead. • An assignment (X = value) creates or changes the name X in the current local scope, by default. If X is declared global within the function, the assignment creates or changes the name X in the enclosing module’s scope instead. If, on the other hand, X is declared nonlocal within the function, the assignment changes the name X in the closest enclosing function’s local scope. Notice that the global declaration still maps variables to the enclosing module. When nested functions are present, variables in enclosing functions may be referenced, but they require nonlocal declarations to be changed. Nested Scope Examples To clarify the prior section’s points, let’s illustrate with some real code. Here is what an enclosing function scope looks like: X = 99 def f1(): X = 88 def f2(): print(X) f2() f1() # Global scope name: not used # Enclosing def local # Reference made in nested def # Prints 88: enclosing def local First off, this is legal Python code: the def is simply an executable statement, which can appear anywhere any other statement can—including nested in another def. Here, the Scopes and Nested Functions | 419 nested def runs while a call to the function f1 is running; it generates a function and assigns it to the name f2, a local variable within f1’s local scope. In a sense, f2 is a temporary function that lives only during the execution of (and is visible only to code in) the enclosing f1. But notice what happens inside f2: when it prints the variable X, it refers to the X that lives in the enclosing f1 function’s local scope. Because functions can access names in all physically enclosing def statements, the X in f2 is automatically mapped to the X in f1, by the LEGB lookup rule. This enclosing scope lookup works even if the enclosing function has already returned. For example, the following code defines a function that makes and returns another function: def f1(): X = 88 def f2(): print(X) return f2 # Remembers X in enclosing def scope # Return f2 but don't call it action = f1() action() # Make, return function # Call it now: prints 88 In this code, the call to action is really running the function we named f2 when f1 ran. f2 remembers the enclosing scope’s X in f1, even though f1 is no longer active. Factory functions Depending on whom you ask, this sort of behavior is also sometimes called a closure or factory function. These terms refer to a function object that remembers values in enclosing scopes regardless of whether those scopes are still present in memory. Although classes (described in Part VI of this book) are usually best at remembering state because they make it explicit with attribute assignments, such functions provide an alternative. For instance, factory functions are sometimes used by programs that need to generate event handlers on the fly in response to conditions at runtime (e.g., user inputs that cannot be anticipated). Look at the following function, for example: >>> def maker(N): ... def action(X): ... return X ** N ... return action ... # Make and return action # action retains N from enclosing scope This defines an outer function that simply generates and returns a nested function, without calling it. If we call the outer function: >>> f = maker(2) >>> f <function action at 0x014720B0> 420 | Chapter 17: Scopes # Pass 2 to N what we get back is a reference to the generated nested function—the one created by running the nested def. If we now call what we got back from the outer function: >>> f(3) 9 >>> f(4) 16 # Pass 3 to X, N remembers 2: 3 ** 2 # 4 ** 2 it invokes the nested function—the one called action within maker. The most unusual part of this is that the nested function remembers integer 2, the value of the variable N in maker, even though maker has returned and exited by the time we call action. In effect, N from the enclosing local scope is retained as state information attached to action, and we get back its argument squared. If we now call the outer function again, we get back a new nested function with different state information attached. That is, we get the argument cubed instead of squared, but the original still squares as before: >>> g = maker(3) >>> g(3) 27 >>> f(3) 9 # g remembers 3, f remembers 2 # 3 ** 3 # 3 ** 2 This works because each call to a factory function like this gets its own set of state information. In our case, the function we assign to name g remembers 3, and f remembers 2, because each has its own state information retained by the variable N in maker. This is an advanced technique that you’re unlikely to see very often in most code, except among programmers with backgrounds in functional programming languages. On the other hand, enclosing scopes are often employed by lambda function-creation expressions (discussed later in this chapter)—because they are expressions, they are almost always nested within a def. Moreover, function nesting is commonly used for decorators (explored in Chapter 38)—in some cases, it’s the most reasonable coding pattern. As a general rule, classes are better at “memory” like this because they make the state retention explicit in attributes. Short of using classes, though, globals, enclosing scope references like these, and default arguments are the main ways that Python functions can retain state information. To see how they compete, Chapter 18 provides complete coverage of defaults, but the next section gives enough of an introduction to get us started. Retaining enclosing scopes’ state with defaults In earlier versions of Python, the sort of code in the prior section failed because nested defs did not do anything about scopes—a reference to a variable within f2 would search only the local (f2), then global (the code outside f1), and then built-in scopes. Because it skipped the scopes of enclosing functions, an error would result. To work around this, programmers typically used default argument values to pass in and remember the objects in an enclosing scope: Scopes and Nested Functions | 421 def f1(): x = 88 def f2(x=x): print(x) f2() # Remember enclosing scope X with defaults f1() # Prints 88 This code works in all Python releases, and you’ll still see this pattern in some existing Python code. In short, the syntax arg = val in a def header means that the argument arg will default to the value val if no real value is passed to arg in a call. In the modified f2 here, the x=x means that the argument x will default to the value of x in the enclosing scope—because the second x is evaluated before Python steps into the nested def, it still refers to the x in f1. In effect, the default remembers what x was in f1 (i.e., the object 88). That’s fairly complex, and it depends entirely on the timing of default value evaluations. In fact, the nested scope lookup rule was added to Python to make defaults unnecessary for this role—today, Python automatically remembers any values required in the enclosing scope for use in nested defs. Of course, the best prescription for most code is simply to avoid nesting defs within defs, as it will make your programs much simpler. The following is an equivalent of the prior example that banishes the notion of nesting. Notice the forward reference in this code—it’s OK to call a function defined after the function that calls it, as long as the second def runs before the first function is actually called. Code inside a def is never evaluated until the function is actually called: >>> def f1(): ... x = 88 ... f2(x) ... >>> def f2(x): ... print(x) ... >>> f1() 88 # Pass x along instead of nesting # Forward reference okay If you avoid nesting this way, you can almost forget about the nested scopes concept in Python, unless you need to code in the factory function style discussed earlier—at least, for def statements. lambdas, which almost naturally appear nested in defs, often rely on nested scopes, as the next section explains. Nested scopes and lambdas While they’re rarely used in practice for defs themselves, you are more likely to care about nested function scopes when you start coding lambda expressions. We won’t cover lambda in depth until Chapter 19, but in short, it’s an expression that generates a new function to be called later, much like a def statement. Because it’s an expression, 422 | Chapter 17: Scopes though, it can be used in places that def cannot, such as within list and dictionary literals. Like a def, a lambda expression introduces a new local scope for the function it creates. Thanks to the enclosing scopes lookup layer, lambdas can see all the variables that live in the functions in which they are coded. Thus, the following code works, but only because the nested scope rules are applied: def func(): x = 4 action = (lambda n: x ** n) return action x = func() print(x(2)) # x remembered from enclosing def # Prints 16, 4 ** 2 Prior to the introduction of nested function scopes, programmers used defaults to pass values from an enclosing scope into lambdas, just as for defs. For instance, the following works on all Python releases: def func(): x = 4 action = (lambda n, x=x: x ** n) return action # Pass x in manually Because lambdas are expressions, they naturally (and even normally) nest inside enclosing defs. Hence, they are perhaps the biggest beneficiaries of the addition of enclosing function scopes in the lookup rules; in most cases, it is no longer necessary to pass values into lambdas with defaults. Scopes versus defaults with loop variables There is one notable exception to the rule I just gave: if a lambda or def defined within a function is nested inside a loop, and the nested function references an enclosing scope variable that is changed by that loop, all functions generated within the loop will have the same value—the value the referenced variable had in the last loop iteration. For instance, the following attempts to build up a list of functions that each remember the current variable i from the enclosing scope: >>> def makeActions(): ... acts = [] ... for i in range(5): ... acts.append(lambda x: i ** x) ... return acts ... >>> acts = makeActions() >>> acts[0] <function <lambda> at 0x012B16B0> # Tries to remember each i # All remember same last i! This doesn’t quite work, though—because the enclosing scope variable is looked up when the nested functions are later called, they all effectively remember the same value Scopes and Nested Functions | 423 (the value the loop variable had on the last loop iteration). That is, we get back 4 to the power of 2 for each function in the list, because i is the same in all of them: # All are 4 ** 2, value of last i >>> acts[0](2) 16 >>> acts[2](2) 16 >>> acts[4](2) 16 # This should be 2 ** 2 # This should be 4 ** 2 This is the one case where we still have to explicitly retain enclosing scope values with default arguments, rather than enclosing scope references. That is, to make this sort of code work, we must pass in the current value of the enclosing scope’s variable with a default. Because defaults are evaluated when the nested function is created (not when it’s later called), each remembers its own value for i: >>> ... ... ... ... ... >>> >>> 0 >>> 4 >>> 16 def makeActions(): acts = [] for i in range(5): acts.append(lambda x, i=i: i ** x) return acts # Use defaults instead # Remember current i acts = makeActions() acts[0](2) # 0 ** 2 acts[2](2) # 2 ** 2 acts[4](2) # 4 ** 2 This is a fairly obscure case, but it can come up in practice, especially in code that generates callback handler functions for a number of widgets in a GUI (e.g., buttonpress handlers). We’ll talk more about defaults in Chapter 18 and lambdas in Chapter 19, so you may want to return and review this section later.‡ Arbitrary scope nesting Before ending this discussion, I should note that scopes may nest arbitrarily, but only enclosing function def statements (not classes, described in Part VI) are searched: >>> def f1(): ... x = 99 ... def f2(): ... def f3(): ... print(x) ... f3() # Found in f1's local scope! ‡ In the section “Function Gotchas” on page 518 at the end of this part of the book, we’ll also see that there is an issue with using mutable objects like lists and dictionaries for default arguments (e.g., def f(a=[]))— because defaults are implemented as single objects attached to functions, mutable defaults retain state from call to call, rather then being initialized anew on each call. Depending on whom you ask, this is either considered a feature that supports state retention, or a strange wart on the language. More on this at the end of Chapter 20. 424 | Chapter 17: Scopes ... f2() ... >>> f1() 99 Python will search the local scopes of all enclosing defs, from inner to outer, after the referencing function’s local scope and before the module’s global scope or built-ins. However, this sort of code is even less likely to pop up in practice. In Python, we say flat is better than nested—except in very limited contexts, your life (and the lives of your coworkers) will generally be better if you minimize nested function definitions. The nonlocal Statement In the prior section we explored the way that nested functions can reference variables in an enclosing function’s scope, even if that function has already returned. It turns out that, as of Python 3.0, we can also change such enclosing scope variables, as long as we declare them in nonlocal statements. With this statement, nested defs can have both read and write access to names in enclosing functions. The nonlocal statement is a close cousin to global, covered earlier. Like global, nonlocal declares that a name will be changed in an enclosing scope. Unlike global, though, nonlocal applies to a name in an enclosing function’s scope, not the global module scope outside all defs. Also unlike global, nonlocal names must already exist in the enclosing function’s scope when declared—they can exist only in enclosing functions and cannot be created by a first assignment in a nested def. In other words, nonlocal both allows assignment to names in enclosing function scopes and limits scope lookups for such names to enclosing defs. The net effect is a more direct and reliable implementation of changeable scope information, for programs that do not desire or need classes with attributes. nonlocal Basics Python 3.0 introduces a new nonlocal statement, which has meaning only inside a function: def func(): nonlocal name1, name2, ... This statement allows a nested function to change one or more names defined in a syntactically enclosing function’s scope. In Python 2.X (including 2.6), when one function def is nested in another, the nested function can reference any of the names defined by assignment in the enclosing def’s scope, but it cannot change them. In 3.0, declaring the enclosing scopes’ names in a nonlocal statement enables nested functions to assign and thus change such names as well. This provides a way for enclosing functions to provide writeable state information, remembered when the nested function is later called. Allowing the state to change The nonlocal Statement | 425 makes it more useful to the nested function (imagine a counter in the enclosing scope, for instance). In 2.X, programmers usually achieve similar goals by using classes or other schemes. Because nested functions have become a more common coding pattern for state retention, though, nonlocal makes it more generally applicable. Besides allowing names in enclosing defs to be changed, the nonlocal statement also forces the issue for references—just like the global statement, nonlocal causes searches for the names listed in the statement to begin in the enclosing defs’ scopes, not in the local scope of the declaring function. That is, nonlocal also means “skip my local scope entirely.” In fact, the names listed in a nonlocal must have been previously defined in an enclosing def when the nonlocal is reached, or an error is raised. The net effect is much like global: global means the names reside in the enclosing module, and nonlocal means they reside in an enclosing def. nonlocal is even more strict, though—scope search is restricted to only enclosing defs. That is, nonlocal names can appear only in enclosing defs, not in the module’s global scope or built-in scopes outside the defs. The addition of nonlocal does not alter name reference scope rules in general; they still work as before, per the “LEGB” rule described earlier. The nonlocal statement mostly serves to allow names in enclosing scopes to be changed rather than just referenced. However, global and nonlocal statements do both restrict the lookup rules somewhat, when coded in a function: • global makes scope lookup begin in the enclosing module’s scope and allows names there to be assigned. Scope lookup continues on to the built-in scope if the name does not exist in the module, but assignments to global names always create or change them in the module’s scope. • nonlocal restricts scope lookup to just enclosing defs, requires that the names already exist there, and allows them to be assigned. Scope lookup does not continue on to the global or built-in scopes. In Python 2.6, references to enclosing def scope names are allowed, but not assignment. However, you can still use classes with explicit attributes to achieve the same changeable state information effect as nonlocals (and you may be better off doing so in some contexts); globals and function attributes can sometimes accomplish similar goals as well. More on this in a moment; first, let’s turn to some working code to make this more concrete. nonlocal in Action On to some examples, all run in 3.0. References to enclosing def scopes work as they do in 2.6. In the following, tester builds and returns the function nested, to be called later, and the state reference in nested maps the local scope of tester using the normal scope lookup rules: 426 | Chapter 17: Scopes C:\\misc>c:\python30\python >>> def tester(start): ... state = start ... def nested(label): ... print(label, state) ... return nested ... >>> F = tester(0) >>> F('spam') spam 0 >>> F('ham') ham 0 # Referencing nonlocals works normally # Remembers state in enclosing scope Changing a name in an enclosing def’s scope is not allowed by default, though; this is the normal case in 2.6 as well: >>> def tester(start): ... state = start ... def nested(label): ... print(label, state) ... state += 1 # Cannot change by default (or in 2.6) ... return nested ... >>> F = tester(0) >>> F('spam') UnboundLocalError: local variable 'state' referenced before assignment Using nonlocal for changes Now, under 3.0, if we declare state in the tester scope as nonlocal within nested, we get to change it inside the nested function, too. This works even though tester has returned and exited by the time we call the returned nested function through the name F: >>> def tester(start): ... state = start ... def nested(label): ... nonlocal state ... print(label, state) ... state += 1 ... return nested ... >>> F = tester(0) >>> F('spam') spam 0 >>> F('ham') ham 1 >>> F('eggs') eggs 2 # Each call gets its own state # Remembers state in enclosing scope # Allowed to change it if nonlocal # Increments state on each call As usual with enclosing scope references, we can call the tester factory function multiple times to get multiple copies of its state in memory. The state object in the enclosing scope is essentially attached to the nested function object returned; each call makes a The nonlocal Statement | 427 new, distinct state object, such that updating one function’s state won’t impact the other. The following continues the prior listing’s interaction: >>> G = tester(42) >>> G('spam') spam 42 # Make a new tester that starts at 42 >>> G('eggs') eggs 43 # My state information updated to 43 >>> F('bacon') bacon 3 # But F's is where it left off: at 3 # Each call has different state information Boundary cases There are a few things to watch out for. First, unlike the global statement, nonlocal names really must have previously been assigned in an enclosing def’s scope when a nonlocal is evaluated, or else you’ll get an error—you cannot create them dynamically by assigning them anew in the enclosing scope: >>> def tester(start): ... def nested(label): ... nonlocal state # Nonlocals must already exist in enclosing def! ... state = 0 ... print(label, state) ... return nested ... SyntaxError: no binding for nonlocal 'state' found >>> ... ... ... ... ... ... >>> >>> abc >>> 0 def tester(start): def nested(label): global state state = 0 print(label, state) return nested # Globals don't have to exist yet when declared # This creates the name in the module now F = tester(0) F('abc') 0 state Second, nonlocal restricts the scope lookup to just enclosing defs; nonlocals are not looked up in the enclosing module’s global scope or the built-in scope outside all defs, even if they are already there: >>> spam = 99 >>> def tester(): ... def nested(): ... nonlocal spam # Must be in a def, not the module! ... print('Current=', spam) ... spam += 1 ... return nested ... SyntaxError: no binding for nonlocal 'spam' found 428 | Chapter 17: Scopes These restrictions make sense once you realize that Python would not otherwise generally know which enclosing scope to create a brand new name in. In the prior listing, should spam be assigned in tester, or the module outside? Because this is ambiguous, Python must resolve nonlocals at function creation time, not function call time. Why nonlocal? Given the extra complexity of nested functions, you might wonder what the fuss is about. Although it’s difficult to see in our small examples, state information becomes crucial in many programs. There are a variety of ways to “remember” information across function and method calls in Python. While there are tradeoffs for all, nonlocal does improve this story for enclosing scope references—the nonlocal statement allows multiple copies of changeable state to be retained in memory and addresses simple state-retention needs where classes may not be warranted. As we saw in the prior section, the following code allows state to be retained and modified in an enclosing scope. Each call to tester creates a little self-contained package of changeable information, whose names do not clash with any other part of the program: def tester(start): state = start def nested(label): nonlocal state print(label, state) state += 1 return nested # Each call gets its own state # Remembers state in enclosing scope # Allowed to change it if nonlocal F = tester(0) F('spam') Unfortunately, this code only works in Python 3.0. If you are using Python 2.6, other options are available, depending on your goals. The next two sections present some alternatives. Shared state with globals One usual prescription for achieving the nonlocal effect in 2.6 and earlier is to simply move the state out to the global scope (the enclosing module): >>> def tester(start): ... global state ... state = start ... def nested(label): ... global state ... print(label, state) ... state += 1 ... return nested ... >>> F = tester(0) >>> F('spam') # Move it out to the module to change it # global allows changes in module scope # Each call increments shared global state The nonlocal Statement | 429 spam 0 >>> F('eggs') eggs 1 This works in this case, but it requires global declarations in both functions and is prone to name collisions in the global scope (what if “state” is already being used?). A worse, and more subtle, problem is that it only allows for a single shared copy of the state information in the module scope—if we call tester again, we’ll wind up resetting the module’s state variable, such that prior calls will see their state overwritten: >>> G = tester(42) >>> G('toast') toast 42 # Resets state's single copy in global scope >>> G('bacon') bacon 43 >>> F('ham') ham 44 # Oops -- my counter has been overwritten! As shown earlier, when using nonlocal instead of global, each call to tester remembers its own unique copy of the state object. State with classes (preview) The other prescription for changeable state information in 2.6 and earlier is to use classes with attributes to make state information access more explicit than the implicit magic of scope lookup rules. As an added benefit, each instance of a class gets a fresh copy of the state information, as a natural byproduct of Python’s object model. We haven’t explored classes in detail yet, but as a brief preview, here is a reformulation of the tester/nested functions used earlier as a class—state is recorded in objects explicitly as they are created. To make sense of this code, you need to know that a def within a class like this works exactly like a def outside of a class, except that the function’s self argument automatically receives the implied subject of the call (an instance object created by calling the class itself): >>> class tester: ... def __init__(self, start): ... self.state = start ... def nested(self, label): ... print(label, self.state) ... self.state += 1 ... >>> F = tester(0) >>> F.nested('spam') spam 0 >>> F.nested('ham') ham 1 # Class-based alternative (see Part VI) # On object construction, # save state explicitly in new object >>> G = tester(42) >>> G.nested('toast') toast 42 # Each instance gets new copy of state # Changing one does not impact others 430 | Chapter 17: Scopes # Reference state explicitly # Changes are always allowed # Create instance, invoke __init__ # F is passed to self >>> G.nested('bacon') bacon 43 >>> F.nested('eggs') eggs 2 >>> F.state 3 # F's state is where it left off # State may be accessed outside class With just slightly more magic, which we’ll delve into later in this book, we could also make our class look like a callable function using operator overloading. __call__ intercepts direct calls on an instance, so we don’t need to call a named method: >>> class tester: ... def __init__(self, start): ... self.state = start ... def __call__(self, label): ... print(label, self.state) ... self.state += 1 ... >>> H = tester(99) >>> H('juice') juice 99 >>> H('pancakes') pancakes 100 # Intercept direct instance calls # So .nested() not required # Invokes __call__ Don’t sweat the details in this code too much at this point in the book; we’ll explore classes in depth in Part VI and will look at specific operator overloading tools like __call__ in Chapter 29, so you may wish to file this code away for future reference. The point here is that classes can make state information more obvious, by leveraging explicit attribute assignment instead of scope lookups. While using classes for state information is generally a good rule of thumb to follow, they might be overkill in cases like this, where state is a single counter. Such trivial state cases are more common than you might think; in such contexts, nested defs are sometimes more lightweight than coding classes, especially if you’re not familiar with OOP yet. Moreover, there are some scenarios in which nested defs may actually work better than classes (see the description of method decorators in Chapter 38 for an example that is far beyond this chapter’s scope). State with function attributes As a final state-retention option, we can also sometimes achieve the same effect as nonlocals with function attributes—user-defined names attached to functions directly. Here’s a final version of our example based on this technique—it replaces a nonlocal with an attribute attached to the nested function. Although this scheme may not be as intuitive to some, it also allows the state variable to be accessed outside the nested function (with nonlocals, we can only see state variables within the nested def): >>> def tester(start): ... def nested(label): ... print(label, nested.state) ... nested.state += 1 # nested is in enclosing scope # Change attr, not nested itself The nonlocal Statement | 431 ... nested.state = start ... return nested ... >>> F = tester(0) >>> F('spam') spam 0 >>> F('ham') ham 1 >>> F.state 2 >>> >>> G = tester(42) >>> G('eggs') eggs 42 >>> F('ham') ham 2 # Initial state after func defined # F is a 'nested' with state attached # Can access state outside functions too # G has own state, doesn't overwrite F's This code relies on the fact that the function name nested is a local variable in the tester scope enclosing nested; as such, it can be referenced freely inside nested. This code also relies on the fact that changing an object in-place is not an assignment to a name; when it increments nested.state, it is changing part of the object nested references, not the name nested itself. Because we’re not really assigning a name in the enclosing scope, no nonlocal is needed. As you can see, globals, nonlocals, classes, and function attributes all offer state-retention options. Globals only support shared data, classes require a basic knowledge of OOP, and both classes and function attributes allow state to be accessed outside the nested function itself. As usual, the best tool for your program depends upon your program’s goals. Chapter Summary In this chapter, we studied one of two key concepts related to functions: scopes (how variables are looked up when they are used). As we learned, variables are considered local to the function definitions in which they are assigned, unless they are specifically declared to be global or nonlocal. We also studied some more advanced scope concepts here, including nested function scopes and function attributes. Finally, we looked at some general design ideas, such as the need to avoid globals and cross-file changes. In the next chapter, we’re going to continue our function tour with the second key function-related concept: argument passing. As we’ll find, arguments are passed into a function by assignment, but Python also provides tools that allow functions to be flexible in how items are passed. Before we move on, let’s take this chapter’s quiz to review the scope concepts we’ve covered here. 432 | Chapter 17: Scopes Test Your Knowledge: Quiz 1. What is the output of the following code, and why? >>> X = 'Spam' >>> def func(): ... print(X) ... >>> func() 2. What is the output of this code, and why? >>> >>> ... ... >>> >>> X = 'Spam' def func(): X = 'NI!' func() print(X) 3. What does this code print, and why? >>> >>> ... ... ... >>> >>> X = 'Spam' def func(): X = 'NI' print(X) func() print(X) 4. What output does this code produce? Why? >>> >>> ... ... ... >>> >>> X = 'Spam' def func(): global X X = 'NI' func() print(X) 5. What about this code—what’s the output, and why? >>> >>> ... ... ... ... ... >>> >>> X = 'Spam' def func(): X = 'NI' def nested(): print(X) nested() func() X Test Your Knowledge: Quiz | 433 6. How about this example: what is its output in Python 3.0, and why? >>> def func(): ... X = 'NI' ... def nested(): ... nonlocal X ... X = 'Spam' ... nested() ... print(X) ... >>> func() 7. Name three or more ways to retain state information in a Python function. Test Your Knowledge: Answers 1. The output here is 'Spam', because the function references a global variable in the enclosing module (because it is not assigned in the function, it is considered global). 2. The output here is 'Spam' again because assigning the variable inside the function makes it a local and effectively hides the global of the same name. The print statement finds the variable unchanged in the global (module) scope. 3. It prints 'NI' on one line and 'Spam' on another, because the reference to the variable within the function finds the assigned local and the reference in the print statement finds the global. 4. This time it just prints 'NI' because the global declaration forces the variable assigned inside the function to refer to the variable in the enclosing global scope. 5. The output in this case is again 'NI' on one line and 'Spam' on another, because the print statement in the nested function finds the name in the enclosing function’s local scope, and the print at the end finds the variable in the global scope. 6. This example prints 'Spam', because the nonlocal statement (available in Python 3.0 but not 2.6) means that the assignment to X inside the nested function changes X in the enclosing function’s local scope. Without this statement, this assignment would classify X as local to the nested function, making it a different variable; the code would then print 'NI' instead. 7. Although the values of local variables go away when a function returns, you can make a Python function retain state information by using shared global variables, enclosing function scope references within nested functions, or using default argument values. Function attributes can sometimes allow state to be attached to the function itself, instead of looked up in scopes. Another alternative, using OOP with classes, sometimes supports state retention better than any of the scope-based techniques because it makes it explicit with attribute assignments; we’ll explore this option in Part VI. 434 | Chapter 17: Scopes CHAPTER 18 Arguments Chapter 17 explored the details behind Python’s scopes—the places where variables are defined and looked up. As we learned, the place where a name is defined in our code determines much of its meaning. This chapter continues the function story by studying the concepts in Python argument passing—the way that objects are sent to functions as inputs. As we’ll see, arguments (a.k.a. parameters) are assigned to names in a function, but they have more to do with object references than with variable scopes. We’ll also find that Python provides extra tools, such as keywords, defaults, and arbitrary argument collectors, that allow for wide flexibility in the way arguments are sent to a function. Argument-Passing Basics Earlier in this part of the book, I noted that arguments are passed by assignment. This has a few ramifications that aren’t always obvious to beginners, which I’ll expand on in this section. Here is a rundown of the key points in passing arguments to functions: • Arguments are passed by automatically assigning objects to local variable names. Function arguments—references to (possibly) shared objects sent by the caller—are just another instance of Python assignment at work. Because references are implemented as pointers, all arguments are, in effect, passed by pointer. Objects passed as arguments are never automatically copied. • Assigning to argument names inside a function does not affect the caller. Argument names in the function header become new, local names when the function runs, in the scope of the function. There is no aliasing between function argument names and variable names in the scope of the caller. • Changing a mutable object argument in a function may impact the caller. On the other hand, as arguments are simply assigned to passed-in objects, functions can change passed-in mutable objects in place, and the results may affect the caller. Mutable arguments can be input and output for functions. 435 For more details on references, see Chapter 6; everything we learned there also applies to function arguments, though the assignment to argument names is automatic and implicit. Python’s pass-by-assignment scheme isn’t quite the same as C++’s reference parameters option, but it turns out to be very similar to the C language’s argument-passing model in practice: • Immutable arguments are effectively passed “by value.” Objects such as integers and strings are passed by object reference instead of by copying, but because you can’t change immutable objects in-place anyhow, the effect is much like making a copy. • Mutable arguments are effectively passed “by pointer.” Objects such as lists and dictionaries are also passed by object reference, which is similar to the way C passes arrays as pointers—mutable objects can be changed in-place in the function, much like C arrays. Of course, if you’ve never used C, Python’s argument-passing mode will seem simpler still—it involves just the assignment of objects to names, and it works the same whether the objects are mutable or not. Arguments and Shared References To illustrate argument-passing properties at work, consider the following code: >>> ... ... >>> >>> >>> 88 def f(a): a = 99 # a is assigned to (references) passed object # Changes local variable a only b = 88 f(b) print(b) # a and b both reference same 88 initially # b is not changed In this example the variable a is assigned the object 88 at the moment the function is called with f(b), but a lives only within the called function. Changing a inside the function has no effect on the place where the function is called; it simply resets the local variable a to a completely different object. That’s what is meant by a lack of name aliasing—assignment to an argument name inside a function (e.g., a=99) does not magically change a variable like b in the scope of the function call. Argument names may share passed objects initially (they are essentially pointers to those objects), but only temporarily, when the function is first called. As soon as an argument name is reassigned, this relationship ends. At least, that’s the case for assignment to argument names themselves. When arguments are passed mutable objects like lists and dictionaries, we also need to be aware that inplace changes to such objects may live on after a function exits, and hence impact callers. Here’s an example that demonstrates this behavior: 436 | Chapter 18: Arguments >>> ... ... ... >>> >>> >>> >>> (1, def changer(a, b): a = 2 b[0] = 'spam' # Arguments assigned references to objects # Changes local name's value only # Changes shared object in-place X = 1 L = [1, 2] changer(X, L) X, L ['spam', 2]) # Caller # Pass immutable and mutable objects # X is unchanged, L is different! In this code, the changer function assigns values to argument a itself, and to a component of the object referenced by argument b. These two assignments within the function are only slightly different in syntax but have radically different results: • Because a is a local variable name in the function’s scope, the first assignment has no effect on the caller—it simply changes the local variable a to reference a completely different object, and does not change the binding of the name X in the caller’s scope. This is the same as in the prior example. • Argument b is a local variable name, too, but it is passed a mutable object (the list that L references in the caller’s scope). As the second assignment is an in-place object change, the result of the assignment to b[0] in the function impacts the value of L after the function returns. Really, the second assignment statement in changer doesn’t change b—it changes part of the object that b currently references. This in-place change impacts the caller only because the changed object outlives the function call. The name L hasn’t changed either—it still references the same, changed object—but it seems as though L differs after the call because the value it references has been modified within the function. Figure 18-1 illustrates the name/object bindings that exist immediately after the function has been called, and before its code has run. If this example is still confusing, it may help to notice that the effect of the automatic assignments of the passed-in arguments is the same as running a series of simple assignment statements. In terms of the first argument, the assignment has no effect on the caller: >>> >>> >>> >>> 1 X = 1 a = X a = 2 print(X) # They share the same object # Resets 'a' only, 'X' is still 1 The assignment through the second argument does affect a variable at the call, though, because it is an in-place object change: >>> L = [1, 2] >>> b = L >>> b[0] = 'spam' >>> print(L) ['spam', 2] # They share the same object # In-place change: 'L' sees the change too Argument-Passing Basics | 437 Figure 18-1. References: arguments. Because arguments are passed by assignment, argument names in the function may share objects with variables in the scope of the call. Hence, in-place changes to mutable arguments in a function can impact the caller. Here, a and b in the function initially reference the objects referenced by variables X and L when the function is first called. Changing the list through variable b makes L appear different after the call returns. If you recall our discussions about shared mutable objects in Chapters 6 and 9, you’ll recognize the phenomenon at work: changing a mutable object in-place can impact other references to that object. Here, the effect is to make one of the arguments work like both an input and an output of the function. Avoiding Mutable Argument Changes This behavior of in-place changes to mutable arguments isn’t a bug—it’s simply the way argument passing works in Python. Arguments are passed to functions by reference (a.k.a. pointer) by default because that is what we normally want. It means we can pass large objects around our programs without making multiple copies along the way, and we can easily update these objects as we go. In fact, as we’ll see in Part VI, Python’s class model depends upon changing a passed-in “self” argument in-place, to update object state. If we don’t want in-place changes within functions to impact objects we pass to them, though, we can simply make explicit copies of mutable objects, as we learned in Chapter 6. For function arguments, we can always copy the list at the point of call: L = [1, 2] changer(X, L[:]) # Pass a copy, so our 'L' does not change We can also copy within the function itself, if we never want to change passed-in objects, regardless of how the function is called: def changer(a, b): b = b[:] 438 | Chapter 18: Arguments # Copy input list so we don't impact caller a = 2 b[0] = 'spam' # Changes our list copy only Both of these copying schemes don’t stop the function from changing the object—they just prevent those changes from impacting the caller. To really prevent changes, we can always convert to immutable objects to force the issue. Tuples, for example, throw an exception when changes are attempted: L = [1, 2] changer(X, tuple(L)) # Pass a tuple, so changes are errors This scheme uses the built-in tuple function, which builds a new tuple out of all the items in a sequence (really, any iterable). It’s also something of an extreme—because it forces the function to be written to never change passed-in arguments, this solution might impose more limitations on the function than it should, and so should generally be avoided (you never know when changing arguments might come in handy for other calls in the future). Using this technique will also make the function lose the ability to call any list-specific methods on the argument, including methods that do not change the object in-place. The main point to remember here is that functions might update mutable objects like lists and dictionaries passed into them. This isn’t necessarily a problem if it’s expected, and often serves useful purposes. Moreover, functions that change passed-in mutable objects in place are probably designed and intended to do so—the change is likely part of a well-defined API that you shouldn’t violate by making copies. However, you do have to be aware of this property—if objects change out from under you unexpectedly, check whether a called function might be responsible, and make copies when objects are passed if needed. Simulating Output Parameters We’ve already discussed the return statement and used it in a few examples. Here’s another way to use this statement: because return can send back any sort of object, it can return multiple values by packaging them in a tuple or other collection type. In fact, although Python doesn’t support what some languages label “call-by-reference” argument passing, we can usually simulate it by returning tuples and assigning the results back to the original argument names in the caller: >>> ... ... ... ... >>> >>> >>> >>> (2, def multiple(x, y): x = 2 y = [3, 4] return x, y X = 1 L = [1, 2] X, L = multiple(X, L) X, L [3, 4]) # Changes local names only # Return new values in a tuple # Assign results to caller's names Argument-Passing Basics | 439 It looks like the code is returning two values here, but it’s really just one—a two-item tuple with the optional surrounding parentheses omitted. After the call returns, we can use tuple assignment to unpack the parts of the returned tuple. (If you’ve forgotten why this works, flip back to “Tuples” on page 225 in Chapter 4, Chapter 9, and “Assignment Statements” on page 279 in Chapter 11.) The net effect of this coding pattern is to simulate the output parameters of other languages by explicit assignments. X and L change after the call, but only because the code said so. Unpacking arguments in Python 2.X: The preceding example unpacks a tuple returned by the function with tuple assignment. In Python 2.6, it’s also possible to automatically unpack tuples in arguments passed to a function. In 2.6, a function defined by this header: def f((a, (b, c))): can be called with tuples that match the expected structure: f((1, (2, 3))) assigns a, b, and c to 1, 2, and 3, respectively. Naturally, the passed tuple can also be an object created before the call (f(T)). This def syntax is no longer supported in Python 3.0. Instead, code this function as: def f(T): (a, (b, c)) = T to unpack in an explicit assignment statement. This explicit form works in both 3.0 and 2.6. Argument unpacking is an obscure and rarely used feature in Python 2.X. Moreover, a function header in 2.6 supports only the tuple form of sequence assignment; more general sequence assignments (e.g., def f((a, [b, c])):) fail on syntax errors in 2.6 as well and require the explicit assignment form. Tuple unpacking argument syntax is also disallowed by 3.0 in lambda function argument lists: see the sidebar “Why You Will Care: List Comprehensions and map” on page 491 for an example. Somewhat asymmetrically, tuple unpacking assignment is still automatic in 3.0 for loops targets, though; see Chapter 13 for examples. Special Argument-Matching Modes As we’ve just seen, arguments are always passed by assignment in Python; names in the def header are assigned to passed-in objects. On top of this model, though, Python provides additional tools that alter the way the argument objects in a call are matched with argument names in the header prior to assignment. These tools are all optional, but they allow us to write functions that support more flexible calling patterns, and you may encounter some libraries that require them. 440 | Chapter 18: Arguments By default, arguments are matched by position, from left to right, and you must pass exactly as many arguments as there are argument names in the function header. However, you can also specify matching by name, default values, and collectors for extra arguments. The Basics Before we go into the syntactic details, I want to stress that these special modes are optional and only have to do with matching objects to names; the underlying passing mechanism after the matching takes place is still assignment. In fact, some of these tools are intended more for people writing libraries than for application developers. But because you may stumble across these modes even if you don’t code them yourself, here’s a synopsis of the available tools: Positionals: matched from left to right The normal case, which we’ve mostly been using so far, is to match passed argument values to argument names in a function header by position, from left to right. Keywords: matched by argument name Alternatively, callers can specify which argument in the function is to receive a value by using the argument’s name in the call, with the name=value syntax. Defaults: specify values for arguments that aren’t passed Functions themselves can specify default values for arguments to receive if the call passes too few values, again using the name=value syntax. Varargs collecting: collect arbitrarily many positional or keyword arguments Functions can use special arguments preceded with one or two * characters to collect an arbitrary number of extra arguments (this feature is often referred to as varargs, after the varargs feature in the C language, which also supports variablelength argument lists). Varargs unpacking: pass arbitrarily many positional or keyword arguments Callers can also use the * syntax to unpack argument collections into discrete, separate arguments. This is the inverse of a * in a function header—in the header it means collect arbitrarily many arguments, while in the call it means pass arbitrarily many arguments. Keyword-only arguments: arguments that must be passed by name In Python 3.0 (but not 2.6), functions can also specify arguments that must be passed by name with keyword arguments, not by position. Such arguments are typically used to define configuration options in addition to actual arguments. Special Argument-Matching Modes | 441 Matching Syntax Table 18-1 summarizes the syntax that invokes the special argument-matching modes. Table 18-1. Function argument-matching forms Syntax Location Interpretation func(value) Caller Normal argument: matched by position func(name=value) Caller Keyword argument: matched by name func(*sequence) Caller Pass all objects in sequence as individual positional arguments func(**dict) Caller Pass all key/value pairs in dict as individual keyword arguments def func(name) Function Normal argument: matches any passed value by position or name def func(name=value) Function Default argument value, if not passed in the call def func(*name) Function Matches and collects remaining positional arguments in a tuple def func(**name) Function Matches and collects remaining keyword arguments in a dictionary def func(*args, name) Function Arguments that must be passed by keyword only in calls (3.0) def func(*, name=value) These special matching modes break down into function calls and definitions as follows: • In a function call (the first four rows of the table), simple values are matched by position, but using the name=value form tells Python to match by name to arguments instead; these are called keyword arguments. Using a *sequence or **dict in a call allows us to package up arbitrarily many positional or keyword objects in sequences and dictionaries, respectively, and unpack them as separate, individual arguments when they are passed to the function. • In a function header (the rest of the table), a simple name is matched by position or name depending on how the caller passes it, but the name=value form specifies a default value. The *name form collects any extra unmatched positional arguments in a tuple, and the **name form collects extra keyword arguments in a dictionary. In Python 3.0 and later, any normal or defaulted argument names following a *name or a bare * are keyword-only arguments and must be passed by keyword in calls. Of these, keyword arguments and defaults are probably the most commonly used in Python code. We’ve informally used both of these earlier in this book: • We’ve already used keywords to specify options to the 3.0 print function, but they are more general—keywords allow us to label any argument with its name, to make calls more informational. 442 | Chapter 18: Arguments • We met defaults earlier, too, as a way to pass in values from the enclosing function’s scope, but they are also more general—they allow us to make any argument optional, providing its default value in a function definition. As we’ll see, the combination of defaults in a function header and keywords in a call further allows us to pick and choose which defaults to override. In short, special argument-matching modes let you be fairly liberal about how many arguments must be passed to a function. If a function specifies defaults, they are used if you pass too few arguments. If a function uses the * variable argument list forms, you can pass too many arguments; the * names collect the extra arguments in data structures for processing in the function. The Gritty Details If you choose to use and combine the special argument-matching modes, Python will ask you to follow these ordering rules: • In a function call, arguments must appear in this order: any positional arguments (value), followed by a combination of any keyword arguments (name=value) and the *sequence form, followed by the **dict form. • In a function header, arguments must appear in this order: any normal arguments (name), followed by any default arguments (name=value), followed by the *name (or * in 3.0) form if present, followed by any name or name=value keyword-only arguments (in 3.0), followed by the **name form. In both the call and header, the **arg form must appear last if present. If you mix arguments in any other order, you will get a syntax error because the combinations can be ambiguous. The steps that Python internally carries out to match arguments before assignment can roughly be described as follows: 1. 2. 3. 4. 5. Assign nonkeyword arguments by position. Assign keyword arguments by matching names. Assign extra nonkeyword arguments to *name tuple. Assign extra keyword arguments to **name dictionary. Assign default values to unassigned arguments in header. After this, Python checks to make sure each argument is passed just one value; if not, an error is raised. When all matching is complete, Python assigns argument names to the objects passed to them. Special Argument-Matching Modes | 443 The actual matching algorithm Python uses is a bit more complex (it must also account for keyword-only arguments in 3.0, for instance), so we’ll defer to Python’s standard language manual for a more exact description. It’s not required reading, but tracing Python’s matching algorithm may help you to understand some convoluted cases, especially when modes are mixed. In Python 3.0, argument names in a function header can also have annotation values, specified as name:value (or name:value=default when defaults are present). This is simply additional syntax for arguments and does not augment or change the argument-ordering rules described here. The function itself can also have an annotation value, given as def f()->value. See the discussion of function annotation in Chapter 19 for more details. Keyword and Default Examples This is all simpler in code than the preceding descriptions may imply. If you don’t use any special matching syntax, Python matches names by position from left to right, like most other languages. For instance, if you define a function that requires three arguments, you must call it with three arguments: >>> def f(a, b, c): print(a, b, c) ... Here, we pass them by position—a is matched to 1, b is matched to 2, and so on (this works the same in Python 3.0 and 2.6, but extra tuple parentheses are displayed in 2.6 because we’re using 3.0 print calls): >>> f(1, 2, 3) 1 2 3 Keywords In Python, though, you can be more specific about what goes where when you call a function. Keyword arguments allow us to match by name, instead of by position: >>> f(c=3, b=2, a=1) 1 2 3 The c=3 in this call, for example, means send 3 to the argument named c. More formally, Python matches the name c in the call to the argument named c in the function definition’s header, and then passes the value 3 to that argument. The net effect of this call is the same as that of the prior call, but notice that the left-to-right order of the arguments no longer matters when keywords are used because arguments are matched by name, not by position. It’s even possible to combine positional and keyword arguments in a single call. In this case, all positionals are matched first from left to right in the header, before keywords are matched by name: 444 | Chapter 18: Arguments >>> f(1, c=3, b=2) 1 2 3 When most people see this the first time, they wonder why one would use such a tool. Keywords typically have two roles in Python. First, they make your calls a bit more selfdocumenting (assuming that you use better argument names than a, b, and c). For example, a call of this form: func(name='Bob', age=40, job='dev') is much more meaningful than a call with three naked values separated by commas— the keywords serve as labels for the data in the call. The second major use of keywords occurs in conjunction with defaults, which we turn to next. Defaults We talked about defaults in brief earlier, when discussing nested function scopes. In short, defaults allow us to make selected function arguments optional; if not passed a value, the argument is assigned its default before the function runs. For example, here is a function that requires one argument and defaults two: >>> def f(a, b=2, c=3): print(a, b, c) ... When we call this function, we must provide a value for a, either by position or by keyword; however, providing values for b and c is optional. If we don’t pass values to b and c, they default to 2 and 3, respectively: >>> 1 2 >>> 1 2 f(1) 3 f(a=1) 3 If we pass two values, only c gets its default, and with three values, no defaults are used: >>> 1 4 >>> 1 4 f(1, 4) 3 f(1, 4, 5) 5 Finally, here is how the keyword and default features interact. Because they subvert the normal left-to-right positional mapping, keywords allow us to essentially skip over arguments with defaults: >>> f(1, c=6) 1 2 6 Here, a gets 1 by position, c gets 6 by keyword, and b, in between, defaults to 2. Be careful not to confuse the special name=value syntax in a function header and a function call; in the call it means a match-by-name keyword argument, while in the header it specifies a default for an optional argument. In both cases, this is not an assignment statement (despite its appearance); it is special syntax for these two contexts, which modifies the default argument-matching mechanics. Special Argument-Matching Modes | 445 Combining keywords and defaults Here is a slightly larger example that demonstrates keywords and defaults in action. In the following, the caller must always pass at least two arguments (to match spam and eggs), but the other two are optional. If they are omitted, Python assigns toast and ham to the defaults specified in the header: def func(spam, eggs, toast=0, ham=0): print((spam, eggs, toast, ham)) # First 2 required func(1, 2) func(1, ham=1, eggs=0) func(spam=1, eggs=0) func(toast=1, eggs=2, spam=3) func(1, 2, 3, 4) # Output: (1, 2, 0, 0) # Output: (1, 0, 0, 1) # Output: (1, 0, 0, 0) # Output: (3, 2, 1, 0) # Output: (1, 2, 3, 4) Notice again that when keyword arguments are used in the call, the order in which the arguments are listed doesn’t matter; Python matches by name, not by position. The caller must supply values for spam and eggs, but they can be matched by position or by name. Again, keep in mind that the form name=value means different things in the call and the def: a keyword in the call and a default in the header. Arbitrary Arguments Examples The last two matching extensions, * and **, are designed to support functions that take any number of arguments. Both can appear in either the function definition or a function call, and they have related purposes in the two locations. Collecting arguments The first use, in the function definition, collects unmatched positional arguments into a tuple: >>> def f(*args): print(args) ... When this function is called, Python collects all the positional arguments into a new tuple and assigns the variable args to that tuple. Because it is a normal tuple object, it can be indexed, stepped through with a for loop, and so on: >>> f() () >>> f(1) (1,) >>> f(1, 2, 3, 4) (1, 2, 3, 4) The ** feature is similar, but it only works for keyword arguments—it collects them into a new dictionary, which can then be processed with normal dictionary tools. In a sense, the ** form allows you to convert from keywords to dictionaries, which you can then step through with keys calls, dictionary iterators, and the like: 446 | Chapter 18: Arguments >>> def f(**args): print(args) ... >>> f() {} >>> f(a=1, b=2) {'a': 1, 'b': 2} Finally, function headers can combine normal arguments, the *, and the ** to implement wildly flexible call signatures. For instance, in the following, 1 is passed to a by position, 2 and 3 are collected into the pargs positional tuple, and x and y wind up in the kargs keyword dictionary: >>> def f(a, *pargs, **kargs): print(a, pargs, kargs) ... >>> f(1, 2, 3, x=1, y=2) 1 (2, 3) {'y': 2, 'x': 1} In fact, these features can be combined in even more complex ways that may seem ambiguous at first glance—an idea we will revisit later in this chapter. First, though, let’s see what happens when * and ** are coded in function calls instead of definitions. Unpacking arguments In recent Python releases, we can use the * syntax when we call a function, too. In this context, its meaning is the inverse of its meaning in the function definition—it unpacks a collection of arguments, rather than building a collection of arguments. For example, we can pass four arguments to a function in a tuple and let Python unpack them into individual arguments: >>> ... >>> >>> >>> 1 2 def func(a, b, c, d): print(a, b, c, d) args = (1, 2) args += (3, 4) func(*args) 3 4 Similarly, the ** syntax in a function call unpacks a dictionary of key/value pairs into separate keyword arguments: >>> >>> >>> 1 2 args = {'a': 1, 'b': 2, 'c': 3} args['d'] = 4 func(**args) 3 4 Again, we can combine normal, positional, and keyword arguments in the call in very flexible ways: >>> func(*(1, 2), **{'d': 4, 'c': 4}) 1 2 4 4 >>> func(1, *(2, 3), **{'d': 4}) 1 2 3 4 >>> func(1, c=3, *(2,), **{'d': 4}) Special Argument-Matching Modes | 447 1 2 3 4 >>> func(1, *(2, 3), d=4) 1 2 3 4 >>> f(1, *(2,), c=3, **{'d':4}) 1 2 3 4 This sort of code is convenient when you cannot predict the number of arguments that will be passed to a function when you write your script; you can build up a collection of arguments at runtime instead and call the function generically this way. Again, don’t confuse the */** syntax in the function header and the function call—in the header it collects any number of arguments, while in the call it unpacks any number of arguments. As we saw in Chapter 14, the *pargs form in a call is an iteration context, so technically it accepts any iterable object, not just tuples or other sequences as shown in the examples here. For instance, a file object works after the *, and unpacks its lines into individual arguments (e.g., func(*open('fname')). This generality is supported in both Python 3.0 and 2.6, but it holds true only for calls—a *pargs in a call allows any iterable, but the same form in a def header always bundles extra arguments into a tuple. This header behavior is similar in spirit and syntax to the * in Python 3.0 extended sequence unpacking assignment forms we met in Chapter 11 (e.g., x, *y = z), though that feature always creates lists, not tuples. Applying functions generically The prior section’s examples may seem obtuse, but they are used more often than you might expect. Some programs need to call arbitrary functions in a generic fashion, without knowing their names or arguments ahead of time. In fact, the real power of the special “varargs” call syntax is that you don’t need to know how many arguments a function call requires before you write a script. For example, you can use if logic to select from a set of functions and argument lists, and call any of them generically: if <test>: action, args = func1, (1,) else: action, args = func2, (1, 2, 3) ... action(*args) # Call func1 with 1 arg in this case # Call func2 with 3 args here # Dispatch generically More generally, this varargs call syntax is useful any time you cannot predict the arguments list. If your user selects an arbitrary function via a user interface, for instance, you may be unable to hardcode a function call when writing your script. To work around this, simply build up the arguments list with sequence operations, and call it with starred names to unpack the arguments: 448 | Chapter 18: Arguments >>> >>> >>> (2, >>> args = (2,3) args += (4,) args 3, 4) func(*args) Because the arguments list is passed in as a tuple here, the program can build it at runtime. This technique also comes in handy for functions that test or time other functions. For instance, in the following code we support any function with any arguments by passing along whatever arguments were sent in: def tracer(func, *pargs, **kargs): print('calling:', func.__name__) return func(*pargs, **kargs) # Accept arbitrary arguments # Pass along arbitrary arguments def func(a, b, c, d): return a + b + c + d print(tracer(func, 1, 2, c=3, d=4)) When this code is run, arguments are collected by the tracer and then propagated with varargs call syntax: calling: func 10 We’ll see larger examples of such roles later in this book; see especially the sequence timing example in Chapter 20 and the various decorator tools we will code in Chapter 38. The defunct apply built-in (Python 2.6) Prior to Python 3.0, the effect of the *args and **args varargs call syntax could be achieved with a built-in function named apply. This original technique has been removed in 3.0 because it is now redundant (3.0 cleans up many such dusty tools that have been subsumed over the years). It’s still available in Python 2.6, though, and you may come across it in older 2.X code. In short, the following are equivalent prior to Python 3.0: func(*pargs, **kargs) # Newer call syntax: func(*sequence, **dict) apply(func, pargs, kargs) # Defunct built-in: apply(func, sequence, dict) For example, consider the following function, which accepts any number of positional or keyword arguments: >>> def echo(*args, **kwargs): print(args, kwargs) ... >>> echo(1, 2, a=3, b=4) (1, 2) {'a': 3, 'b': 4} Special Argument-Matching Modes | 449 In Python 2.6, we can call it generically with apply, or with the call syntax that is now required in 3.0: >>> pargs = (1, 2) >>> kargs = {'a':3, 'b':4} >>> apply(echo, pargs, kargs) (1, 2) {'a': 3, 'b': 4} >>> echo(*pargs, **kargs) (1, 2) {'a': 3, 'b': 4} The unpacking call syntax form is newer than the apply function, is preferred in general, and is required in 3.0. Apart from its symmetry with the *pargs and **kargs collector forms in def headers, and the fact that it requires fewer keystrokes overall, the newer call syntax also allows us to pass along additional arguments without having to manually extend argument sequences or dictionaries: >>> echo(0, c=5, *pargs, **kargs) (0, 1, 2) {'a': 3, 'c': 5, 'b': 4} # Normal, keyword, *sequence, **dictionary That is, the call syntax form is more general. Since it’s required in 3.0, you should now disavow all knowledge of apply (unless, of course, it appears in 2.X code you must use or maintain...). Python 3.0 Keyword-Only Arguments Python 3.0 generalizes the ordering rules in function headers to allow us to specify keyword-only arguments—arguments that must be passed by keyword only and will never be filled in by a positional argument. This is useful if we want a function to both process any number of arguments and accept possibly optional configuration options. Syntactically, keyword-only arguments are coded as named arguments that appear after *args in the arguments list. All such arguments must be passed using keyword syntax in the call. For example, in the following, a may be passed by name or position, b collects any extra positional arguments, and c must be passed by keyword only: >>> def kwonly(a, *b, c): ... print(a, b, c) ... >>> kwonly(1, 2, c=3) 1 (2,) 3 >>> kwonly(a=1, c=3) 1 () 3 >>> kwonly(1, 2, 3) TypeError: kwonly() needs keyword-only argument c We can also use a * character by itself in the arguments list to indicate that a function does not accept a variable-length argument list but still expects all arguments following the * to be passed as keywords. In the next function, a may be passed by position or name again, but b and c must be keywords, and no extra positionals are allowed: 450 | Chapter 18: Arguments >>> def kwonly(a, *, b, c): ... print(a, b, c) ... >>> kwonly(1, c=3, b=2) 1 2 3 >>> kwonly(c=3, b=2, a=1) 1 2 3 >>> kwonly(1, 2, 3) TypeError: kwonly() takes exactly 1 positional argument (3 given) >>> kwonly(1) TypeError: kwonly() needs keyword-only argument b You can still use defaults for keyword-only arguments, even though they appear after the * in the function header. In the following code, a may be passed by name or position, and b and c are optional but must be passed by keyword if used: >>> def kwonly(a, *, b='spam', c='ham'): ... print(a, b, c) ... >>> kwonly(1) 1 spam ham >>> kwonly(1, c=3) 1 spam 3 >>> kwonly(a=1) 1 spam ham >>> kwonly(c=3, b=2, a=1) 1 2 3 >>> kwonly(1, 2) TypeError: kwonly() takes exactly 1 positional argument (2 given) In fact, keyword-only arguments with defaults are optional, but those without defaults effectively become required keywords for the function: >>> def kwonly(a, *, b, c='spam'): ... print(a, b, c) ... >>> kwonly(1, b='eggs') 1 eggs spam >>> kwonly(1, c='eggs') TypeError: kwonly() needs keyword-only argument b >>> kwonly(1, 2) TypeError: kwonly() takes exactly 1 positional argument (2 given) >>> def kwonly(a, *, b=1, c, d=2): ... print(a, b, c, d) ... >>> kwonly(3, c=4) 3 1 4 2 >>> kwonly(3, c=4, b=5) 3 5 4 2 >>> kwonly(3) TypeError: kwonly() needs keyword-only argument c >>> kwonly(1, 2, 3) TypeError: kwonly() takes exactly 1 positional argument (3 given) Special Argument-Matching Modes | 451 Ordering rules Finally, note that keyword-only arguments must be specified after a single star, not two—named arguments cannot appear after the **args arbitrary keywords form, and a ** can’t appear by itself in the arguments list. Both attempts generate a syntax error: >>> def kwonly(a, **pargs, b, c): SyntaxError: invalid syntax >>> def kwonly(a, **, b, c): SyntaxError: invalid syntax This means that in a function header, keyword-only arguments must be coded before the **args arbitrary keywords form and after the *args arbitrary positional form, when both are present. Whenever an argument name appears before *args, it is a possibly default positional argument, not keyword-only: >>> def f(a, *b, **d, c=6): print(a, b, c, d) SyntaxError: invalid syntax # Keyword-only before **! >>> def f(a, *b, c=6, **d): print(a, b, c, d) ... >>> f(1, 2, 3, x=4, y=5) 1 (2, 3) 6 {'y': 5, 'x': 4} # Collect args in header >>> f(1, 2, 3, x=4, y=5, c=7) 1 (2, 3) 7 {'y': 5, 'x': 4} # Override default >>> f(1, 2, 3, c=7, x=4, y=5) 1 (2, 3) 7 {'y': 5, 'x': 4} # Anywhere in keywords >>> def f(a, c=6, *b, **d): print(a, b, c, d) ... >>> f(1, 2, 3, x=4) 1 (3,) 2 {'x': 4} # c is not keyword-only! # Default used In fact, similar ordering rules hold true in function calls: when keyword-only arguments are passed, they must appear before a **args form. The keyword-only argument can be coded either before or after the *args, though, and may be included in **args: >>> def f(a, *b, c=6, **d): print(a, b, c, d) ... >>> f(1, *(2, 3), **dict(x=4, y=5)) 1 (2, 3) 6 {'y': 5, 'x': 4} # KW-only between * and ** >>> f(1, *(2, 3), **dict(x=4, y=5), c=7) SyntaxError: invalid syntax # Keywords before **args! >>> f(1, *(2, 3), c=7, **dict(x=4, y=5)) 1 (2, 3) 7 {'y': 5, 'x': 4} # Override default >>> f(1, c=7, *(2, 3), **dict(x=4, y=5)) 1 (2, 3) 7 {'y': 5, 'x': 4} # After or before * >>> f(1, *(2, 3), **dict(x=4, y=5, c=7)) 1 (2, 3) 7 {'y': 5, 'x': 4} # Keyword-only in ** 452 | Chapter 18: Arguments # Unpack args at call Trace through these cases on your own, in conjunction with the general argumentordering rules described formally earlier. They may appear to be worst cases in the artificial examples here, but they can come up in real practice, especially for people who write libraries and tools for other Python programmers to use. Why keyword-only arguments? So why care about keyword-only arguments? In short, they make it easier to allow a function to accept both any number of positional arguments to be processed, and configuration options passed as keywords. While their use is optional, without keywordonly arguments extra work may be required to provide defaults for such options and to verify that no superfluous keywords were passed. Imagine a function that processes a set of passed-in objects and allows a tracing flag to be passed: process(X, Y, Z) process(X, Y, notify=True) # use flag's default # override flag default Without keyword-only arguments we have to use both *args and **args and manually inspect the keywords, but with keyword-only arguments less code is required. The following guarantees that no positional argument will be incorrectly matched against notify and requires that it be a keyword if passed: def process(*args, notify=False): ... Since we’re going to see a more realistic example of this later in this chapter, in “Emulating the Python 3.0 print Function” on page 457, I’ll postpone the rest of this story until then. For an additional example of keyword-only arguments in action, see the iteration options timing case study in Chapter 20. And for additional function definition enhancements in Python 3.0, stay tuned for the discussion of function annotation syntax in Chapter 19. The min Wakeup Call! Time for something more realistic. To make this chapter’s concepts more concrete, let’s work through an exercise that demonstrates a practical application of argumentmatching tools. Suppose you want to code a function that is able to compute the minimum value from an arbitrary set of arguments and an arbitrary set of object data types. That is, the function should accept zero or more arguments, as many as you wish to pass. Moreover, the function should work for all kinds of Python object types: numbers, strings, lists, lists of dictionaries, files, and even None. The first requirement provides a natural example of how the * feature can be put to good use—we can collect arguments into a tuple and step over each of them in turn with a simple for loop. The second part of the problem definition is easy: because every The min Wakeup Call! | 453 object type supports comparisons, we don’t have to specialize the function per type (an application of polymorphism); we can simply compare objects blindly and let Python worry about what sort of comparison to perform. Full Credit The following file shows three ways to code this operation, at least one of which was suggested by a student in one of my courses: • The first function fetches the first argument (args is a tuple) and traverses the rest by slicing off the first (there’s no point in comparing an object to itself, especially if it might be a large structure). • The second version lets Python pick off the first and rest of the arguments automatically, and so avoids an index and slice. • The third converts from a tuple to a list with the built-in list call and employs the list sort method. The sort method is coded in C, so it can be quicker than the other approaches at times, but the linear scans of the first two techniques will make them faster most of the time.* The file mins.py contains the code for all three solutions: def min1(*args): res = args[0] for arg in args[1:]: if arg < res: res = arg return res def min2(first, *rest): for arg in rest: if arg < first: first = arg return first def min3(*args): tmp = list(args) tmp.sort() return tmp[0] # Or, in Python 2.4+: return sorted(args)[0] print(min1(3,4,1,2)) * Actually, this is fairly complicated. The Python sort routine is coded in C and uses a highly optimized algorithm that attempts to take advantage of partial ordering in the items to be sorted. It’s named “timsort” after Tim Peters, its creator, and in its documentation it claims to have “supernatural performance” at times (pretty good, for a sort!). Still, sorting is an inherently exponential operation (it must chop up the sequence and put it back together many times), and the other versions simply perform one linear left-to-right scan. The net effect is that sorting is quicker if the arguments are partially ordered, but is likely to be slower otherwise. Even so, Python performance can change over time, and the fact that sorting is implemented in the C language can help greatly; for an exact analysis, you should time the alternatives with the time or timeit modules we’ll meet in Chapter 20. 454 | Chapter 18: Arguments print(min2("bb", "aa")) print(min3([2,2], [1,1], [3,3])) All three solutions produce the same result when the file is run. Try typing a few calls interactively to experiment with these on your own: % python mins.py 1 aa [1, 1] Notice that none of these three variants tests for the case where no arguments are passed in. They could, but there’s no point in doing so here—in all three solutions, Python will automatically raise an exception if no arguments are passed in. The first variant raises an exception when we try to fetch item 0, the second when Python detects an argument list mismatch, and the third when we try to return item 0 at the end. This is exactly what we want to happen—because these functions support any data type, there is no valid sentinel value that we could pass back to designate an error. There are exceptions to this rule (e.g., if you have to run expensive actions before you reach the error), but in general it’s better to assume that arguments will work in your functions’ code and let Python raise errors for you when they do not. Bonus Points You can get can get bonus points here for changing these functions to compute the maximum, rather than minimum, values. This one’s easy: the first two versions only require changing < to >, and the third simply requires that we return tmp[−1] instead of tmp[0]. For an extra point, be sure to set the function name to “max” as well (though this part is strictly optional). It’s also possible to generalize a single function to compute either a minimum or a maximum value, by evaluating comparison expression strings with a tool like the eval built-in function (see the library manual) or passing in an arbitrary comparison function. The file minmax.py shows how to implement the latter scheme: def minmax(test, *args): res = args[0] for arg in args[1:]: if test(arg, res): res = arg return res def lessthan(x, y): return x < y def grtrthan(x, y): return x > y # See also: lambda print(minmax(lessthan, 4, 2, 1, 5, 6, 3)) print(minmax(grtrthan, 4, 2, 1, 5, 6, 3)) # Self-test code % python minmax.py The min Wakeup Call! | 455 1 6 Functions are another kind of object that can be passed into a function like this one. To make this a max (or other) function, for example, we could simply pass in the right sort of test function. This may seem like extra work, but the main point of generalizing functions this way (instead of cutting and pasting to change just a single character) is that we’ll only have one version to change in the future, not two. The Punch Line... Of course, all this was just a coding exercise. There’s really no reason to code min or max functions, because both are built-ins in Python! We met them briefly in Chapter 5 in conjunction with numeric tools, and again in Chapter 14 when exploring iteration contexts. The built-in versions work almost exactly like ours, but they’re coded in C for optimal speed and accept either a single iterable or multiple arguments. Still, though it’s superfluous in this context, the general coding pattern we used here might be useful in other scenarios. Generalized Set Functions Let’s look at a more useful example of special argument-matching modes at work. At the end of Chapter 16, we wrote a function that returned the intersection of two sequences (it picked out items that appeared in both). Here is a version that intersects an arbitrary number of sequences (one or more) by using the varargs matching form *args to collect all the passed-in arguments. Because the arguments come in as a tuple, we can process them in a simple for loop. Just for fun, we’ll code a union function that also accepts an arbitrary number of arguments to collect items that appear in any of the operands: def intersect(*args): res = [] for x in args[0]: for other in args[1:]: if x not in other: break else: res.append(x) return res def union(*args): res = [] for seq in args: for x in seq: if not x in res: res.append(x) return res 456 | Chapter 18: Arguments # Scan first sequence # For all other args # Item in each one? # No: break out of loop # Yes: add items to end # For all args # For all nodes # Add new items to result Because these are tools worth reusing (and they’re too big to retype interactively), we’ll store the functions in a module file called inter2.py (if you’ve forgotten how modules and imports work, see the introduction in Chapter 3, or stay tuned for in-depth coverage in Part V). In both functions, the arguments passed in at the call come in as the args tuple. As in the original intersect, both work on any kind of sequence. Here, they are processing strings, mixed types, and more than two sequences: % python >>> from inter2 import intersect, union >>> s1, s2, s3 = "SPAM", "SCAM", "SLAM" >>> intersect(s1, s2), union(s1, s2) (['S', 'A', 'M'], ['S', 'P', 'A', 'M', 'C']) # Two operands >>> intersect([1,2,3], (1,4)) [1] # Mixed types >>> intersect(s1, s2, s3) ['S', 'A', 'M'] # Three operands >>> union(s1, s2, s3) ['S', 'P', 'A', 'M', 'C', 'L'] I should note that because Python now has a set object type (described in Chapter 5), none of the set-processing examples in this book are strictly required anymore; they are included only as demonstrations of coding techniques. Because it’s constantly improving, Python has an uncanny way of conspiring to make my book examples obsolete over time! Emulating the Python 3.0 print Function To round out the chapter, let’s look at one last example of argument matching at work. The code you’ll see here is intended for use in Python 2.6 or earlier (it works in 3.0, too, but is pointless there): it uses both the *args arbitrary positional tuple and the **args arbitrary keyword-arguments dictionary to simulate most of what the Python 3.0 print function does. As we learned in Chapter 11, this isn’t actually required, because 2.6 programmers can always enable the 3.0 print function with an import of this form: from __future__ import print_function To demonstrate argument matching in general, though, the following file, print30.py, does the same job in a small amount of reusable code: Emulating the Python 3.0 print Function | 457 """ Emulate most of the 3.0 print function for use in 2.X call signature: print30(*args, sep=' ', end='\n', file=None) """ import sys def print30(*args, **kargs): sep = kargs.get('sep', ' ') # Keyword arg defaults end = kargs.get('end', '\n') file = kargs.get('file', sys.stdout) output = '' first = True for arg in args: output += ('' if first else sep) + str(arg) first = False file.write(output + end) To test it, import this into another file or the interactive prompt, and use it like the 3.0 print function. Here is a test script, testprint30.py (notice that the function must be called “print30”, because “print” is a reserved word in 2.6): from print30 import print30 print30(1, 2, 3) print30(1, 2, 3, sep='') print30(1, 2, 3, sep='...') print30(1, [2], (3,), sep='...') print30(4, 5, 6, sep='', end='') print30(7, 8, 9) print30() # Suppress separator # Various object types # Suppress newline # Add newline (or blank line) import sys print30(1, 2, 3, sep='??', end='.\n', file=sys.stderr) # Redirect to file When run under 2.6, we get the same results as 3.0’s print function: C:\misc> c:\python26\python testprint30.py 1 2 3 123 1...2...3 1...[2]...(3,) 4567 8 9 1??2??3. Although pointless in 3.0, the results are the same when run there. As usual, the generality of Python’s design allows us to prototype or develop concepts in the Python language itself. In this case, argument-matching tools are as flexible in Python code as they are in Python’s internal implementation. 458 | Chapter 18: Arguments Using Keyword-Only Arguments It’s interesting to notice that this example could be coded with Python 3.0 keyword-only arguments, described earlier in this chapter, to automatically validate configuration arguments: # Use keyword-only args def print30(*args, sep=' ', end='\n', file=sys.stdout): output = '' first = True for arg in args: output += ('' if first else sep) + str(arg) first = False file.write(output + end) This version works the same as the original, and it’s a prime example of how keywordonly arguments come in handy. The original version assumes that all positional arguments are to be printed, and all keywords are for options only. That’s almost sufficient, but any extra keyword arguments are silently ignored. A call like the following, for instance, will generate an exception with the keyword-only form: >>> print30(99, name='bob') TypeError: print30() got an unexpected keyword argument 'name' but will silently ignore the name argument in the original version. To detect superfluous keywords manually, we could use dict.pop() to delete fetched entries, and check if the dictionary is not empty. Here is an equivalent to the keyword-only version: # Use keyword args deletion with defaults def print30(*args, **kargs): sep = kargs.pop('sep', ' ') end = kargs.pop('end', '\n') file = kargs.pop('file', sys.stdout) if kargs: raise TypeError('extra keywords: %s' % kargs) output = '' first = True for arg in args: output += ('' if first else sep) + str(arg) first = False file.write(output + end) This works as before, but it now catches extraneous keyword arguments, too: >>> print30(99, name='bob') TypeError: extra keywords: {'name': 'bob'} Emulating the Python 3.0 print Function | 459 This version of the function runs under Python 2.6, but it requires four more lines of code than the keyword-only version. Unfortunately, the extra code is required in this case—the keyword-only version only works on 3.0, which negates most of the reason that I wrote this example in the first place (a 3.0 emulator that only works on 3.0 isn’t incredibly useful!). In programs written to run on 3.0, though, keyword-only arguments can simplify a specific category of functions that accept both arguments and options. For another example of 3.0 keyword-only arguments, be sure to see the upcoming iteration timing case study in Chapter 20. Why You Will Care: Keyword Arguments As you can probably tell, advanced argument-matching modes can be complex. They are also entirely optional; you can get by with just simple positional matching, and it’s probably a good idea to do so when you’re starting out. However, because some Python tools make use of them, some general knowledge of these modes is important. For example, keyword arguments play an important role in tkinter, the de facto standard GUI API for Python (this module’s name is Tkinter in Python 2.6). We touch on tkinter only briefly at various points in this book, but in terms of its call patterns, keyword arguments set configuration options when GUI components are built. For instance, a call of the form: from tkinter import * widget = Button(text="Press me", command=someFunction) creates a new button and specifies its text and callback function, using the text and command keyword arguments. Since the number of configuration options for a widget can be large, keyword arguments let you pick and choose which to apply. Without them, you might have to either list all the possible options by position or hope for a judicious positional argument defaults protocol that would handle every possible option arrangement. Many built-in functions in Python expect us to use keywords for usage-mode options as well, which may or may not have defaults. As we learned in Chapter 8, for instance, the sorted built-in: sorted(iterable, key=None, reverse=False) expects us to pass an iterable object to be sorted, but also allows us to pass in optional keyword arguments to specify a dictionary sort key and a reversal flag, which default to None and False, respectively. Since we normally don’t use these options, they may be omitted to use defaults. Chapter Summary In this chapter, we studied the second of two key concepts related to functions: arguments (how objects are passed into a function). As we learned, arguments are passed into a function by assignment, which means by object reference, which really means 460 | Chapter 18: Arguments by pointer. We also studied some more advanced extensions, including default and keyword arguments, tools for using arbitrarily many arguments, and keyword-only arguments in 3.0. Finally, we saw how mutable arguments can exhibit the same behavior as other shared references to objects—unless the object is explicitly copied when it’s sent in, changing a passed-in mutable in a function can impact the caller. The next chapter continues our look at functions by exploring some more advanced function-related ideas: function annotations, lambdas, and functional tools such as map and filter. Many of these concepts stem from the fact that functions are normal objects in Python, and so support some advanced and very flexible processing modes. Before diving into those topics, however, take this chapter’s quiz to review the argument ideas we’ve studied here. Test Your Knowledge: Quiz 1. What is the output of the following code, and why? >>> def func(a, b=4, c=5): print(a, b, c) ... ... >>> func(1, 2) 2. What is the output of this code, and why? >>> def func(a, b, c=5): ... print(a, b, c) ... >>> func(1, c=3, b=2) 3. How about this code: what is its output, and why? >>> def func(a, *pargs): ... print(a, pargs) ... >>> func(1, 2, 3) 4. What does this code print, and why? >>> def func(a, **kargs): ... print(a, kargs) ... >>> func(a=1, c=3, b=2) 5. One last time: what is the output of this code, and why? >>> def func(a, b, c=3, d=4): print(a, b, c, d) ... >>> func(1, *(5,6)) 6. Name three or more ways that functions can communicate results to a caller. Test Your Knowledge: Quiz | 461 Test Your Knowledge: Answers 1. The output here is '1 2 5', because 1 and 2 are passed to a and b by position, and c is omitted in the call and defaults to 5. 2. The output this time is '1 2 3': 1 is passed to a by position, and b and c are passed 2 and 3 by name (the left-to-right order doesn’t matter when keyword arguments are used like this). 3. This code prints '1 (2, 3)', because 1 is passed to a and the *pargs collects the remaining positional arguments into a new tuple object. We can step through the extra positional arguments tuple with any iteration tool (e.g., for arg in pargs: ...). 4. This time the code prints '1, {'c': 3, 'b': 2}', because 1 is passed to a by name and the **kargs collects the remaining keyword arguments into a dictionary. We could step through the extra keyword arguments dictionary by key with any iteration tool (e.g., for key in kargs: ...). 5. The output here is '1 5 6 4': 1 matches a by position, 5 and 6 match b and c by *name positionals (6 overrides c’s default), and d defaults to 4 because it was not passed a value. 6. Functions can send back results with return statements, by changing passed-in mutable arguments, and by setting global variables. Globals are generally frowned upon (except for very special cases, like multithreaded programs) because they can make code more difficult to understand and use. return statements are usually best, but changing mutables is fine, if expected. Functions may also communicate with system devices such as files and sockets, but these are beyond our scope here. 462 | Chapter 18: Arguments CHAPTER 19 Advanced Function Topics This chapter introduces a collection of more advanced function-related topics: recursive functions, function attributes and annotations, the lambda expression, and functional programming tools such as map and filter. These are all somewhat advanced tools that, depending on your job description, you may not encounter on a regular basis. Because of their roles in some domains, though, a basic understanding can be useful; lambdas, for instance, are regular customers in GUIs. Part of the art of using functions lies in the interfaces between them, so we will also explore some general function design principles here. The next chapter continues this advanced theme with an exploration of generator functions and expressions and a revival of list comprehensions in the context of the functional tools we will study here. Function Design Concepts Now that we’ve had a chance to study function basics in Python, let’s begin this chapter with a few words of context. When you start using functions in earnest, you’re faced with choices about how to glue components together—for instance, how to decompose a task into purposeful functions (known as cohesion), how your functions should communicate (called coupling), and so on. You also need to take into account concepts such as the size of your functions, because they directly impact code usability. Some of this falls into the category of structured analysis and design, but it applies to Python code as to any other. We introduced some ideas related to function and module coupling in the Chapter 17 when studying scopes, but here is a review of a few general guidelines for function beginners: • Coupling: use arguments for inputs and return for outputs. Generally, you should strive to make a function independent of things outside of it. Arguments and return statements are often the best ways to isolate external dependencies to a small number of well-known places in your code. 463 • Coupling: use global variables only when truly necessary. Global variables (i.e., names in the enclosing module) are usually a poor way for functions to communicate. They can create dependencies and timing issues that make programs difficult to debug and change. • Coupling: don’t change mutable arguments unless the caller expects it. Functions can change parts of passed-in mutable objects, but (as with global variables) this creates lots of coupling between the caller and callee, which can make a function too specific and brittle. • Cohesion: each function should have a single, unified purpose. When designed well, each of your functions should do one thing—something you can summarize in a simple declarative sentence. If that sentence is very broad (e.g., “this function implements my whole program”), or contains lots of conjunctions (e.g., “this function gives employee raises and submits a pizza order”), you might want to think about splitting it into separate and simpler functions. Otherwise, there is no way to reuse the code behind the steps mixed together in the function. • Size: each function should be relatively small. This naturally follows from the preceding goal, but if your functions start spanning multiple pages on your display, it’s probably time to split them. Especially given that Python code is so concise to begin with, a long or deeply nested function is often a symptom of design problems. Keep it simple, and keep it short. • Coupling: avoid changing variables in another module file directly. We introduced this concept in Chapter 17, and we’ll revisit it in the next part of the book when we focus on modules. For reference, though, remember that changing variables across file boundaries sets up a coupling between modules similar to how global variables couple functions—the modules become difficult to understand and reuse. Use accessor functions whenever possible, instead of direct assignment statements. Figure 19-1 summarizes the ways functions can talk to the outside world; inputs may come from items on the left side, and results may be sent out in any of the forms on the right. Good function designers prefer to use only arguments for inputs and return statements for outputs, whenever possible. Of course, there are plenty of exceptions to the preceding design rules, including some related to Python’s OOP support. As you’ll see in Part VI, Python classes depend on changing a passed-in mutable object—class functions set attributes of an automatically passed-in argument called self to change per-object state information (e.g., self.name='bob'). Moreover, if classes are not used, global variables are often the most straightforward way for functions in modules to retain state between calls. Side effects are dangerous only if they’re unexpected. In general though, you should strive to minimize external dependencies in functions and other program components. The more self-contained a function is, the easier it will be to understand, reuse, and modify. 464 | Chapter 19: Advanced Function Topics Figure 19-1. Function execution environment. Functions may obtain input and produce output in a variety of ways, though functions are usually easier to understand and maintain if you use arguments for input and return statements and anticipated mutable argument changes for output. In Python 3, outputs may also take the form of declared nonlocal names that exist in an enclosing function scope. Recursive Functions While discussing scope rules near the start of Chapter 17, we briefly noted that Python supports recursive functions—functions that call themselves either directly or indirectly in order to loop. Recursion is a somewhat advanced topic, and it’s relatively rare to see in Python. Still, it’s a useful technique to know about, as it allows programs to traverse structures that have arbitrary and unpredictable shapes. Recursion is even an alternative for simple loops and iterations, though not necessarily the simplest or most efficient one. Summation with Recursion Let’s look at some examples. To sum a list (or other sequence) of numbers, we can either use the built-in sum function or write a more custom version of our own. Here’s what a custom summing function might look like when coded with recursion: >>> def mysum(L): ... if not L: ... return 0 ... else: ... return L[0] + mysum(L[1:]) # Call myself >>> mysum([1, 2, 3, 4, 5]) 15 At each level, this function calls itself recursively to compute the sum of the rest of the list, which is later added to the item at the front. The recursive loop ends and zero is returned when the list becomes empty. When using recursion like this, each open level Recursive Functions | 465 of call to the function has its own copy of the function’s local scope on the runtime call stack—here, that means L is different in each level. If this is difficult to understand (and it often is for new programmers), try adding a print of L to the function and run it again, to trace the current list at each call level: >>> ... ... ... ... ... ... >>> [1, [2, [3, [4, [5] [] 15 def mysum(L): print(L) if not L: return 0 else: return L[0] + mysum(L[1:]) # Trace recursive levels # L shorter at each level mysum([1, 2, 3, 4, 5]) 2, 3, 4, 5] 3, 4, 5] 4, 5] 5] As you can see, the list to be summed grows smaller at each recursive level, until it becomes empty—the termination of the recursive loop. The sum is computed as the recursive calls unwind. Coding Alternatives Interestingly, we can also use Python’s if/else ternary expression (described in Chapter 12) to save some code real-estate here. We can also generalize for any summable type (which is easier if we assume at least one item in the input, as we did in Chapter 18’s minimum value example) and use Python 3.0’s extended sequence assignment to make the first/rest unpacking simpler (as covered in Chapter 11): def mysum(L): return 0 if not L else L[0] + mysum(L[1:]) # Use ternary expression def mysum(L): return L[0] if len(L) == 1 else L[0] + mysum(L[1:]) # Any type, assume one def mysum(L): first, *rest = L return first if not rest else first + mysum(rest) # Use 3.0 ext seq assign The latter two of these fail for empty lists but allow for sequences of any object type that supports +, not just numbers: >>> mysum([1]) 1 >>> mysum([1, 2, 3, 4, 5]) 15 >>> mysum(('s', 'p', 'a', 'm')) 'spam' 466 | Chapter 19: Advanced Function Topics # mysum([]) fails in last 2 # But various types now work >>> mysum(['spam', 'ham', 'eggs']) 'spamhameggs' If you study these three variants, you’ll find that the latter two also work on a single string argument (e.g., mysum ('spam')), because strings are sequences of one-character strings; the third variant works on arbitary iterables, including open input files, but the others do not because they index; and the function header def mysum(first, * rest), although similar to the third variant, wouldn’t work at all, because it expects individual arguments, not a single iterable. Keep in mind that recursion can be direct, as in the examples so far, or indirect, as in the following (a function that calls another function, which calls back to its caller). The net effect is the same, though there are two function calls at each level instead of one: >>> def mysum(L): ... if not L: return 0 ... return nonempty(L) ... >>> def nonempty(L): ... return L[0] + mysum(L[1:]) ... >>> mysum([1.1, 2.2, 3.3, 4.4]) 11.0 # Call a function that calls me # Indirectly recursive Loop Statements Versus Recursion Though recursion works for summing in the prior sections’ examples, it’s probably overkill in this context. In fact, recursion is not used nearly as often in Python as in more esoteric languages like Prolog or Lisp, because Python emphasizes simpler procedural statements like loops, which are usually more natural. The while, for example, often makes things a bit more concrete, and it doesn’t require that a function be defined to allow recursive calls: >>> >>> >>> ... ... ... >>> 15 L = [1, 2, 3, 4, 5] sum = 0 while L: sum += L[0] L = L[1:] sum Better yet, for loops iterate for us automatically, making recursion largely extraneous in most cases (and, in all likelihood, less efficient in terms of memory space and execution time): >>> >>> >>> ... >>> 15 L = [1, 2, 3, 4, 5] sum = 0 for x in L: sum += x sum Recursive Functions | 467 With looping statements, we don’t require a fresh copy of a local scope on the call stack for each iteration, and we avoid the speed costs associated with function calls in general. (Stay tuned for Chapter 20’s timer case study for ways to compare the execution times of alternatives like these.) Handling Arbitrary Structures On the other hand, recursion (or equivalent explicit stack-based algorithms, which we’ll finesse here) can be required to traverse arbitrarily shaped structures. As a simple example of recursion’s role in this context, consider the task of computing the sum of all the numbers in a nested sublists structure like this: [1, [2, [3, 4], 5], 6, [7, 8]] # Arbitrarily nested sublists Simple looping statements won’t work here because this not a linear iteration. Nested looping statements do not suffice either, because the sublists may be nested to arbitrary depth and in an arbitrary shape. Instead, the following code accommodates such general nesting by using recursion to visit sublists along the way: def sumtree(L): tot = 0 for x in L: if not isinstance(x, list): tot += x else: tot += sumtree(x) return tot L = [1, [2, [3, 4], 5], 6, [7, 8]] print(sumtree(L)) # For each item at this level # Add numbers directly # Recur for sublists # Arbitrary nesting # Prints 36 # Pathological cases print(sumtree([1, [2, [3, [4, [5]]]]])) # Prints 15 (right-heavy) print(sumtree([[[[[1], 2], 3], 4], 5])) # Prints 15 (left-heavy) Trace through the test cases at the bottom of this script to see how recursion traverses their nested lists. Although this example is artificial, it is representative of a larger class of programs; inheritance trees and module import chains, for example, can exhibit similarly general structures. In fact, we will use recursion again in such roles in more realistic examples later in this book: • In Chapter 24’s reloadall.py, to traverse import chains • In Chapter 28’s classtree.py, to traverse class inheritance trees • In Chapter 30’s lister.py, to traverse class inheritance trees again 468 | Chapter 19: Advanced Function Topics Although you should generally prefer looping statements to recursion for linear iterations on the grounds of simplicity and efficiency, we’ll find that recursion is essential in scenarios like those in these later examples. Moreover, you sometimes need to be aware of the potential of unintended recursion in your programs. As you’ll also see later in the book, some operator overloading methods in classes such as __setattr__ and __getattribute__ have the potential to recursively loop if used incorrectly. Recursion is a powerful tool, but it tends to be best when expected! Function Objects: Attributes and Annotations Python functions are more flexible than you might think. As we’ve seen in this part of the book, functions in Python are much more than code-generation specifications for a compiler—Python functions are full-blown objects, stored in pieces of memory all their own. As such, they can be freely passed around a program and called indirectly. They also support operations that have little to do with calls at all—attribute storage and annotation. Indirect Function Calls Because Python functions are objects, you can write programs that process them generically. Function objects may be assigned to other names, passed to other functions, embedded in data structures, returned from one function to another, and more, as if they were simple numbers or strings. Function objects also happen to support a special operation: they can be called by listing arguments in parentheses after a function expression. Still, functions belong to the same general category as other objects. We’ve seen some of these generic use cases for functions in earlier examples, but a quick review helps to underscore the object model. For example, there’s really nothing special about the name used in a def statement: it’s just a variable assigned in the current scope, as if it had appeared on the left of an = sign. After a def runs, the function name is simply a reference to an object—you can reassign that object to other names freely and call it through any reference: >>> def echo(message): ... print(message) ... >>> echo('Direct call') Direct call # Name echo assigned to function object >>> x = echo >>> x('Indirect call!') Indirect call! # Now x references the function too # Call object through name by adding () # Call object through original name Function Objects: Attributes and Annotations | 469 Because arguments are passed by assigning objects, it’s just as easy to pass functions to other functions as arguments. The callee may then call the passed-in function just by adding arguments in parentheses: >>> def indirect(func, arg): ... func(arg) ... >>> indirect(echo, 'Argument call!') Argument call! # Call the passed-in object by adding () # Pass the function to another function You can even stuff function objects into data structures, as though they were integers or strings. The following, for example, embeds the function twice in a list of tuples, as a sort of actions table. Because Python compound types like these can contain any sort of object, there’s no special case here, either: >>> schedule = [ (echo, 'Spam!'), (echo, 'Ham!') ] >>> for (func, arg) in schedule: ... func(arg) # Call functions embedded in containers ... Spam! Ham! This code simply steps through the schedule list, calling the echo function with one argument each time through (notice the tuple-unpacking assignment in the for loop header, introduced in Chapter 13). As we saw in Chapter 17’s examples, functions can also be created and returned for use elsewhere: >>> def make(label): # Make a function but don't call it ... def echo(message): ... print(label + ':' + message) ... return echo ... >>> F = make('Spam') # Label in enclosing scope is retained >>> F('Ham!') # Call the function that make returned Spam:Ham! >>> F('Eggs!') Spam:Eggs! Python’s universal object model and lack of type declarations make for an incredibly flexible programming language. Function Introspection Because they are objects, we can also process functions with normal object tools. In fact, functions are more flexible than you might expect. For instance, once we make a function, we can call it as usual: >>> def func(a): ... b = 'spam' ... return b * a ... >>> func(8) 'spamspamspamspamspamspamspamspam' 470 | Chapter 19: Advanced Function Topics But the call expression is just one operation defined to work on function objects. We can also inspect their attributes generically (the following is run in Python 3.0, but 2.6 results are similar): >>> func.__name__ 'func' >>> dir(func) ['__annotations__', '__call__', '__class__', '__closure__', '__code__', ...more omitted... '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__'] Introspection tools allow us to explore implementation details too—functions have attached code objects, for example, which provide details on aspects such as the functions’ local variables and arguments: >>> func.__code__ <code object func at 0x0257C9B0, file "<stdin>", line 1> >>> dir(func.__code__) ['__class__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', ...more omitted... 'co_argcount', 'co_cellvars', 'co_code', 'co_consts', 'co_filename', 'co_firstlineno', 'co_flags', 'co_freevars', 'co_kwonlyargcount', 'co_lnotab', 'co_name', 'co_names', 'co_nlocals', 'co_stacksize', 'co_varnames'] >>> func.__code__.co_varnames ('a', 'b') >>> func.__code__.co_argcount 1 Tool writers can make use of such information to manage functions (in fact, we will too in Chapter 38, to implement validation of function arguments in decorators). Function Attributes Function objects are not limited to the system-defined attributes listed in the prior section, though. As we learned in Chapter 17, it’s possible to attach arbitrary userdefined attributes to them as well: >>> func <function func at 0x0257C738> >>> func.count = 0 >>> func.count += 1 >>> func.count 1 >>> func.handles = 'Button-Press' >>> func.handles 'Button-Press' >>> dir(func) ['__annotations__', '__call__', '__class__', '__closure__', '__code__', ...more omitted... __str__', '__subclasshook__', 'count', 'handles'] Function Objects: Attributes and Annotations | 471 As we saw in that chapter, such attributes can be used to attach state information to function objects directly, instead of using other techniques such as globals, nonlocals, and classes. Unlike nonlocals, such attributes are accessible anywhere the function itself is. In a sense, this is also a way to emulate “static locals” in other languages—variables whose names are local to a function, but whose values are retained after a function exits. Attributes are related to objects instead of scopes, but the net effect is similar. Function Annotations in 3.0 In Python 3.0 (but not 2.6), it’s also possible to attach annotation information— arbitrary user-defined data about a function’s arguments and result—to a function object. Python provides special syntax for specifying annotations, but it doesn’t do anything with them itself; annotations are completely optional, and when present are simply attached to the function object’s __annotations__ attribute for use by other tools. We met Python 3.0’s keyword-only arguments in the prior chapter; annotations generalize function header syntax further. Consider the following nonannotated function, which is coded with three arguments and returns a result: >>> def func(a, b, c): ... return a + b + c ... >>> func(1, 2, 3) 6 Syntactically, function annotations are coded in def header lines, as arbitrary expressions associated with arguments and return values. For arguments, they appear after a colon immediately following the argument’s name; for return values, they are written after a -> following the arguments list. This code, for example, annotates all three of the prior function’s arguments, as well as its return value: >>> def func(a: 'spam', b: (1, 10), c: float) -> int: ... return a + b + c ... >>> func(1, 2, 3) 6 Calls to an annotated function work as usual, but when annotations are present Python collects them in a dictionary and attaches it to the function object itself. Argument names become keys, the return value annotation is stored under key “return” if coded, and the values of annotation keys are assigned to the results of the annotation expressions: >>> func.__annotations__ {'a': 'spam', 'c': <class 'float'>, 'b': (1, 10), 'return': <class 'int'>} Because they are just Python objects attached to a Python object, annotations are straightforward to process. The following annotates just two of three arguments and steps through the attached annotations generically: 472 | Chapter 19: Advanced Function Topics >>> def func(a: 'spam', b, c: 99): ... return a + b + c ... >>> func(1, 2, 3) 6 >>> func.__annotations__ {'a': 'spam', 'c': 99} >>> for arg in func.__annotations__: ... print(arg, '=>', func.__annotations__[arg]) ... a => spam c => 99 There are two fine points to note here. First, you can still use defaults for arguments if you code annotations—the annotation (and its : character) appear before the default (and its = character). In the following, for example, a: 'spam' = 4 means that argument a defaults to 4 and is annotated with the string 'spam': >>> def func(a: 'spam' = 4, b: (1, 10) = 5, c: float = 6) -> int: ... return a + b + c ... >>> func(1, 2, 3) 6 >>> func() # 4 + 5 + 6 (all defaults) 15 >>> func(1, c=10) # 1 + 5 + 10 (keywords work normally) 16 >>> func.__annotations__ {'a': 'spam', 'c': <class 'float'>, 'b': (1, 10), 'return': <class 'int'>} Second, note that the blank spaces in the prior example are all optional—you can use spaces between components in function headers or not, but omitting them might degrade your code’s readability to some observers: >>> def func(a:'spam'=4, b:(1,10)=5, c:float=6)->int: ... return a + b + c ... >>> func(1, 2) #1+2+6 9 >>> func.__annotations__ {'a': 'spam', 'c': <class 'float'>, 'b': (1, 10), 'return': <class 'int'>} Annotations are a new feature in 3.0, and some of their potential uses remain to be uncovered. It’s easy to imagine annotations being used to specify constraints for argument types or values, though, and larger APIs might use this feature as a way to register function interface information. In fact, we’ll see a potential application in Chapter 38, where we’ll look at annotations as an alternative to function decorator arguments (a more general concept in which information is coded outside the function header and so is not limited to a single role). Like Python itself, annotation is a tool whose roles are shaped by your imagination. Function Objects: Attributes and Annotations | 473 Finally, note that annotations work only in def statements, not lambda expressions, because lambda’s syntax already limits the utility of the functions it defines. Coincidentally, this brings us to our next topic. Anonymous Functions: lambda Besides the def statement, Python also provides an expression form that generates function objects. Because of its similarity to a tool in the Lisp language, it’s called lambda.* Like def, this expression creates a function to be called later, but it returns the function instead of assigning it to a name. This is why lambdas are sometimes known as anonymous (i.e., unnamed) functions. In practice, they are often used as a way to inline a function definition, or to defer execution of a piece of code. lambda Basics The lambda’s general form is the keyword lambda, followed by one or more arguments (exactly like the arguments list you enclose in parentheses in a def header), followed by an expression after a colon: lambda argument1, argument2,... argumentN :expression using arguments Function objects returned by running lambda expressions work exactly the same as those created and assigned by defs, but there are a few differences that make lambdas useful in specialized roles: • lambda is an expression, not a statement. Because of this, a lambda can appear in places a def is not allowed by Python’s syntax—inside a list literal or a function call’s arguments, for example. As an expression, lambda returns a value (a new function) that can optionally be assigned a name. In contrast, the def statement always assigns the new function to the name in the header, instead of returning it as a result. • lambda’s body is a single expression, not a block of statements. The lambda’s body is similar to what you’d put in a def body’s return statement; you simply type the result as a naked expression, instead of explicitly returning it. Because it is limited to an expression, a lambda is less general than a def—you can only squeeze so much logic into a lambda body without using statements such as if. This is by design, to limit program nesting: lambda is designed for coding simple functions, and def handles larger tasks. * The lambda tends to intimidate people more than it should. This reaction seems to stem from the name “lambda” itself—a name that comes from the Lisp language, which got it from lambda calculus, which is a form of symbolic logic. In Python, though, it’s really just a keyword that introduces the expression syntactically. Obscure mathematical heritage aside, lambda is simpler to use than you may think. 474 | Chapter 19: Advanced Function Topics Apart from those distinctions, defs and lambdas do the same sort of work. For instance, we’ve seen how to make a function with a def statement: >>> def func(x, y, z): return x + y + z ... >>> func(2, 3, 4) 9 But you can achieve the same effect with a lambda expression by explicitly assigning its result to a name through which you can later call the function: >>> f = lambda x, y, z: x + y + z >>> f(2, 3, 4) 9 Here, f is assigned the function object the lambda expression creates; this is how def works, too, but its assignment is automatic. Defaults work on lambda arguments, just like in a def: >>> x = (lambda a="fee", b="fie", c="foe": a + b + c) >>> x("wee") 'weefiefoe' The code in a lambda body also follows the same scope lookup rules as code inside a def. lambda expressions introduce a local scope much like a nested def, which automatically sees names in enclosing functions, the module, and the built-in scope (via the LEGB rule): >>> def knights(): ... title = 'Sir' ... action = (lambda x: title + ' ' + x) ... return action ... >>> act = knights() >>> act('robin') 'Sir robin' # Title in enclosing def # Return a function In this example, prior to Release 2.2, the value for the name title would typically have been passed in as a default argument value instead; flip back to the scopes coverage in Chapter 17 if you’ve forgotten why. Why Use lambda? Generally speaking, lambdas come in handy as a sort of function shorthand that allows you to embed a function’s definition within the code that uses it. They are entirely optional (you can always use defs instead), but they tend to be simpler coding constructs in scenarios where you just need to embed small bits of executable code. For instance, we’ll see later that callback handlers are frequently coded as inline lambda expressions embedded directly in a registration call’s arguments list, instead of being defined with a def elsewhere in a file and referenced by name (see the sidebar “Why You Will Care: Callbacks” on page 479 for an example). Anonymous Functions: lambda | 475 lambdas are also commonly used to code jump tables, which are lists or dictionaries of actions to be performed on demand. For example: # Inline function definition L = [lambda x: x ** 2, lambda x: x ** 3, lambda x: x ** 4] # A list of 3 callable functions for f in L: print(f(2)) # Prints 4, 8, 16 print(L[0](3)) # Prints 9 The lambda expression is most useful as a shorthand for def, when you need to stuff small pieces of executable code into places where statements are illegal syntactically. This code snippet, for example, builds up a list of three functions by embedding lambda expressions inside a list literal; a def won’t work inside a list literal like this because it is a statement, not an expression. The equivalent def coding would require temporary function names and function definitions outside the context of intended use: def f1(x): return x ** 2 def f2(x): return x ** 3 def f3(x): return x ** 4 # Define named functions L = [f1, f2, f3] # Reference by name for f in L: print(f(2)) # Prints 4, 8, 16 print(L[0](3)) # Prints 9 In fact, you can do the same sort of thing with dictionaries and other data structures in Python to build up more general sorts of action tables. Here’s another example to illustrate, at the interactive prompt: >>> key = 'got' >>> {'already': (lambda: 2 + 2), ... 'got': (lambda: 2 * 4), ... 'one': (lambda: 2 ** 6)}[key]() 8 Here, when Python makes the temporary dictionary, each of the nested lambdas generates and leaves behind a function to be called later. Indexing by key fetches one of those functions, and parentheses force the fetched function to be called. When coded this way, a dictionary becomes a more general multiway branching tool than what I could show you in Chapter 12’s coverage of if statements. To make this work without lambda, you’d need to instead code three def statements somewhere else in your file, outside the dictionary in which the functions are to be used, and reference the functions by name: >>> def f1(): return 2 + 2 ... >>> def f2(): return 2 * 4 ... 476 | Chapter 19: Advanced Function Topics >>> def f3(): return 2 ** 6 ... >>> key = 'one' >>> {'already': f1, 'got': f2, 'one': f3}[key]() 64 This works, too, but your defs may be arbitrarily far away in your file, even if they are just little bits of code. The code proximity that lambdas provide is especially useful for functions that will only be used in a single context—if the three functions here are not useful anywhere else, it makes sense to embed their definitions within the dictionary as lambdas. Moreover, the def form requires you to make up names for these little functions that may clash with other names in this file (perhaps unlikely, but always possible). lambdas also come in handy in function-call argument lists as a way to inline temporary function definitions not used anywhere else in your program; we’ll see some examples of such other uses later in this chapter, when we study map. How (Not) to Obfuscate Your Python Code The fact that the body of a lambda has to be a single expression (not a series of statements) would seem to place severe limits on how much logic you can pack into a lambda. If you know what you’re doing, though, you can code most statements in Python as expression-based equivalents. For example, if you want to print from the body of a lambda function, simply say sys.stdout.write(str(x)+'\n'), instead of print(x) (recall from Chapter 11 that this is what print really does). Similarly, to nest logic in a lambda, you can use the if/else ternary expression introduced in Chapter 12, or the equivalent but trickier and/or combination also described there. As you learned earlier, the following statement: if a: b else: c can be emulated by either of these roughly equivalent expressions: b if a else c ((a and b) or c) Because expressions like these can be placed inside a lambda, they may be used to implement selection logic within a lambda function: >>> lower = (lambda x, y: x if x < y else y) >>> lower('bb', 'aa') 'aa' >>> lower('aa', 'bb') 'aa' Anonymous Functions: lambda | 477 Furthermore, if you need to perform loops within a lambda, you can also embed things like map calls and list comprehension expressions (tools we met in earlier chapters and will revisit in this and the next chapter): >>> import sys >>> showall = lambda x: list(map(sys.stdout.write, x)) # Use list in 3.0 >>> t = showall(['spam\n', 'toast\n', 'eggs\n']) spam toast eggs >>> showall = lambda x: [sys.stdout.write(line) for line in x] >>> t = showall(('bright\n', 'side\n', 'of\n', 'life\n')) bright side of life Now that I’ve shown you these tricks, I am required by law to ask you to please only use them as a last resort. Without due care, they can lead to unreadable (a.k.a. obfuscated) Python code. In general, simple is better than complex, explicit is better than implicit, and full statements are better than arcane expressions. That’s why lambda is limited to expressions. If you have larger logic to code, use def; lambda is for small pieces of inline code. On the other hand, you may find these techniques useful in moderation. Nested lambdas and Scopes lambdas are the main beneficiaries of nested function scope lookup (the E in the LEGB scope rule we studied in Chapter 17). In the following, for example, the lambda appears inside a def—the typical case—and so can access the value that the name x had in the enclosing function’s scope at the time that the enclosing function was called: >>> def action(x): ... return (lambda y: x + y) ... >>> act = action(99) >>> act <function <lambda> at 0x00A16A88> >>> act(2) 101 # Make and return function, remember x # Call what action returned What wasn’t illustrated in the prior discussion of nested function scopes is that a lambda also has access to the names in any enclosing lambda. This case is somewhat obscure, but imagine if we recoded the prior def with a lambda: >>> action = (lambda x: (lambda y: x + y)) >>> act = action(99) >>> act(3) 102 478 | Chapter 19: Advanced Function Topics >>> ((lambda x: (lambda y: x + y))(99))(4) 103 Here, the nested lambda structure makes a function that makes a function when called. In both cases, the nested lambda’s code has access to the variable x in the enclosing lambda. This works, but it’s fairly convoluted code; in the interest of readability, nested lambdas are generally best avoided. Why You Will Care: Callbacks Another very common application of lambda is to define inline callback functions for Python’s tkinter GUI API (this module is named Tkinter in Python 2.6). For example, the following creates a button that prints a message on the console when pressed, assuming tkinter is available on your computer (it is by default on Windows and other OSs): import sys from tkinter import Button, mainloop # Tkinter in 2.6 x = Button( text ='Press me', command=(lambda:sys.stdout.write('Spam\n'))) x.pack() mainloop() Here, the callback handler is registered by passing a function generated with a lambda to the command keyword argument. The advantage of lambda over def here is that the code that handles a button press is right here, embedded in the button-creation call. In effect, the lambda defers execution of the handler until the event occurs: the write call happens on button presses, not when the button is created. Because the nested function scope rules apply to lambdas as well, they are also easier to use as callback handlers, as of Python 2.2—they automatically see names in the functions in which they are coded and no longer require passed-in defaults in most cases. This is especially handy for accessing the special self instance argument that is a local variable in enclosing class method functions (more on classes in Part VI): class MyGui: def makewidgets(self): Button(command=(lambda: self.onPress("spam"))) def onPress(self, message): ...use message... In prior releases, even self had to be passed in to a lambda with defaults. Mapping Functions over Sequences: map One of the more common things programs do with lists and other sequences is apply an operation to each item and collect the results. For instance, updating all the counters in a list can be done easily with a for loop: Mapping Functions over Sequences: map | 479 >>> counters = [1, 2, 3, 4] >>> >>> updated = [] >>> for x in counters: ... updated.append(x + 10) ... >>> updated [11, 12, 13, 14] # Add 10 to each item But because this is such a common operation, Python actually provides a built-in that does most of the work for you. The map function applies a passed-in function to each item in an iterable object and returns a list containing all the function call results. For example: >>> def inc(x): return x + 10 ... >>> list(map(inc, counters)) [11, 12, 13, 14] # Function to be run # Collect results We met map briefly in Chapters 13 and 14, as a way to apply a built-in function to items in an iterable. Here, we make better use of it by passing in a user-defined function to be applied to each item in the list—map calls inc on each list item and collects all the return values into a new list. Remember that map is an iterable in Python 3.0, so a list call is used to force it to produce all its results for display here; this isn’t necessary in 2.6. Because map expects a function to be passed in, it also happens to be one of the places where lambda commonly appears: >>> list(map((lambda x: x + 3), counters)) [4, 5, 6, 7] # Function expression Here, the function adds 3 to each item in the counters list; as this little function isn’t needed elsewhere, it was written inline as a lambda. Because such uses of map are equivalent to for loops, with a little extra code you can always code a general mapping utility yourself: >>> def mymap(func, seq): ... res = [] ... for x in seq: res.append(func(x)) ... return res Assuming the function inc is still as it was when it was shown previously, we can map it across a sequence with the built-in or our equivalent: >>> list(map(inc, [1, 2, 3])) [11, 12, 13] >>> mymap(inc, [1, 2, 3]) [11, 12, 13] # Built-in is an iterator # Ours builds a list (see generators) However, as map is a built-in, it’s always available, always works the same way, and has some performance benefits (as we’ll prove in the next chapter, it’s usually faster than a manually coded for loop). Moreover, map can be used in more advanced ways than 480 | Chapter 19: Advanced Function Topics shown here. For instance, given multiple sequence arguments, it sends items taken from sequences in parallel as distinct arguments to the function: >>> pow(3, 4) 81 >>> list(map(pow, [1, 2, 3], [2, 3, 4])) [1, 8, 81] # 3**4 # 1**2, 2**3, 3**4 With multiple sequences, map expects an N-argument function for N sequences. Here, the pow function takes two arguments on each call—one from each sequence passed to map. It’s not much extra work to simulate this multiple-sequence generality in code, too, but we’ll postpone doing so until later in the next chapter, after we’ve met some additional iteration tools. The map call is similar to the list comprehension expressions we studied in Chapter 14 and will meet again in the next chapter, but map applies a function call to each item instead of an arbitrary expression. Because of this limitation, it is a somewhat less general tool. However, in some cases map may be faster to run than a list comprehension (e.g., when mapping a built-in function), and it may also require less coding. Functional Programming Tools: filter and reduce The map function is the simplest representative of a class of Python built-ins used for functional programming—tools that apply functions to sequences and other iterables. Its relatives filter out items based on a test function (filter) and apply functions to pairs of items and running results (reduce). Because they return iterables, range and filter both require list calls to display all their results in 3.0. For example, the following filter call picks out items in a sequence that are greater than zero: >>> list(range(−5, 5)) [−5, −4, −3, −2, −1, 0, 1, 2, 3, 4] # An iterator in 3.0 >>> list(filter((lambda x: x > 0), range(−5, 5))) [1, 2, 3, 4] # An iterator in 3.0 Items in the sequence or iterable for which the function returns a true result are added to the result list. Like map, this function is roughly equivalent to a for loop, but it is built-in and fast: >>> >>> ... ... ... >>> [1, res = [] for x in range(−5, 5): if x > 0: res.append(x) res 2, 3, 4] reduce, which is a simple built-in function in 2.6 but lives in the functools module in 3.0, is more complex. It accepts an iterator to process, but it’s not an iterator itself—it Functional Programming Tools: filter and reduce | 481 returns a single result. Here are two reduce calls that compute the sum and product of the items in a list: >>> from functools import reduce # Import in 3.0, not in 2.6 >>> reduce((lambda x, y: x + y), [1, 2, 3, 4]) 10 >>> reduce((lambda x, y: x * y), [1, 2, 3, 4]) 24 At each step, reduce passes the current sum or product, along with the next item from the list, to the passed-in lambda function. By default, the first item in the sequence initializes the starting value. To illustrate, here’s the for loop equivalent to the first of these calls, with the addition hardcoded inside the loop: >>> >>> >>> ... ... >>> 10 L = [1,2,3,4] res = L[0] for x in L[1:]: res = res + x res Coding your own version of reduce is actually fairly straightforward. The following function emulates most of the built-in’s behavior and helps demystify its operation in general: >>> def myreduce(function, sequence): ... tally = sequence[0] ... for next in sequence[1:]: ... tally = function(tally, next) ... return tally ... >>> myreduce((lambda x, y: x + y), [1, 2, 3, 4, 5]) 15 >>> myreduce((lambda x, y: x * y), [1, 2, 3, 4, 5]) 120 The built-in reduce also allows an optional third argument placed before the items in the sequence to serve as a default result when the sequence is empty, but we’ll leave this extension as a suggested exercise. If this coding technique has sparked your interest, you might also be interested in the standard library operator module, which provides functions that correspond to builtin expressions and so comes in handy for some uses of functional tools (see Python’s library manual for more details on this module): >>> import operator, functools >>> functools.reduce(operator.add, [2, 4, 6]) # Function-based + 12 >>> functools.reduce((lambda x, y: x + y), [2, 4, 6]) 12 482 | Chapter 19: Advanced Function Topics Together with map, filter and reduce support powerful functional programming techniques. Some observers might also extend the functional programming toolset in Python to include lambda, discussed earlier, as well as list comprehensions—a topic we will return to in the next chapter. Chapter Summary This chapter took us on a tour of advanced function-related concepts: recursive functions; function annotations; lambda expression functions; functional tools such as map, filter, and reduce; and general function design ideas. The next chapter continues the advanced topics motif with a look at generators and a reprisal of iterators and list comprehensions—tools that are just as related to functional programming as to looping statements. Before you move on, though, make sure you’ve mastered the concepts covered here by working through this chapter’s quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. How are lambda expressions and def statements related? What’s the point of using lamba? Compare and contrast map, filter, and reduce. What are function annotations, and how are they used? What are recursive functions, and how are they used? What are some general design guidelines for coding functions? Test Your Knowledge: Answers 1. Both lambda and def create function objects to be called later. Because lambda is an expression, though, it returns a function object instead of assigning it to a name, and it can be used to nest a function definition in places where a def will not work syntactically. A lambda only allows for a single implicit return value expression, though; because it does not support a block of statements, it is not ideal for larger functions. 2. lambdas allow us to “inline” small units of executable code, defer its execution, and provide it with state in the form of default arguments and enclosing scope variables. Using a lambda is never required; you can always code a def instead and reference the function by name. lambdas come in handy, though, to embed small pieces of deferred code that are unlikely to be used elsewhere in a program. They commonly appear in callback-based program such as GUIs, and they have a natural affinity with function tools like map and filter that expect a processing function. Test Your Knowledge: Answers | 483 3. These three built-in functions all apply another function to items in a sequence (iterable) object and collect results. map passes each item to the function and collects all results, filter collects items for which the function returns a True value, and reduce computes a single value by applying the function to an accumulator and successive items. Unlike the other two, reduce is available in the functools module in 3.0, not the built-in scope. 4. Function annotations, available in 3.0 and later, are syntactic embellishments of a function’s arguments and result, which are collected into a dictionary assigned to the function’s __annotations__ attribute. Python places no semantic meaning on these annotations, but simply packages them for potential use by other tools. 5. Recursive functions call themselves either directly or indirectly in order to loop. They may be used to traverse arbitrarily shaped structures, but they can also be used for iteration in general (though the latter role is often more simply and efficiently coded with looping statements). 6. Functions should generally be small, as self-contained as possible, have a single unified purpose, and communicate with other components through input arguments and return values. They may use mutable arguments to communicate results too if changes are expected, and some types of programs imply other communication mechanisms. 484 | Chapter 19: Advanced Function Topics CHAPTER 20 Iterations and Comprehensions, Part 2 This chapter continues the advanced function topics theme, with a reprisal of the comprehension and iteration concepts introduced in Chapter 14. Because list comprehensions are as much related to the prior chapter’s functional tools (e.g., map and filter) as they are to for loops, we’ll revisit them in this context here. We’ll also take a second look at iterators in order to study generator functions and their generator expression relatives—user-defined ways to produce results on demand. Iteration in Python also encompasses user-defined classes, but we’ll defer that final part of this story until Part VI, when we study operator overloading. As this is the last pass we’ll make over built-in iteration tools, though, we will summarize the various tools we’ve met thus far, and time the relative performance of some of them. Finally, because this is the last chapter in the part of the book, we’ll close with the usual sets of “gotchas” and exercises to help you start coding the ideas you’ve read about. List Comprehensions Revisited: Functional Tools In the prior chapter, we studied functional programming tools like map and filter, which map operations over sequences and collect results. Because this is such a common task in Python coding, Python eventually sprouted a new expression—the list comprehension—that is even more flexible than the tools we just studied. In short, list comprehensions apply an arbitrary expression to items in an iterable, rather than applying a function. As such, they can be more general tools. We met list comprehensions in Chapter 14, in conjunction with looping statements. Because they’re also related to functional programming tools like the map and filter calls, though, we’ll resurrect the topic here for one last look. Technically, this feature is not tied to functions—as we’ll see, list comprehensions can be a more general tool than map and filter—but it is sometimes best understood by analogy to function-based alternatives. 485 List Comprehensions Versus map Let’s work through an example that demonstrates the basics. As we saw in Chapter 7, Python’s built-in ord function returns the ASCII integer code of a single character (the chr built-in is the converse—it returns the character for an ASCII integer code): >>> ord('s') 115 Now, suppose we wish to collect the ASCII codes of all characters in an entire string. Perhaps the most straightforward approach is to use a simple for loop and append the results to a list: >>> res = [] >>> for x in 'spam': ... res.append(ord(x)) ... >>> res [115, 112, 97, 109] Now that we know about map, though, we can achieve similar results with a single function call without having to manage list construction in the code: >>> res = list(map(ord, 'spam')) >>> res [115, 112, 97, 109] # Apply function to sequence However, we can get the same results from a list comprehension expression—while map maps a function over a sequence, list comprehensions map an expression over a sequence: >>> res = [ord(x) for x in 'spam'] >>> res [115, 112, 97, 109] # Apply expression to sequence List comprehensions collect the results of applying an arbitrary expression to a sequence of values and return them in a new list. Syntactically, list comprehensions are enclosed in square brackets (to remind you that they construct lists). In their simple form, within the brackets you code an expression that names a variable followed by what looks like a for loop header that names the same variable. Python then collects the expression’s results for each iteration of the implied loop. The effect of the preceding example is similar to that of the manual for loop and the map call. List comprehensions become more convenient, though, when we wish to apply an arbitrary expression to a sequence: >>> [x ** 2 for x in range(10)] [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] Here, we’ve collected the squares of the numbers 0 through 9 (we’re just letting the interactive prompt print the resulting list; assign it to a variable if you need to retain it). To do similar work with a map call, we would probably need to invent a little function to implement the square operation. Because we won’t need this function elsewhere, 486 | Chapter 20: Iterations and Comprehensions, Part 2 we’d typically (but not necessarily) code it inline, with a lambda, instead of using a def statement elsewhere: >>> list(map((lambda x: x ** 2), range(10))) [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] This does the same job, and it’s only a few keystrokes longer than the equivalent list comprehension. It’s also only marginally more complex (at least, once you understand the lambda). For more advanced kinds of expressions, though, list comprehensions will often require considerably less typing. The next section shows why. Adding Tests and Nested Loops: filter List comprehensions are even more general than shown so far. For instance, as we learned in Chapter 14, you can code an if clause after the for to add selection logic. List comprehensions with if clauses can be thought of as analogous to the filter builtin discussed in the prior chapter—they skip sequence items for which the if clause is not true. To demonstrate, here are both schemes picking up even numbers from 0 to 4; like the map list comprehension alternative of the prior section, the filter version here must invent a little lambda function for the test expression. For comparison, the equivalent for loop is shown here as well: >>> [x for x in range(5) if x % 2 == 0] [0, 2, 4] >>> list(filter((lambda x: x % 2 == 0), range(5))) [0, 2, 4] >>> >>> ... ... ... >>> [0, res = [] for x in range(5): if x % 2 == 0: res.append(x) res 2, 4] All of these use the modulus (remainder of division) operator, %, to detect even numbers: if there is no remainder after dividing a number by 2, it must be even. The filter call here is not much longer than the list comprehension either. However, we can combine an if clause and an arbitrary expression in our list comprehension, to give it the effect of a filter and a map, in a single expression: >>> [x ** 2 for x in range(10) if x % 2 == 0] [0, 4, 16, 36, 64] This time, we collect the squares of the even numbers from 0 through 9: the for loop skips numbers for which the attached if clause on the right is false, and the expression on the left computes the squares. The equivalent map call would require a lot more work List Comprehensions Revisited: Functional Tools | 487 on our part—we would have to combine filter selections with map iteration, making for a noticeably more complex expression: >>> list( map((lambda x: x**2), filter((lambda x: x % 2 == 0), range(10))) ) [0, 4, 16, 36, 64] In fact, list comprehensions are more general still. You can code any number of nested for loops in a list comprehension, and each may have an optional associated if test. The general structure of list comprehensions looks like this: [ expression for target1 in iterable1 [if condition1] for target2 in iterable2 [if condition2] ... for targetN in iterableN [if conditionN] ] When for clauses are nested within a list comprehension, they work like equivalent nested for loop statements. For example, the following: >>> res = [x + y for x in [0, 1, 2] for y in [100, 200, 300]] >>> res [100, 200, 300, 101, 201, 301, 102, 202, 302] has the same effect as this substantially more verbose equivalent: >>> res = [] >>> for x in [0, 1, 2]: ... for y in [100, 200, 300]: ... res.append(x + y) ... >>> res [100, 200, 300, 101, 201, 301, 102, 202, 302] Although list comprehensions construct lists, remember that they can iterate over any sequence or other iterable type. Here’s a similar bit of code that traverses strings instead of lists of numbers, and so collects concatenation results: >>> [x + y for x in 'spam' for y in 'SPAM'] ['sS', 'sP', 'sA', 'sM', 'pS', 'pP', 'pA', 'pM', 'aS', 'aP', 'aA', 'aM', 'mS', 'mP', 'mA', 'mM'] Finally, here is a much more complex list comprehension that illustrates the effect of attached if selections on nested for clauses: >>> [(x, y) for x in range(5) if x % 2 == 0 for y in range(5) if y % 2 == 1] [(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)] This expression permutes even numbers from 0 through 4 with odd numbers from 0 through 4. The if clauses filter out items in each sequence iteration. Here is the equivalent statement-based code: >>> res = [] >>> for x in range(5): ... if x % 2 == 0: ... for y in range(5): ... if y % 2 == 1: ... res.append((x, y)) ... 488 | Chapter 20: Iterations and Comprehensions, Part 2 >>> res [(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)] Recall that if you’re confused about what a complex list comprehension does, you can always nest the list comprehension’s for and if clauses inside each other (indenting successively further to the right) to derive the equivalent statements. The result is longer, but perhaps clearer. The map and filter equivalent would be wildly complex and deeply nested, so I won’t even try showing it here. I’ll leave its coding as an exercise for Zen masters, ex-Lisp programmers, and the criminally insane.... List Comprehensions and Matrixes Not all list comprehensions are so artificial, of course. Let’s look at one more application to stretch a few synapses. One basic way to code matrixes (a.k.a. multidimensional arrays) in Python is with nested list structures. The following, for example, defines two 3 × 3 matrixes as lists of nested lists: >>> M = [[1, 2, 3], ... [4, 5, 6], ... [7, 8, 9]] >>> N = [[2, 2, 2], ... [3, 3, 3], ... [4, 4, 4]] Given this structure, we can always index rows, and columns within rows, using normal index operations: >>> M[1] [4, 5, 6] >>> M[1][2] 6 List comprehensions are powerful tools for processing such structures, though, because they automatically scan rows and columns for us. For instance, although this structure stores the matrix by rows, to collect the second column we can simply iterate across the rows and pull out the desired column, or iterate through positions in the rows and index as we go: >>> [row[1] for row in M] [2, 5, 8] >>> [M[row][1] for row in (0, 1, 2)] [2, 5, 8] Given positions, we can also easily perform tasks such as pulling out a diagonal. The following expression uses range to generate the list of offsets and then indexes with the row and column the same, picking out M[0][0], then M[1][1], and so on (we assume the matrix has the same number of rows and columns): List Comprehensions Revisited: Functional Tools | 489 >>> [M[i][i] for i in range(len(M))] [1, 5, 9] Finally, with a bit of creativity, we can also use list comprehensions to combine multiple matrixes. The following first builds a flat list that contains the result of multiplying the matrixes pairwise, and then builds a nested list structure having the same values by nesting list comprehensions: >>> [M[row][col] * N[row][col] for row in range(3) for col in range(3)] [2, 4, 6, 12, 15, 18, 28, 32, 36] >>> [[M[row][col] * N[row][col] for col in range(3)] for row in range(3)] [[2, 4, 6], [12, 15, 18], [28, 32, 36]] This last expression works because the row iteration is an outer loop: for each row, it runs the nested column iteration to build up one row of the result matrix. It’s equivalent to this statement-based code: >>> res >>> for ... ... ... ... ... >>> res [[2, 4, = [] row in range(3): tmp = [] for col in range(3): tmp.append(M[row][col] * N[row][col]) res.append(tmp) 6], [12, 15, 18], [28, 32, 36]] Compared to these statements, the list comprehension version requires only one line of code, will probably run substantially faster for large matrixes, and just might make your head explode! Which brings us to the next section. Comprehending List Comprehensions With such generality, list comprehensions can quickly become, well, incomprehensible, especially when nested. Consequently, my advice is typically to use simple for loops when getting started with Python, and map or comprehensions in isolated cases where they are easy to apply. The “keep it simple” rule applies here, as always: code conciseness is a much less important goal than code readability. However, in this case, there is currently a substantial performance advantage to the extra complexity: based on tests run under Python today, map calls are roughly twice as fast as equivalent for loops, and list comprehensions are usually slightly faster than map calls.* This speed difference is generally due to the fact that map and list * These performance generalizations can depend on call patterns, as well as changes and optimizations in Python itself. Recent Python releases have sped up the simple for loop statement, for example. Usually, though, list comprehensions are still substantially faster than for loops and even faster than map (though map can still win for built-in functions). To time these alternatives yourself, see the standard library’s time module’s time.clock and time.time calls, the newer timeit module added in Release 2.4, or this chapter’s upcoming section “Timing Iteration Alternatives” on page 509. 490 | Chapter 20: Iterations and Comprehensions, Part 2 comprehensions run at C language speed inside the interpreter, which is much faster than stepping through Python for loop code within the PVM. Because for loops make logic more explicit, I recommend them in general on the grounds of simplicity. However, map and list comprehensions are worth knowing and using for simpler kinds of iterations, and if your application’s speed is an important consideration. In addition, because map and list comprehensions are both expressions, they can show up syntactically in places that for loop statements cannot, such as in the bodies of lambda functions, within list and dictionary literals, and more. Still, you should try to keep your map calls and list comprehensions simple; for more complex tasks, use full statements instead. Why You Will Care: List Comprehensions and map Here’s a more realistic example of list comprehensions and map in action (we solved this problem with list comprehensions in Chapter 14, but we’ll revive it here to add map-based alternatives). Recall that the file readlines method returns lines with \n endof-line characters at the ends: >>> open('myfile').readlines() ['aaa\n', 'bbb\n', 'ccc\n'] If you don’t want the end-of-line characters, you can slice them off all the lines in a single step with a list comprehension or a map call (map results are iterables in Python 3.0, so we must run them through list to see all their results at once): >>> [line.rstrip() for line in open('myfile').readlines()] ['aaa', 'bbb', 'ccc'] >>> [line.rstrip() for line in open('myfile')] ['aaa', 'bbb', 'ccc'] >>> list(map((lambda line: line.rstrip()), open('myfile'))) ['aaa', 'bbb', 'ccc'] The last two of these make use of file iterators (which essentially means that you don’t need a method call to grab all the lines in iteration contexts such as these). The map call is slightly longer than the list comprehension, but neither has to manage result list construction explicitly. A list comprehension can also be used as a sort of column projection operation. Python’s standard SQL database API returns query results as a list of tuples much like the following—the list is the table, tuples are rows, and items in tuples are column values: listoftuple = [('bob', 35, 'mgr'), ('mel', 40, 'dev')] A for loop could pick up all the values from a selected column manually, but map and list comprehensions can do it in a single step, and faster: >>> [age for (name, age, job) in listoftuple] [35, 40] >>> list(map((lambda row: row[1]), listoftuple)) [35, 40] List Comprehensions Revisited: Functional Tools | 491 The first of these makes use of tuple assignment to unpack row tuples in the list, and the second uses indexing. In Python 2.6 (but not in 3.0—see the note on 2.6 argument unpacking in Chapter 18), map can use tuple unpacking on its argument, too: # 2.6 only >>> list(map((lambda (name, age, job): age), listoftuple)) [35, 40] See other books and resources for more on Python’s database API. Beside the distinction between running functions versus expressions, the biggest difference between map and list comprehensions in Python 3.0 is that map is an iterator, generating results on demand; to achieve the same memory economy, list comprehensions must be coded as generator expressions (one of the topics of this chapter). Iterators Revisited: Generators Python today supports procrastination much more than it did in the past—it provides tools that produce results only when needed, instead of all at once. In particular, two language constructs delay result creation whenever possible: • Generator functions are coded as normal def statements but use yield statements to return results one at a time, suspending and resuming their state between each. • Generator expressions are similar to the list comprehensions of the prior section, but they return an object that produces results on demand instead of building a result list. Because neither constructs a result list all at once, they save memory space and allow computation time to be split across result requests. As we’ll see, both of these ultimately perform their delayed-results magic by implementing the iteration protocol we studied in Chapter 14. Generator Functions: yield Versus return In this part of the book, we’ve learned about coding normal functions that receive input parameters and send back a single result immediately. It is also possible, however, to write functions that may send back a value and later be resumed, picking up where they left off. Such functions are known as generator functions because they generate a sequence of values over time. Generator functions are like normal functions in most respects, and in fact are coded with normal def statements. However, when created, they are automatically made to implement the iteration protocol so that they can appear in iteration contexts. We studied iterators in Chapter 14; here, we’ll revisit them to see how they relate to generators. 492 | Chapter 20: Iterations and Comprehensions, Part 2 State suspension Unlike normal functions that return a value and exit, generator functions automatically suspend and resume their execution and state around the point of value generation. Because of that, they are often a useful alternative to both computing an entire series of values up front and manually saving and restoring state in classes. Because the state that generator functions retain when they are suspended includes their entire local scope, their local variables retain information and make it available when the functions are resumed. The chief code difference between generator and normal functions is that a generator yields a value, rather than returning one—the yield statement suspends the function and sends a value back to the caller, but retains enough state to enable the function to resume from where it left off. When resumed, the function continues execution immediately after the last yield run. From the function’s perspective, this allows its code to produce a series of values over time, rather than computing them all at once and sending them back in something like a list. Iteration protocol integration To truly understand generator functions, you need to know that they are closely bound up with the notion of the iteration protocol in Python. As we’ve seen, iterable objects define a __next__ method, which either returns the next item in the iteration, or raises the special StopIteration exception to end the iteration. An object’s iterator is fetched with the iter built-in function. Python for loops, and all other iteration contexts, use this iteration protocol to step through a sequence or value generator, if the protocol is supported; if not, iteration falls back on repeatedly indexing sequences instead. To support this protocol, functions containing a yield statement are compiled specially as generators. When called, they return a generator object that supports the iteration interface with an automatically created method named __next__ to resume execution. Generator functions may also have a return statement that, along with falling off the end of the def block, simply terminates the generation of values—technically, by raising a StopIteration exception after any normal function exit actions. From the caller’s perspective, the generator’s __next__ method resumes the function and runs until either the next yield result is returned or a StopIteration is raised. The net effect is that generator functions, coded as def statements containing yield statements, are automatically made to support the iteration protocol and thus may be used in any iteration context to produce results over time and on demand. Iterators Revisited: Generators | 493 As noted in Chapter 14, in Python 2.6 and earlier, iterable objects define a method named next instead of __next__. This includes the generator objects we are using here. In 3.0 this method is renamed to __next__. The next built-in function is provided as a convenience and portability tool: next(I) is the same as I.__next__() in 3.0 and I.next() in 2.6. Prior to 2.6, programs simply call I.next() instead to iterate manually. Generator functions in action To illustrate generator basics, let’s turn to some code. The following code defines a generator function that can be used to generate the squares of a series of numbers over time: >>> def gensquares(N): ... for i in range(N): ... yield i ** 2 ... # Resume here later This function yields a value, and so returns to its caller, each time through the loop; when it is resumed, its prior state is restored and control picks up again immediately after the yield statement. For example, when it’s used in the body of a for loop, control returns to the function after its yield statement each time through the loop: # Resume the function # Print last yielded value >>> for i in gensquares(5): ... print(i, end=' : ') ... 0 : 1 : 4 : 9 : 16 : >>> To end the generation of values, functions either use a return statement with no value or simply allow control to fall off the end of the function body. If you want to see what is going on inside the for, call the generator function directly: >>> x = gensquares(4) >>> x <generator object at 0x0086C378> You get back a generator object that supports the iteration protocol we met in Chapter 14—the generator object has a __next__ method that starts the function, or resumes it from where it last yielded a value, and raises a StopIteration exception when the end of the series of values is reached. For convenience, the next(X) built-in calls an object’s X.__next__() method for us: >>> 0 >>> 1 >>> 4 >>> 9 >>> next(x) # Same as x.__next__() in 3.0 next(x) # Use x.next() or next() in 2.6 next(x) next(x) next(x) 494 | Chapter 20: Iterations and Comprehensions, Part 2 Traceback (most recent call last): ...more text omitted... StopIteration As we learned in Chapter 14, for loops (and other iteration contexts) work with generators in the same way—by calling the __next__ method repeatedly, until an exception is caught. If the object to be iterated over does not support this protocol, for loops instead use the indexing protocol to iterate. Note that in this example, we could also simply build the list of yielded values all at once: >>> def buildsquares(n): ... res = [] ... for i in range(n): res.append(i ** 2) ... return res ... >>> for x in buildsquares(5): print(x, end=' : ') ... 0 : 1 : 4 : 9 : 16 : For that matter, we could use any of the for loop, map, or list comprehension techniques: >>> for x in [n ** 2 for n in range(5)]: ... print(x, end=' : ') ... 0 : 1 : 4 : 9 : 16 : >>> for x in map((lambda n: n ** 2), range(5)): ... print(x, end=' : ') ... 0 : 1 : 4 : 9 : 16 : However, generators can be better in terms of both memory use and performance. They allow functions to avoid doing all the work up front, which is especially useful when the result lists are large or when it takes a lot of computation to produce each value. Generators distribute the time required to produce the series of values among loop iterations. Moreover, for more advanced uses, generators can provide a simpler alternative to manually saving the state between iterations in class objects—with generators, variables accessible in the function’s scopes are saved and restored automatically.† We’ll discuss class-based iterators in more detail in Part VI. † Interestingly, generator functions are also something of a “poor man’s” multithreading device—they interleave a function’s work with that of its caller, by dividing its operation into steps run between yields. Generators are not threads, though: the program is explicitly directed to and from the function within a single thread of control. In one sense, threading is more general (producers can run truly independently and post results to a queue), but generators may be simpler to code. See the second footnote in Chapter 17 for a brief introduction to Python multithreading tools. Note that because control is routed explicitly at yield and next calls, generators are also not backtracking, but are more strongly related to coroutines—formal concepts that are both beyond this chapter’s scope. Iterators Revisited: Generators | 495 Extended generator function protocol: send versus next In Python 2.5, a send method was added to the generator function protocol. The send method advances to the next item in the series of results, just like __next__, but also provides a way for the caller to communicate with the generator, to affect its operation. Technically, yield is now an expression form that returns the item passed to send, not a statement (though it can be called either way—as yield X, or A = (yield X)). The expression must be enclosed in parentheses unless it’s the only item on the right side of the assignment statement. For example, X = yield Y is OK, as is X = (yield Y) + 42. When this extra protocol is used, values are sent into a generator G by calling G.send(value). The generator’s code is then resumed, and the yield expression in the generator returns the value passed to send. If the regular G.__next__() method (or its next(G) equivalent) is called to advance, the yield simply returns None. For example: >>> def gen(): ... for i in range(10): ... X = yield i ... print(X) ... >>> G = gen() >>> next(G) # Must call next() first, to start generator 0 >>> G.send(77) # Advance, and send value to yield expression 77 1 >>> G.send(88) 88 2 >>> next(G) # next() and X.__next__() send None None 3 The send method can be used, for example, to code a generator that its caller can terminate by sending a termination code, or redirect by passing a new position. In addition, generators in 2.5 also support a throw(type) method to raise an exception inside the generator at the latest yield, and a close method that raises a special Generator Exit exception inside the generator to terminate the iteration. These are advanced features that we won’t delve into in more detail here; see reference texts and Python’s standard manuals for more information. Note that while Python 3.0 provides a next(X) convenience built-in that calls the X.__next__() method of an object, other generator methods, like send, must be called as methods of generator objects directly (e.g., G.send(X)). This makes sense if you realize that these extra methods are implemented on built-in generator objects only, whereas the __next__ method applies to all iterable objects (both built-in types and user-defined classes). 496 | Chapter 20: Iterations and Comprehensions, Part 2 Generator Expressions: Iterators Meet Comprehensions In all recent versions of Python, the notions of iterators and list comprehensions are combined in a new feature of the language, generator expressions. Syntactically, generator expressions are just like normal list comprehensions, but they are enclosed in parentheses instead of square brackets: >>> [x ** 2 for x in range(4)] [0, 1, 4, 9] # List comprehension: build a list >>> (x ** 2 for x in range(4)) <generator object at 0x011DC648> # Generator expression: make an iterable In fact, at least on a function basis, coding a list comprehension is essentially the same as wrapping a generator expression in a list built-in call to force it to produce all its results in a list at once: >>> list(x ** 2 for x in range(4)) [0, 1, 4, 9] # List comprehension equivalence Operationally, however, generator expressions are very different—instead of building the result list in memory, they return a generator object, which in turn supports the iteration protocol to yield one piece of the result list at a time in any iteration context: >>> >>> 0 >>> 1 >>> 4 >>> 9 >>> G = (x ** 2 for x in range(4)) next(G) next(G) next(G) next(G) next(G) Traceback (most recent call last): ...more text omitted... StopIteration We don’t typically see the next iterator machinery under the hood of a generator expression like this because for loops trigger it for us automatically: >>> for num in (x ** 2 for x in range(4)): ... print('%s, %s' % (num, num / 2.0)) ... 0, 0.0 1, 0.5 4, 2.0 9, 4.5 As we’ve already learned, every iteration context does this, including the sum, map, and sorted built-in functions; list comprehensions; and other iteration contexts we learned about in Chapter 14, such as the any, all, and list built-in functions. Iterators Revisited: Generators | 497 Notice that the parentheses are not required around a generator expression if they are the sole item enclosed in other parentheses, like those of a function call. Extra parentheses are required, however, in the second call to sorted: >>> sum(x ** 2 for x in range(4)) 14 >>> sorted(x ** 2 for x in range(4)) [0, 1, 4, 9] >>> sorted((x ** 2 for x in range(4)), reverse=True) [9, 4, 1, 0] >>> import math >>> list( map(math.sqrt, (x ** 2 for x in range(4))) ) [0.0, 1.0, 2.0, 3.0] Generator expressions are primarily a memory-space optimization—they do not require the entire result list to be constructed all at once, as the square-bracketed list comprehension does. They may also run slightly slower in practice, so they are probably best used only for very large result sets. A more authoritative statement about performance, though, will have to await the timing script we’ll code later in this chapter. Generator Functions Versus Generator Expressions Interestingly, the same iteration can often be coded with either a generator function or a generator expression. The following generator expression, for example, repeats each character in a string four times: >>> G = (c * 4 for c in 'SPAM') >>> list(G) ['SSSS', 'PPPP', 'AAAA', 'MMMM'] # Generator expression # Force generator to produce all results The equivalent generator function requires slightly more code, but as a multistatement function it will be able to code more logic and use more state information if needed: >>> def timesfour(S): ... for c in S: ... yield c * 4 ... >>> G = timesfour('spam') >>> list(G) ['ssss', 'pppp', 'aaaa', 'mmmm'] # Generator function # Iterate automatically Both expressions and functions support both automatic and manual iteration—the prior list call iterates automatically, and the following iterate manually: >>> G = (c * 4 for c in 'SPAM') >>> I = iter(G) >>> next(I) 'SSSS' >>> next(I) 'PPPP' 498 | Chapter 20: Iterations and Comprehensions, Part 2 # Iterate manually >>> G = timesfour('spam') >>> I = iter(G) >>> next(I) 'ssss' >>> next(I) 'pppp' Notice that we make new generators here to iterate again—as explained in the next section, generators are one-shot iterators. Generators Are Single-Iterator Objects Both generator functions and generator expressions are their own iterators and thus support just one active iteration—unlike some built-in types, you can’t have multiple iterators of either positioned at different locations in the set of results. For example, using the prior section’s generator expression, a generator’s iterator is the generator itself (in fact, calling iter on a generator is a no-op): >>> G = (c * 4 for c in 'SPAM') >>> iter(G) is G True # My iterator is myself: G has __next__ If you iterate over the results stream manually with multiple iterators, they will all point to the same position: >>> G = (c * 4 for c in 'SPAM') >>> I1 = iter(G) >>> next(I1) 'SSSS' >>> next(I1) 'PPPP' >>> I2 = iter(G) >>> next(I2) 'AAAA' # Make a new generator # Iterate manually # Second iterator at same position! Moreover, once any iteration runs to completion, all are exhausted—we have to make a new generator to start again: >>> list(I1) ['MMMM'] >>> next(I2) StopIteration # Collect the rest of I1's items >>> I3 = iter(G) >>> next(I3) StopIteration # Ditto for new iterators >>> I3 = iter(c * 4 for c in 'SPAM') >>> next(I3) 'SSSS' # New generator to start over # Other iterators exhausted too Iterators Revisited: Generators | 499 The same holds true for generator functions—the following def statement-based equivalent supports just one active iterator and is exhausted after one pass: >>> def timesfour(S): ... for c in S: ... yield c * 4 ... >>> G = timesfour('spam') >>> iter(G) is G True >>> I1, I2 = iter(G), iter(G) >>> next(I1) 'ssss' >>> next(I1) 'pppp' >>> next(I2) 'aaaa' # Generator functions work the same way # I2 at same position as I1 This is different from the behavior of some built-in types, which support multiple iterators and passes and reflect their in-place changes in active iterators: >>> L = [1, 2, 3, 4] >>> I1, I2 = iter(L), iter(L) >>> next(I1) 1 >>> next(I1) 2 >>> next(I2) 1 >>> del L[2:] >>> next(I1) StopIteration # Lists support multiple iterators # Changes reflected in iterators When we begin coding class-based iterators in Part VI, we’ll see that it’s up to us to decide how any iterations we wish to support for our objects, if any. Emulating zip and map with Iteration Tools To demonstrate the power of iteration tools in action, let’s turn to some more advanced use case examples. Once you know about list comprehensions, generators, and other iteration tools, it turns out that emulating many of Python’s functional built-ins is both straightforward and instructive. For example, we’ve already seen how the built-in zip and map functions combine iterables and project functions across them, respectively. With multiple sequence arguments, map projects the function across items taken from each sequence in much the same way that zip pairs them up: >>> S1 = 'abc' >>> S2 = 'xyz123' >>> list(zip(S1, S2)) [('a', 'x'), ('b', 'y'), ('c', 'z')] 500 | Chapter 20: Iterations and Comprehensions, Part 2 # zip pairs items from iterables # zip pairs items, truncates at shortest >>> list(zip([−2, −1, 0, 1, 2])) [(−2,), (−1,), (0,), (1,), (2,)] # Single sequence: 1-ary tuples >>> list(zip([1, 2, 3], [2, 3, 4, 5])) [(1, 2), (2, 3), (3, 4)] # N sequences: N-ary tuples # map passes paired itenms to a function, truncates >>> list(map(abs, [−2, −1, 0, 1, 2])) [2, 1, 0, 1, 2] # Single sequence: 1-ary function >>> list(map(pow, [1, 2, 3], [2, 3, 4, 5])) [1, 8, 81] # N sequences: N-ary function Though they’re being used for different purposes, if you study these examples long enough, you might notice a relationship between zip results and mapped function arguments that our next example can exploit. Coding your own map(func, ...) Although the map and zip built-ins are fast and convenient, it’s always possible to emulate them in code of our own. In the preceding chapter, for example, we saw a function that emulated the map built-in for a single sequence argument. It doesn’t take much more work to allow for multiple sequences, as the built-in does: # map(func, seqs...) workalike with zip def mymap(func, *seqs): res = [] for args in zip(*seqs): res.append(func(*args)) return res print(mymap(abs, [−2, −1, 0, 1, 2])) print(mymap(pow, [1, 2, 3], [2, 3, 4, 5])) This version relies heavily upon the special *args argument-passing syntax—it collects multiple sequence (really, iterable) arguments, unpacks them as zip arguments to combine, and then unpacks the paired zip results as arguments to the passed-in function. That is, we’re using the fact that the zipping is essentially a nested operation in mapping. The test code at the bottom applies this to both one and two sequences to produce this output (the same we would get with the built-in map): [2, 1, 0, 1, 2] [1, 8, 81] Really, though, the prior version exhibits the classic list comprehension pattern, building a list of operation results within a for loop. We can code our map more concisely as an equivalent one-line list comprehension: Iterators Revisited: Generators | 501 # Using a list comprehension def mymap(func, *seqs): return [func(*args) for args in zip(*seqs)] print(mymap(abs, [−2, −1, 0, 1, 2])) print(mymap(pow, [1, 2, 3], [2, 3, 4, 5])) When this is run the result is the same as before, but the code is more concise and might run faster (more on performance in the section “Timing Iteration Alternatives” on page 509). Both of the preceding mymap versions build result lists all at once, though, and this can waste memory for larger lists. Now that we know about generator functions and expressions, it’s simple to recode both these alternatives to produce results on demand instead: # Using generators: yield and (...) def mymap(func, *seqs): res = [] for args in zip(*seqs): yield func(*args) def mymap(func, *seqs): return (func(*args) for args in zip(*seqs)) These versions produce the same results but return generators designed to support the iteration protocol—the first yields one result at a time, and the second returns a generator expression’s result to do the same. They produce the same results if we wrap them in list calls to force them to produce their values all at once: print(list(mymap(abs, [−2, −1, 0, 1, 2]))) print(list(mymap(pow, [1, 2, 3], [2, 3, 4, 5]))) No work is really done here until the list calls force the generators to run, by activating the iteration protocol. The generators returned by these functions themselves, as well as that returned by the Python 3.0 flavor of the zip built-in they use, produce results only on demand. Coding your own zip(...) and map(None, ...) Of course, much of the magic in the examples shown so far lies in their use of the zip built-in to pair arguments from multiple sequences. You’ll also note that our map workalikes are really emulating the behavior of the Python 3.0 map—they truncate at the length of the shortest sequence, and they do not support the notion of padding results when lengths differ, as map does in Python 2.X with a None argument: C:\misc> c:\python26\python >>> map(None, [1, 2, 3], [2, 3, 4, 5]) [(1, 2), (2, 3), (3, 4), (None, 5)] >>> map(None, 'abc', 'xyz123') [('a', 'x'), ('b', 'y'), ('c', 'z'), (None, '1'), (None, '2'), (None, '3')] 502 | Chapter 20: Iterations and Comprehensions, Part 2 Using iteration tools, we can code workalikes that emulate both truncating zip and 2.6’s padding map—these turn out to be nearly the same in code: # zip(seqs...) and 2.6 map(None, seqs...) workalikes def myzip(*seqs): seqs = [list(S) for S in seqs] res = [] while all(seqs): res.append(tuple(S.pop(0) for S in seqs)) return res def mymapPad(*seqs, pad=None): seqs = [list(S) for S in seqs] res = [] while any(seqs): res.append(tuple((S.pop(0) if S else pad) for S in seqs)) return res S1, S2 = 'abc', 'xyz123' print(myzip(S1, S2)) print(mymapPad(S1, S2)) print(mymapPad(S1, S2, pad=99)) Both of the functions coded here work on any type of iterable object, because they run their arguments through the list built-in to force result generation (e.g., files would work as arguments, in addition to sequences like strings). Notice the use of the all and any built-ins here—these return True if all and any items in an iterable are True (or equivalently, nonempty), respectively. These built-ins are used to stop looping when any or all of the listified arguments become empty after deletions. Also note the use of the Python 3.0 keyword-only argument, pad; unlike the 2.6 map, our version will allow any pad object to be specified (if you’re using 2.6, use a **kargs form to support this option instead; see Chapter 18 for details). When these functions are run, the following results are printed—a zip, and two padding maps: [('a', 'x'), ('b', 'y'), ('c', 'z')] [('a', 'x'), ('b', 'y'), ('c', 'z'), (None, '1'), (None, '2'), (None, '3')] [('a', 'x'), ('b', 'y'), ('c', 'z'), (99, '1'), (99, '2'), (99, '3')] These functions aren’t amenable to list comprehension translation because their loops are too specific. As before, though, while our zip and map workalikes currently build and return result lists, it’s just as easy to turn them into generators with yield so that they each return one piece of their result set at a time. The results are the same as before, but we need to use list again to force the generators to yield their values for display: # Using generators: yield def myzip(*seqs): seqs = [list(S) for S in seqs] while all(seqs): yield tuple(S.pop(0) for S in seqs) Iterators Revisited: Generators | 503 def mymapPad(*seqs, pad=None): seqs = [list(S) for S in seqs] while any(seqs): yield tuple((S.pop(0) if S else pad) for S in seqs) S1, S2 = 'abc', 'xyz123' print(list(myzip(S1, S2))) print(list(mymapPad(S1, S2))) print(list(mymapPad(S1, S2, pad=99))) Finally, here’s an alternative implementation of our zip and map emulators—rather than deleting arguments from lists with the pop method, the following versions do their job by calculating the minimum and maximum argument lengths. Armed with these lengths, it’s easy to code nested list comprehensions to step through argument index ranges: # Alternate implementation with lengths def myzip(*seqs): minlen = min(len(S) for S in seqs) return [tuple(S[i] for S in seqs) for i in range(minlen)] def mymapPad(*seqs, pad=None): maxlen = max(len(S) for S in seqs) index = range(maxlen) return [tuple((S[i] if len(S) > i else pad) for S in seqs) for i in index] S1, S2 = 'abc', 'xyz123' print(myzip(S1, S2)) print(mymapPad(S1, S2)) print(mymapPad(S1, S2, pad=99)) Because these use len and indexing, they assume that arguments are sequences or similar, not arbitrary iterables. The outer comprehensions here step through argument index ranges, and the inner comprehensions (passed to tuple) step through the passedin sequences to pull out arguments in parallel. When they’re run, the results are as before. Most strikingly, generators and iterators seem to run rampant in this example. The arguments passed to min and max are generator expressions, which run to completion before the nested comprehensions begin iterating. Moreover, the nested list comprehensions employ two levels of delayed evaluation—the Python 3.0 range built-in is an iterable, as is the generator expression argument to tuple. In fact, no results are produced here until the square brackets of the list comprehensions request values to place in the result list—they force the comprehensions and generators to run. To turn these functions themselves into generators instead of list builders, use parentheses instead of square brackets again. Here’s the case for our zip: # Using generators: (...) def myzip(*seqs): minlen = min(len(S) for S in seqs) 504 | Chapter 20: Iterations and Comprehensions, Part 2 return (tuple(S[i] for S in seqs) for i in range(minlen)) print(list(myzip(S1, S2))) In this case, it takes a list call to activate the generators and iterators to produce their results. Experiment with these on your own for more details. Developing further coding alternatives is left as a suggested exercise (see also the sidebar “Why You Will Care: One-Shot Iterations” for investigation of one such option). Why You Will Care: One-Shot Iterations In Chapter 14, we saw how some built-ins (like map) support only a single traversal and are empty after it occurs, and I promised to show you an example of how that can become subtle but important in practice. Now that we’ve studied a few more iteration topics, I can make good on this promise. Consider the following clever alternative coding for this chapter’s zip emulation examples, adapted from one in Python’s manuals: def myzip(*args): iters = map(iter, args) while iters: res = [next(i) for i in iters] yield tuple(res) Because this code uses iter and next, it works on any type of iterable. Note that there is no reason to catch the StopIteration raised by the next(it) inside the comprehension here when any one of the arguments’ iterators is exhausted—allowing it to pass ends this generator function and has the same effect that a return statement would. The while iters: suffices to loop if at least one argument is passed, and avoids an infinite loop otherwise (the list comprehension would always return an empty list). This code works fine in Python 2.6 as is: >>> list(myzip('abc', 'lmnop')) [('a', 'l'), ('b', 'm'), ('c', 'n')] But it falls into an infinite loop and fails in Python 3.0, because the 3.0 map returns a one-shot iterable object instead of a list as in 2.6. In 3.0, as soon as we’ve run the list comprehension inside the loop once, iters will be empty (and res will be []) forever. To make this work in 3.0, we need to use the list built-in function to create an object that can support multiple iterations: def myzip(*args): iters = list(map(iter, args)) ...rest as is... Run this on your own to trace its operation. The lesson here: wrapping map calls in list calls in 3.0 is not just for display! Iterators Revisited: Generators | 505 Value Generation in Built-in Types and Classes Finally, although we’ve focused on coding value generators ourselves in this section, don’t forget that many built-in types behave in similar ways—as we saw in Chapter 14, for example, dictionaries have iterators that produce keys on each iteration: >>> >>> >>> 'a' >>> 'c' D = {'a':1, 'b':2, 'c':3} x = iter(D) next(x) next(x) Like the values produced by handcoded generators, dictionary keys may be iterated over both manually and with automatic iteration tools including for loops, map calls, list comprehensions, and the many other contexts we met in Chapter 14: >>> for key in D: ... print(key, D[key]) ... a 1 c 3 b 2 As we’ve also seen, for file iterators, Python simply loads lines from the file on demand: >>> for line in open('temp.txt'): ... print(line, end='') ... Tis but a flesh wound. While built-in type iterators are bound to a specific type of value generation, the concept is similar to generators we code with expressions and functions. Iteration contexts like for loops accept any iterable, whether user-defined or built-in. Although beyond the scope of this chapter, it is also possible to implement arbitrary user-defined generator objects with classes that conform to the iteration protocol. Such classes define a special __iter__ method run by the iter built-in function that returns an object having a __next__ method run by the next built-in function (a __getitem__ indexing method is also available as a fallback option for iteration). The instance objects created from such a class are considered iterable and may be used in for loops and all other iteration contexts. With classes, though, we have access to richer logic and data structuring options than other generator constructs can offer. The iterator story won’t really be complete until we’ve seen how it maps to classes, too. For now, we’ll have to settle for postponing its conclusion until we study class-based iterators in Chapter 29. 506 | Chapter 20: Iterations and Comprehensions, Part 2 3.0 Comprehension Syntax Summary We’ve been focusing on list comprehensions and generators in this chapter, but keep in mind that there are two other comprehension expression forms: set and dictionary comprehensions are also available as of Python 3.0. We met these briefly in Chapters 5 and 8, but with our new knowledge of comprehensions and generators, you should now be able to grasp these 3.0 extensions in full: • For sets, the new literal form {1, 3, 2} is equivalent to set([1, 3, 2]), and the new set comprehension syntax {f(x) for x in S if P(x)} is like the generator expression set(f(x) for x in S if P(x)), where f(x) is an arbitrary expression. • For dictionaries, the new dictionary comprehension syntax {key: val for (key, val) in zip(keys, vals)} works like the form dict(zip(keys, vals)), and {x: f(x) for x in items} is like the generator expression dict((x, f(x)) for x in items). Here’s a summary of all the comprehension alternatives in 3.0. The last two are new and are not available in 2.6: >>> [x * x for x in range(10)] [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] # List comprehension: builds list # like list(generator expr) >>> (x * x for x in range(10)) <generator object at 0x009E7328> # Generator expression: produces items # Parens are often optional >>> {x * x for x in range(10)} {0, 1, 4, 81, 64, 9, 16, 49, 25, 36} # Set comprehension, new in 3.0 # {x, y} is a set in 3.0 too >>> {x: x * x for x in range(10)} # Dictionary comprehension, new in 3.0 {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81} Comprehending Set and Dictionary Comprehensions In a sense, set and dictionary comprehensions are just syntactic sugar for passing generator expressions to the type names. Because both accept any iterable, a generator works well here: # Comprehension >>> {0, >>> {0, {x * x for x in range(10)} 1, 4, 81, 64, 9, 16, 49, 25, 36} set(x * x for x in range(10)) 1, 4, 81, 64, 9, 16, 49, 25, 36} >>> {0: >>> {0: {x: x * x for x in range(10)} 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81} dict((x, x * x) for x in range(10)) 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81} # Generator and type name As for list comprehensions, though, we can always build the result objects with manual code, too. Here are statement-based equivalents of the last two comprehensions: 3.0 Comprehension Syntax Summary | 507 >>> >>> ... ... >>> {0, res = set() for x in range(10): res.add(x * x) >>> >>> ... ... >>> {0: res = {} for x in range(10): res[x] = x * x # Set comprehension equivalent res 1, 4, 81, 64, 9, 16, 49, 25, 36} # Dict comprehension equivalent res 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81} Notice that although both forms accept iterators, they have no notion of generating results on demand—both forms build objects all at once. If you mean to produce keys and values upon request, a generator expression is more appropriate: >>> >>> (0, >>> (1, G = ((x, x * x) for x in range(10)) next(G) 0) next(G) 1) Extended Comprehension Syntax for Sets and Dictionaries Like list comprehensions and generator expressions, both set and dictionary comprehensions support nested associated if clauses to filter items out of the result—the following collect squares of even items (i.e., items having no remainder for division by 2) in a range: >>> [0, >>> {0, >>> {0: [x * x for x in 4, 16, 36, 64] {x * x for x in 16, 4, 64, 36} {x: x * x for x 0, 8: 64, 2: 4, range(10) if x % 2 == 0] # Lists are ordered range(10) if x % 2 == 0} # But sets are not in range(10) if x % 2 == 0} 4: 16, 6: 36} # Neither are dict keys Nested for loops work as well, though the unordered and no-duplicates nature of both types of objects can make the results a bit less straightforward to decipher: >>> [5, >>> {8, >>> {1: [x + y for x in [1, 2, 3] for y in [4, 5, 6]] 6, 7, 6, 7, 8, 7, 8, 9] {x + y for x in [1, 2, 3] for y in [4, 5, 6]} 9, 5, 6, 7} {x: y for x in [1, 2, 3] for y in [4, 5, 6]} 6, 2: 6, 3: 6} # Lists keep duplicates # But sets do not # Neither do dict keys Like list comprehensions, the set and dictionary varieties can also iterate over any type of iterator—lists, strings, files, ranges, and anything else that supports the iteration protocol: >>> {x + y for x in 'ab' for y in 'cd'} {'bd', 'ac', 'ad', 'bc'} 508 | Chapter 20: Iterations and Comprehensions, Part 2 >>> {x + y: (ord(x), ord(y)) for x in 'ab' for y in 'cd'} {'bd': (98, 100), 'ac': (97, 99), 'ad': (97, 100), 'bc': (98, 99)} >>> {k * 2 for k in ['spam', 'ham', 'sausage'] if k[0] == 's'} {'sausagesausage', 'spamspam'} >>> {k.upper(): k * 2 for k in ['spam', 'ham', 'sausage'] if k[0] == 's'} {'SAUSAGE': 'sausagesausage', 'SPAM': 'spamspam'} For more details, experiment with these tools on your own. They may or may not have a performance advantage over the generator or for loop alternatives, but we would have to time their performance explicitly to be sure—which seems a natural segue to the next section. Timing Iteration Alternatives We’ve met quite a few iteration alternatives in this book. To summarize, let’s work through a larger case study that pulls together some of the things we’ve learned about iteration and functions. I’ve mentioned a few times that list comprehensions have a speed performance advantage over for loop statements, and that map performance can be better or worse depending on call patterns. The generator expressions of the prior sections tend to be slightly slower than list comprehensions, though they minimize memory space requirements. All that’s true today, but relative performance can vary over time because Python’s internals are constantly being changed and optimized. If you want to verify their performance for yourself, you need to time these alternatives on your own computer and your own version of Python. Timing Module Luckily, Python makes it easy to time code. To see how the iteration options stack up, let’s start with a simple but general timer utility function coded in a module file, so it can be used in a variety of programs: # File mytimer.py import time reps = 1000 repslist = range(reps) def timer(func, *pargs, **kargs): start = time.clock() for i in repslist: ret = func(*pargs, **kargs) elapsed = time.clock() - start return (elapsed, ret) Timing Iteration Alternatives | 509 Operationally, this module times calls to any function with any positional and keyword arguments by fetching the start time, calling the function a fixed number of times, and subtracting the start time from the stop time. Points to notice: • Python’s time module gives access to the current time, with precision that varies per platform. On Windows, this call is claimed to give microsecond granularity and so is very accurate. • The range call is hoisted out of the timing loop, so its construction cost is not charged to the timed function in Python 2.6. In 3.0 range is an iterator, so this step isn’t required (but doesn’t hurt). • The reps count is a global that importers can change if needed: mytimer.reps = N. When complete, the total elapsed time for all calls is returned in a tuple, along with the timed function’s final return value so callers can verify its operation. From a larger perspective, because this function is coded in a module file, it becomes a generally useful tool anywhere we wish to import it. You’ll learn more about modules and imports in the next part of this book, but you’ve already seen enough of the basics to make sense of this code—simply import the module and call the function to use this file’s timer (and see Chapter 3’s coverage of module attributes if you need a refresher). Timing Script Now, to time iteration tool speed, run the following script—it uses the timer module we just wrote to time the relative speeds of the various list construction techniques we’ve studied: # File timeseqs.py import sys, mytimer reps = 10000 repslist = range(reps) # Import timer function # Hoist range out in 2.6 def forLoop(): res = [] for x in repslist: res.append(abs(x)) return res def listComp(): return [abs(x) for x in repslist] def mapCall(): return list(map(abs, repslist)) # Use list in 3.0 only def genExpr(): return list(abs(x) for x in repslist) # list forces results def genFunc(): def gen(): 510 | Chapter 20: Iterations and Comprehensions, Part 2 for x in repslist: yield abs(x) return list(gen()) print(sys.version) for test in (forLoop, listComp, mapCall, genExpr, genFunc): elapsed, result = mytimer.timer(test) print ('-' * 33) print ('%-9s: %.5f => [%s...%s]' % (test.__name__, elapsed, result[0], result[-1])) This script tests five alternative ways to build lists of results and, as shown, executes on the order of 10 million steps for each—that is, each of the five tests builds a list of 10,000 items 1,000 times. Notice how we have to run the generator expression and function results through the built-in list call to force them to yield all of their values; if we did not, we would just produce generators that never do any real work. In Python 3.0 (only) we must do the same for the map result, since it is now an iterable object as well. Also notice how the code at the bottom steps through a tuple of four function objects and prints the __name__ of each: as we’ve seen, this is a built-in attribute that gives a function’s name. Timing Results When the script of the prior section is run under Python 3.0, I get the following results on my Windows Vista laptop—map is slightly faster than list comprehensions, both are substantially quicker than for loops, and generator expressions and functions place in the middle: C:\misc> c:\python30\python timeseqs.py 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] --------------------------------forLoop : 2.64441 => [0...9999] --------------------------------listComp : 1.60110 => [0...9999] --------------------------------mapCall : 1.41977 => [0...9999] --------------------------------genExpr : 2.21758 => [0...9999] --------------------------------genFunc : 2.18696 => [0...9999] If you study this code and its output long enough, you’ll notice that generator expressions run slower than list comprehensions. Although wrapping a generator expression in a list call makes it functionally equivalent to a square-bracketed list comprehension, the internal implementations of the two expressions appear to differ (though we’re also effectively timing the list call for the generator test): return [abs(x) for x in range(size)] return list(abs(x) for x in range(size)) # 1.6 seconds # 2.2 seconds: differs internally Timing Iteration Alternatives | 511 Interestingly, when I ran this on Windows XP with Python 2.5 for the prior edition of this book, the results were relatively similar—list comprehensions were nearly twice as fast as equivalent for loop statements, and map was slightly quicker than list comprehensions when mapping a built-in function such as abs (absolute value). I didn’t test generator functions then, and the output format wasn’t quite as grandiose: 2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit (Intel)] forStatement => 6.10899996758 listComprehension => 3.51499986649 mapFunction => 2.73399996758 generatorExpression => 4.11600017548 The fact that the actual 2.5 test times listed here are over two times as slow as the output I showed earlier is likely due to my using a quicker laptop for the more recent test, not due to improvements in Python 3.0. In fact, all the 2.6 results for this script are slightly quicker than 3.0 on this same machine if the list call is removed from the map test to avoid creating the results list twice (try this on your own to verify). Watch what happens, though, if we change this script to perform a real operation on each iteration, such as addition, instead of calling a trivial built-in function like abs (the omitted parts of the following are the same as before): # File timeseqs.py ... ... def forLoop(): res = [] for x in repslist: res.append(x + 10) return res def listComp(): return [x + 10 for x in repslist] def mapCall(): return list(map((lambda x: x + 10), repslist)) # list in 3.0 only def genExpr(): return list(x + 10 for x in repslist) # list in 2.6 + 3.0 def genFunc(): def gen(): for x in repslist: yield x + 10 return list(gen()) ... ... Now the need to call a user-defined function for the map call makes it slower than the for loop statements, despite the fact that the looping statements version is larger in terms of code. On Python 3.0: C:\misc> c:\python30\python timeseqs.py 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] 512 | Chapter 20: Iterations and Comprehensions, Part 2 --------------------------------forLoop : 2.60754 => [10...10009] --------------------------------listComp : 1.57585 => [10...10009] --------------------------------mapCall : 3.10276 => [10...10009] --------------------------------genExpr : 1.96482 => [10...10009] --------------------------------genFunc : 1.95340 => [10...10009] The Python 2.5 results on a slower machine were again relatively similar in the prior edition, but twice as slow due to test machine differences: 2.5 (r25:51908, Sep 19 2006, 09:52:17) [MSC v.1310 32 bit (Intel)] forStatement => 5.25699996948 listComprehension => 2.68400001526 mapFunction => 5.96900010109 generatorExpression => 3.37400007248 Because the interpreter optimizes so much internally, performance analysis of Python code like this is a very tricky affair. It’s virtually impossible to guess which method will perform the best—the best you can do is time your own code, on your computer, with your version of Python. In this case, all we should say for certain is that on this Python, using a user-defined function in map calls can slow performance by at least a factor of 2, and that list comprehensions run quickest for this test. As I’ve mentioned before, however, performance should not be your primary concern when writing Python code—the first thing you should do to optimize Python code is to not optimize Python code! Write for readability and simplicity first, then optimize later, if and only if needed. It could very well be that any of the five alternatives is quick enough for the data sets your program needs to process; if so, program clarity should be the chief goal. Timing Module Alternatives The timing module of the prior section works, but it’s a bit primitive on multiple fronts: • It always uses the time.clock call to time code. While that option is best on Windows, the time.time call may provide better resolution on some Unix platforms. • Adjusting the number of repetitions requires changing module-level globals—a less than ideal arrangement if the timer function is being used and shared by multiple importers. • As is, the timer works by running the test function a large number of times. To account for random system load fluctuations, it might be better to select the best time among all the tests, instead of the total time. The following alternative implements a more sophisticated timer module that addresses all three points by selecting a timer call based on platform, allowing the repeat count Timing Iteration Alternatives | 513 to be passed in as a keyword argument named _reps, and providing a best-of-N alternative timing function: # File mytimer.py (2.6 and 3.0) """ timer(spam, 1, 2, a=3, b=4, _reps=1000) calls and times spam(1, 2, a=3) _reps times, and returns total time for all runs, with final result; best(spam, 1, 2, a=3, b=4, _reps=50) runs best-of-N timer to filter out any system load variation, and returns best time among _reps tests """ import time, sys if sys.platform[:3] == 'win': timefunc = time.clock else: timefunc = time.time # Use time.clock on Windows def trace(*args): pass # Or: print args def timer(func, *pargs, **kargs): _reps = kargs.pop('_reps', 1000) trace(func, pargs, kargs, _reps) repslist = range(_reps) start = timefunc() for i in repslist: ret = func(*pargs, **kargs) elapsed = timefunc() - start return (elapsed, ret) # Better resolution on some Unix platforms # Passed-in or default reps # Hoist range out for 2.6 lists def best(func, *pargs, **kargs): _reps = kargs.pop('_reps', 50) best = 2 ** 32 for i in range(_reps): (time, ret) = timer(func, *pargs, _reps=1, **kargs) if time < best: best = time return (best, ret) This module’s docstring at the top of the file describes its intended usage. It uses dictionary pop operations to remove the _reps argument from arguments intended for the test function and provide it with a default, and it traces arguments during development if you change its trace function to print. To test with this new timer module on either Python 3.0 or 2.6, change the timing script as follows (the omitted code in the test functions of this version use the x + 1 operation for each test, as coded in the prior section): # File timeseqs.py import sys, mytimer reps = 10000 repslist = range(reps) def forLoop(): ... 514 | Chapter 20: Iterations and Comprehensions, Part 2 def listComp(): ... def mapCall(): ... def genExpr(): ... def genFunc(): ... print(sys.version) for tester in (mytimer.timer, mytimer.best): print('<%s>' % tester.__name__) for test in (forLoop, listComp, mapCall, genExpr, genFunc): elapsed, result = tester(test) print ('-' * 35) print ('%-9s: %.5f => [%s...%s]' % (test.__name__, elapsed, result[0], result[-1])) When run under Python 3.0, the timing results are essentially the same as before, and relatively the same for both to the total-of-N and best-of-N timing techniques—running tests many times seems to do as good a job filtering out system load fluctuations as taking the best case, but the best-of-N scheme may be better when testing a longrunning function. The results on my machine are as follows: C:\misc> c:\python30\python timeseqs.py 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] <timer> ----------------------------------forLoop : 2.35371 => [10...10009] ----------------------------------listComp : 1.29640 => [10...10009] ----------------------------------mapCall : 3.16556 => [10...10009] ----------------------------------genExpr : 1.97440 => [10...10009] ----------------------------------genFunc : 1.95072 => [10...10009] <best> ----------------------------------forLoop : 0.00193 => [10...10009] ----------------------------------listComp : 0.00124 => [10...10009] ----------------------------------mapCall : 0.00268 => [10...10009] ----------------------------------genExpr : 0.00164 => [10...10009] ----------------------------------genFunc : 0.00165 => [10...10009] The times reported by the best-of-N timer here are small, of course, but they might become significant if your program iterates many times over large data sets. At least in terms of relative performance, list comprehensions appear best in most cases; map is only slightly better when built-ins are applied. Timing Iteration Alternatives | 515 Using keyword-only arguments in 3.0 We can also make use of Python 3.0 keyword-only arguments here to simplify the timer module’s code. As we learned in Chapter 19, keyword-only arguments are ideal for configuration options such as our functions’ _reps argument. They must be coded after a * and before a ** in the function header, and in a function call they must be passed by keyword and appear before the ** if used. Here’s a keyword-only-based alternative to the prior module. Though simpler, it compiles and runs under Python 3.X only, not 2.6: # File mytimer.py (3.X only) """ Use 3.0 keyword-only default arguments, instead of ** and dict pops. No need to hoist range() out of test in 3.0: a generator, not a list """ import time, sys trace = lambda *args: None # or print timefunc = time.clock if sys.platform == 'win32' else time.time def timer(func, *pargs, _reps=1000, **kargs): trace(func, pargs, kargs, _reps) start = timefunc() for i in range(_reps): ret = func(*pargs, **kargs) elapsed = timefunc() - start return (elapsed, ret) def best(func, *pargs, _reps=50, **kargs): best = 2 ** 32 for i in range(_reps): (time, ret) = timer(func, *pargs, _reps=1, **kargs) if time < best: best = time return (best, ret) This version is used the same way as and produces results identical to the prior version, not counting negligible test time differences from run to run: C:\misc> c:\python30\python timeseqs.py ...same results as before... In fact, for variety we can also test this version of the module from the interactive prompt, completely independent of the sequence timer script—it’s a general-purpose tool: C:\misc> c:\python30\python >>> from mytimer import timer, best >>> >>> def power(X, Y): return X ** Y ... >>> timer(power, 2, 32) (0.002625403507987747, 4294967296) >>> timer(power, 2, 32, _reps=1000000) 516 | Chapter 20: Iterations and Comprehensions, Part 2 # Test function # Total time, last result # Override defult reps (1.1822605247314932, 4294967296) >>> timer(power, 2, 100000)[0] 2.2496919999608878 >>> best(power, 2, 32) (5.58730229727189e-06, 4294967296) >>> best(power, 2, 100000)[0] 0.0019937589833460834 >>> best(power, 2, 100000, _reps=500)[0] 0.0019845399345541637 # 2 ** 100,000 tot time @1,000 reps # Best time, last result # 2 ** 100,000 best time # Override default reps For trivial functions like the one tested in this interactive session, the costs of the timer’s code are probably as significant as those of the timed function, so you should not take timer results too absolutely (we are timing more than just X ** Y here). The timer’s results can help you judge relative speeds of coding alternatives, though, and may be more meaningful for longer-running operations like the following—calculating 2 to the power one million takes an order of magnitude (power of 10) longer than the preceding 2**100,000: >>> timer(power, 2, 1000000, _reps=1)[0] 0.088112804839710179 >>> timer(power, 2, 1000000, _reps=10)[0] 0.40922470593329763 # 2 ** 1,000,000: total time >>> best(power, 2, 1000000, _reps=1)[0] 0.086550036387279761 >>> best(power, 2, 1000000, _reps=10)[0] 0.029616752967200455 >>> best(power, 2, 1000000, _reps=50)[0] 0.029486918030102061 # 2 ** 1,000,000: best time # 10 is sometimes as good as 50 # Best resolution Again, although the times measured here are small, the differences can be significant in programs that compute powers often. See Chapter 19 for more on keyword-only arguments in 3.0; they can simplify code for configurable tools like this one but are not backward compatible with 2.X Pythons. If you want to compare 2.X and 3.X speed, for example, or support programmers using either Python line, the prior version is likely a better choice. If you’re using Python 2.6, the above session runs the same with the prior version of the timer module. Other Suggestions For more insight, try modifying the repetition counts used by these modules, or explore the alternative timeit module in Python’s standard library, which automates timing of code, supports command-line usage modes, and finesses some platform-specific issues. Python’s manuals document its use. You might also want to look at the profile standard library module for a complete source code profiler tool—we’ll learn more about it in Chapter 35 in the context of development tools for large projects. In general, you should profile code to isolate bottlenecks before recoding and timing alternatives as we’ve done here. Timing Iteration Alternatives | 517 It might be useful as well to experiment with using the new str.format method in Python 2.6 and 3.0 instead of the % formatting expression (which could potentially be deprecated in the future!), by changing the timing script’s formatted print lines as follows: print('<%s>' % tester.__name__) print('<{0}>'.format(tester.__name__)) # From expression # To method call print ('%-9s: %.5f => [%s...%s]' % (test.__name__, elapsed, result[0], result[-1])) print('{0:<9}: {1:.5f} => [{2}...{3}]'.format( test.__name__, elapsed, result[0], result[-1])) You can judge the difference between these techniques yourself. If you feel ambitious, you might also try modifying or emulating the timing script to measure the speed of the 3.0 set and dictionary comprehensions illustrated in this chapter, and their for loop equivalents. Since using them is much less common in Python programs than building lists of results, we’ll leave this task in the suggested exercise column (and please, no wagering...). Finally, keep the timing module we wrote here filed away for future reference—we’ll repurpose it to measure performance of alternative numeric square root operations in an exercise at the end of this chapter. If you’re interested in pursuing this topic further, we’ll also experiment with techniques for timing dictionary comprehensions versus for loops interactively. Function Gotchas Now that we’ve reached the end of the function story, let’s review some common pitfalls. Functions have some jagged edges that you might not expect. They’re all obscure, and a few have started to fall away from the language completely in recent releases, but most have been known to trip up new users. Local Names Are Detected Statically As you know, Python classifies names assigned in a function as locals by default; they live in the function’s scope and exist only while the function is running. What you may not realize is that Python detects locals statically, when it compiles the def’s code, rather than by noticing assignments as they happen at runtime. This leads to one of the most common oddities posted on the Python newsgroup by beginners. Normally, a name that isn’t assigned in a function is looked up in the enclosing module: 518 | Chapter 20: Iterations and Comprehensions, Part 2 >>> X = 99 >>> def selector(): ... print(X) ... >>> selector() 99 # X used but not assigned # X found in global scope Here, the X in the function resolves to the X in the module. But watch what happens if you add an assignment to X after the reference: >>> def selector(): ... print(X) # Does not yet exist! ... X = 88 # X classified as a local name (everywhere) ... # Can also happen for "import X", "def X"... >>> selector() ...error text omitted... UnboundLocalError: local variable 'X' referenced before assignment You get the name usage error shown here, but the reason is subtle. Python reads and compiles this code when it’s typed interactively or imported from a module. While compiling, Python sees the assignment to X and decides that X will be a local name everywhere in the function. But when the function is actually run, because the assignment hasn’t yet happened when the print executes, Python says you’re using an undefined name. According to its name rules, it should say this; the local X is used before being assigned. In fact, any assignment in a function body makes a name local. Imports, =, nested defs, nested classes, and so on are all susceptible to this behavior. The problem occurs because assigned names are treated as locals everywhere in a function, not just after the statements where they are assigned. Really, the previous example is ambiguous at best: was the intention to print the global X and then create a local X, or is this a genuine programming error? Because Python treats X as a local everywhere, it is viewed as an error; if you really mean to print the global X, you need to declare it in a global statement: >>> def selector(): ... global X ... print(X) ... X = 88 ... >>> selector() 99 # Force X to be global (everywhere) Remember, though, that this means the assignment also changes the global X, not a local X. Within a function, you can’t use both local and global versions of the same simple name. If you really meant to print the global and then set a local of the same name, you’d need to import the enclosing module and use module attribute notation to get to the global version: >>> X = >>> def ... ... ... 99 selector(): import __main__ print(__main__.X) X = 88 # Import enclosing module # Qualify to get to global version of name # Unqualified X classified as local Function Gotchas | 519 ... print(X) ... >>> selector() 99 88 # Prints local version of name Qualification (the .X part) fetches a value from a namespace object. The interactive namespace is a module called __main__, so __main__.X reaches the global version of X. If that isn’t clear, check out Chapter 17. In recent versions Python has improved on this story somewhat by issuing for this case the more specific “unbound local” error message shown in the example listing (it used to simply raise a generic name error); this gotcha is still present in general, though. Defaults and Mutable Objects Default argument values are evaluated and saved when a def statement is run, not when the resulting function is called. Internally, Python saves one object per default argument attached to the function itself. That’s usually what you want—because defaults are evaluated at def time, it lets you save values from the enclosing scope, if needed. But because a default retains an object between calls, you have to be careful about changing mutable defaults. For instance, the following function uses an empty list as a default value, and then changes it in-place each time the function is called: >>> ... ... ... >>> [2, >>> [1] >>> [1, >>> [1, def saver(x=[]): x.append(1) print(x) # Saves away a list object # Changes same object each time! saver([2]) 1] saver() # Default not used saver() 1] saver() 1, 1] # Grows on each call! # Default used Some see this behavior as a feature—because mutable default arguments retain their state between function calls, they can serve some of the same roles as static local function variables in the C language. In a sense, they work sort of like global variables, but their names are local to the functions and so will not clash with names elsewhere in a program. To most observers, though, this seems like a gotcha, especially the first time they run into it. There are better ways to retain state between calls in Python (e.g., using classes, which will be discussed in Part VI). Moreover, mutable defaults are tricky to remember (and to understand at all). They depend upon the timing of default object construction. In the prior example, there is 520 | Chapter 20: Iterations and Comprehensions, Part 2 just one list object for the default value—the one created when the def is executed. You don’t get a new list every time the function is called, so the list grows with each new append; it is not reset to empty on each call. If that’s not the behavior you want, simply make a copy of the default at the start of the function body, or move the default value expression into the function body. As long as the value resides in code that’s actually executed each time the function runs, you’ll get a new object each time through: >>> ... ... ... ... ... >>> [2, >>> [1] >>> [1] def saver(x=None): if x is None: x = [] x.append(1) print(x) saver([2]) 1] saver() # No argument passed? # Run code to make a new list # Changes new list object # Doesn't grow here saver() By the way, the if statement in this example could almost be replaced by the assignment x = x or [], which takes advantage of the fact that Python’s or returns one of its operand objects: if no argument was passed, x would default to None, so the or would return the new empty list on the right. However, this isn’t exactly the same. If an empty list were passed in, the or expression would cause the function to extend and return a newly created list, rather than extending and returning the passed-in list like the if version. (The expression becomes [] or [], which evaluates to the new empty list on the right; see the section “Truth Tests” on page 320 if you don’t recall why). Real program requirements may call for either behavior. Today, another way to achieve the effect of mutable defaults in a possibly less confusing way is to use the function attributes we discussed in Chapter 19: >>> ... ... ... >>> >>> [1] >>> [1, >>> [1, def saver(): saver.x.append(1) print(saver.x) saver.x = [] saver() saver() 1] saver() 1, 1] The function name is global to the function itself, but it need not be declared because it isn’t changed directly within the function. This isn’t used in exactly the same way, Function Gotchas | 521 but when coded like this, the attachment of an object to the function is much more explicit (and arguably less magical). Functions Without returns In Python functions, return (and yield) statements are optional. When a function doesn’t return a value explicitly, the function exits when control falls off the end of the function body. Technically, all functions return a value; if you don’t provide a return statement, your function returns the None object automatically: >>> def proc(x): ... print(x) ... >>> x = proc('testing 123...') testing 123... >>> print(x) None # No return is a None return Functions such as this without a return are Python’s equivalent of what are called “procedures” in some languages. They’re usually invoked as statements, and the None results are ignored, as they do their business without computing a useful result. This is worth knowing, because Python won’t tell you if you try to use the result of a function that doesn’t return one. For instance, assigning the result of a list append method won’t raise an error, but you’ll get back None, not the modified list: >>> list = [1, 2, 3] >>> list = list.append(4) >>> print(list) None # append is a "procedure" # append changes list in-place As mentioned in “Common Coding Gotchas” on page 387 in Chapter 15, such functions do their business as a side effect and are usually designed to be run as statements, not expressions. Enclosing Scope Loop Variables We described this gotcha in Chapter 17’s discussion of enclosing function scopes, but as a reminder, be careful about relying on enclosing function scope lookup for variables that are changed by enclosing loops—all such references will remember the value of the last loop iteration. Use defaults to save loop variable values instead (see Chapter 17 for more details on this topic). Chapter Summary This chapter wrapped up our coverage of built-in comprehension and iteration tools. It explored list comprehensions in the context of functional tools and presented generator functions and expressions as additional iteration protocol tools. As a finale, we 522 | Chapter 20: Iterations and Comprehensions, Part 2 also measured the performance of iteration alternatives, and we closed with a review of common function-related mistakes to help you avoid pitfalls. This concludes the functions part of this book. In the next part, we will study modules— the topmost organizational structure in Python, and the structure in which our functions always live. After that, we will explore classes, tools that are largely packages of functions with special first arguments. As we’ll see, user-defined classes can implement objects that tap into the iteration protocol, just like the generators and iterables we met here. Everything we have learned in this part of the book will apply when functions pop up later in the context of class methods. Before moving on to modules, though, be sure to work through this chapter’s quiz and the exercises for this part of the book, to practice what we’ve learned about functions here. Test Your Knowledge: Quiz 1. What is the difference between enclosing a list comprehension in square brackets and parentheses? 2. How are generators and iterators related? 3. How can you tell if a function is a generator function? 4. What does a yield statement do? 5. How are map calls and list comprehensions related? Compare and contrast the two. Test Your Knowledge: Answers 1. List comprehensions in square brackets produce the result list all at once in memory. When they are enclosed in parentheses instead, they are actually generator expressions—they have a similar meaning but do not produce the result list all at once. Instead, generator expressions return a generator object, which yields one item in the result at a time when used in an iteration context. 2. Generators are objects that support the iteration protocol—they have a __next__ method that repeatedly advances to the next item in a series of results and raises an exception at the end of the series. In Python, we can code generator functions with def, generator expressions with parenthesized list comprehensions, and generator objects with classes that define a special method named __iter__ (discussed later in the book). 3. A generator function has a yield statement somewhere in its code. Generator functions are otherwise identical to normal functions syntactically, but they are compiled specially by Python so as to return an iterable object when called. Test Your Knowledge: Answers | 523 4. When present, this statement makes Python compile the function specially as a generator; when called, the function returns a generator object that supports the iteration protocol. When the yield statement is run, it sends a result back to the caller and suspends the function’s state; the function can then be resumed after the last yield statement, in response to a next built-in or __next__ method call issued by the caller. Generator functions may also have a return statement, which terminates the generator. 5. The map call is similar to a list comprehension—both build a new list by collecting the results of applying an operation to each item in a sequence or other iterable, one item at a time. The main difference is that map applies a function call to each item, and list comprehensions apply arbitrary expressions. Because of this, list comprehensions are more general; they can apply a function call expression like map, but map requires a function to apply other kinds of expressions. List comprehensions also support extended syntax such as nested for loops and if clauses that subsume the filter built-in. Test Your Knowledge: Part IV Exercises In these exercises, you’re going to start coding more sophisticated programs. Be sure to check the solutions in “Part IV, Functions” on page 1111 in Appendix B, and be sure to start writing your code in module files. You won’t want to retype these exercises from scratch if you make a mistake. 1. The basics. At the Python interactive prompt, write a function that prints its single argument to the screen and call it interactively, passing a variety of object types: string, integer, list, dictionary. Then, try calling it without passing any argument. What happens? What happens when you pass two arguments? 2. Arguments. Write a function called adder in a Python module file. The function should accept two arguments and return the sum (or concatenation) of the two. Then, add code at the bottom of the file to call the adder function with a variety of object types (two strings, two lists, two floating points), and run this file as a script from the system command line. Do you have to print the call statement results to see results on your screen? 3. varargs. Generalize the adder function you wrote in the last exercise to compute the sum of an arbitrary number of arguments, and change the calls to pass more or fewer than two arguments. What type is the return value sum? (Hints: a slice such as S[:0] returns an empty sequence of the same type as S, and the type builtin function can test types; but see the manually coded min examples in Chapter 18 for a simpler approach.) What happens if you pass in arguments of different types? What about passing in dictionaries? 524 | Chapter 20: Iterations and Comprehensions, Part 2 4. Keywords. Change the adder function from exercise 2 to accept and sum/concatenate three arguments: def adder(good, bad, ugly). Now, provide default values for each argument, and experiment with calling the function interactively. Try passing one, two, three, and four arguments. Then, try passing keyword arguments. Does the call adder(ugly=1, good=2) work? Why? Finally, generalize the new adder to accept and sum/concatenate an arbitrary number of keyword arguments. This is similar to what you did in exercise 3, but you’ll need to iterate over a dictionary, not a tuple. (Hint: the dict.keys method returns a list you can step through with a for or while, but be sure to wrap it in a list call to index it in 3.0!) 5. Write a function called copyDict(dict) that copies its dictionary argument. It should return a new dictionary containing all the items in its argument. Use the dictionary keys method to iterate (or, in Python 2.2, step over a dictionary’s keys without calling keys). Copying sequences is easy (X[:] makes a top-level copy); does this work for dictionaries, too? 6. Write a function called addDict(dict1, dict2) that computes the union of two dictionaries. It should return a new dictionary containing all the items in both its arguments (which are assumed to be dictionaries). If the same key appears in both arguments, feel free to pick a value from either. Test your function by writing it in a file and running the file as a script. What happens if you pass lists instead of dictionaries? How could you generalize your function to handle this case, too? (Hint: see the type built-in function used earlier.) Does the order of the arguments passed in matter? 7. More argument-matching examples. First, define the following six functions (either interactively or in a module file that can be imported): def f1(a, b): print(a, b) def f2(a, *b): print(a, b) # Normal args # Positional varargs def f3(a, **b): print(a, b) # Keyword varargs def f4(a, *b, **c): print(a, b, c) # Mixed modes def f5(a, b=2, c=3): print(a, b, c) # Defaults def f6(a, b=2, *c): print(a, b, c) # Defaults and positional varargs Now, test the following calls interactively, and try to explain each result; in some cases, you’ll probably need to fall back on the matching algorithm shown in Chapter 18. Do you think mixing matching modes is a good idea in general? Can you think of cases where it would be useful? >>> f1(1, 2) >>> f1(b=2, a=1) >>> f2(1, 2, 3) >>> f3(1, x=2, y=3) >>> f4(1, 2, 3, x=2, y=3) Test Your Knowledge: Part IV Exercises | 525 >>> f5(1) >>> f5(1, 4) >>> f6(1) >>> f6(1, 3, 4) 8. Primes revisited. Recall the following code snippet from Chapter 13, which simplistically determines whether a positive integer is prime: x = y // 2 while x > 1: if y % x == 0: print(y, 'has factor', x) break x -= 1 else: print(y, 'is prime') # For some y > 1 # Remainder # Skip else # Normal exit Package this code as a reusable function in a module file (y should be a passed-in argument), and add some calls to the function at the bottom of your file. While you’re at it, experiment with replacing the first line’s // operator with / to see how true division changes the / operator in Python 3.0 and breaks this code (refer back to Chapter 5 if you need a refresher). What can you do about negatives, and the values 0 and 1? How about speeding this up? Your outputs should look something like this: 13 is prime 13.0 is prime 15 has factor 5 15.0 has factor 5.0 9. List comprehensions. Write code to build a new list containing the square roots of all the numbers in this list: [2, 4, 9, 16, 25]. Code this as a for loop first, then as a map call, and finally as a list comprehension. Use the sqrt function in the builtin math module to do the calculation (i.e., import math and say math.sqrt(x)). Of the three, which approach do you like best? 10. Timing tools. In Chapter 5, we saw three ways to compute square roots: math.sqrt(X), X ** .5, and pow(X, .5). If your programs run a lot these, their relative performance might become important. To see which is quickest, repurpose the timerseqs.py script we wrote in this chapter to time each of these three tools. Use the mytimer.py timer module with the best function (you can use either the 3.0-ony keyword-only variant, or the 2.6/3.0 version). You might also want to repackage the testing code in this script for better reusability—by passing a test functions tuple to a general tester function, for example (for this exercise a copy-and-modify approach is fine). Which of the three square root tools seems to run fastest on your machine and Python in general? Finally, how might you go about interactively timing the speed of dictionary comprehensions versus for loops? 526 | Chapter 20: Iterations and Comprehensions, Part 2 PART V Modules CHAPTER 21 Modules: The Big Picture This chapter begins our in-depth look at the Python module, the highest-level program organization unit, which packages program code and data for reuse. In concrete terms, modules usually correspond to Python program files (or extensions coded in external languages such as C, Java, or C#). Each file is a module, and modules import other modules to use the names they define. Modules are processed with two statements and one important function: import Lets a client (importer) fetch a module as a whole from Allows clients to fetch particular names from a module imp.reload Provides a way to reload a module’s code without stopping Python Chapter 3 introduced module fundamentals, and we’ve been using them ever since. This part of the book begins by expanding on core module concepts, then moves on to explore more advanced module usage. This first chapter offers a general look at the role of modules in overall program structure. In the following chapters, we’ll dig into the coding details behind the theory. Along the way, we’ll flesh out module details omitted so far: you’ll learn about reloads, the __name__ and __all__ attributes, package imports, relative import syntax, and so on. Because modules and classes are really just glorified namespaces, we’ll formalize namespace concepts here as well. Why Use Modules? In short, modules provide an easy way to organize components into a system by serving as self-contained packages of variables known as namespaces. All the names defined at the top level of a module file become attributes of the imported module object. As we saw in the last part of this book, imports give access to names in a module’s global 529 scope. That is, the module file’s global scope morphs into the module object’s attribute namespace when it is imported. Ultimately, Python’s modules allow us to link individual files into a larger program system. More specifically, from an abstract perspective, modules have at least three roles: Code reuse As discussed in Chapter 3, modules let you save code in files permanently. Unlike code you type at the Python interactive prompt, which goes away when you exit Python, code in module files is persistent—it can be reloaded and rerun as many times as needed. More to the point, modules are a place to define names, known as attributes, which may be referenced by multiple external clients. System namespace partitioning Modules are also the highest-level program organization unit in Python. Fundamentally, they are just packages of names. Modules seal up names into self-contained packages, which helps avoid name clashes—you can never see a name in another file, unless you explicitly import that file. In fact, everything “lives” in a module—code you execute and objects you create are always implicitly enclosed in modules. Because of that, modules are natural tools for grouping system components. Implementing shared services or data From an operational perspective, modules also come in handy for implementing components that are shared across a system and hence require only a single copy. For instance, if you need to provide a global object that’s used by more than one function or file, you can code it in a module that can then be imported by many clients. For you to truly understand the role of modules in a Python system, though, we need to digress for a moment and explore the general structure of a Python program. Python Program Architecture So far in this book, I’ve sugarcoated some of the complexity in my descriptions of Python programs. In practice, programs usually involve more than just one file; for all but the simplest scripts, your programs will take the form of multifile systems. And even if you can get by with coding a single file yourself, you will almost certainly wind up using external files that someone else has already written. This section introduces the general architecture of Python programs—the way you divide a program into a collection of source files (a.k.a. modules) and link the parts into a whole. Along the way, we’ll also explore the central concepts of Python modules, imports, and object attributes. 530 | Chapter 21: Modules: The Big Picture How to Structure a Program Generally, a Python program consists of multiple text files containing Python statements. The program is structured as one main, top-level file, along with zero or more supplemental files known as modules in Python. In Python, the top-level (a.k.a. script) file contains the main flow of control of your program—this is the file you run to launch your application. The module files are libraries of tools used to collect components used by the top-level file (and possibly elsewhere). Top-level files use tools defined in module files, and modules use tools defined in other modules. Module files generally don’t do anything when run directly; rather, they define tools intended for use in other files. In Python, a file imports a module to gain access to the tools it defines, which are known as its attributes (i.e., variable names attached to objects such as functions). Ultimately, we import modules and access their attributes to use their tools. Imports and Attributes Let’s make this a bit more concrete. Figure 21-1 sketches the structure of a Python program composed of three files: a.py, b.py, and c.py. The file a.py is chosen to be the top-level file; it will be a simple text file of statements, which is executed from top to bottom when launched. The files b.py and c.py are modules; they are simple text files of statements as well, but they are not usually launched directly. Instead, as explained previously, modules are normally imported by other files that wish to use the tools they define. Figure 21-1. Program architecture in Python. A program is a system of modules. It has one top-level script file (launched to run the program), and multiple module files (imported libraries of tools). Scripts and modules are both text files containing Python statements, though the statements in modules usually just create objects to be used later. Python’s standard library provides a collection of precoded modules. Python Program Architecture | 531 For instance, suppose the file b.py in Figure 21-1 defines a function called spam, for external use. As we learned when studying functions in Part IV, b.py will contain a Python def statement to generate the function, which can later be run by passing zero or more values in parentheses after the function’s name: def spam(text): print(text, 'spam') Now, suppose a.py wants to use spam. To this end, it might contain Python statements such as the following: import b b.spam('gumby') The first of these, a Python import statement, gives the file a.py access to everything defined by top-level code in the file b.py. It roughly means “load the file b.py (unless it’s already loaded), and give me access to all its attributes through the name b.” import (and, as you’ll see later, from) statements execute and load other files at runtime. In Python, cross-file module linking is not resolved until such import statements are executed at runtime; their net effect is to assign module names—simple variables—to loaded module objects. In fact, the module name used in an import statement serves two purposes: it identifies the external file to be loaded, but it also becomes a variable assigned to the loaded module. Objects defined by a module are also created at runtime, as the import is executing: import literally runs statements in the target file one at a time to create its contents. The second of the statements in a.py calls the function spam defined in the module b, using object attribute notation. The code b.spam means “fetch the value of the name spam that lives within the object b.” This happens to be a callable function in our example, so we pass a string in parentheses ('gumby'). If you actually type these files, save them, and run a.py, the words “gumby spam” will be printed. You’ll see the object.attribute notation used throughout Python scripts—most objects have useful attributes that are fetched with the “.” operator. Some are callable things like functions, and others are simple data values that give object properties (e.g., a person’s name). The notion of importing is also completely general throughout Python. Any file can import tools from any other file. For instance, the file a.py may import b.py to call its function, but b.py might also import c.py to leverage different tools defined there. Import chains can go as deep as you like: in this example, the module a can import b, which can import c, which can import b again, and so on. Besides serving as the highest organizational structure, modules (and module packages, described in Chapter 23) are also the highest level of code reuse in Python. Coding components in module files makes them useful in your original program, and in any other programs you may write. For instance, if after coding the program in Figure 21-1 we discover that the function b.spam is a general-purpose tool, we can reuse 532 | Chapter 21: Modules: The Big Picture it in a completely different program; all we have to do is import the file b.py again from the other program’s files. Standard Library Modules Notice the rightmost portion of Figure 21-1. Some of the modules that your programs will import are provided by Python itself and are not files you will code. Python automatically comes with a large collection of utility modules known as the standard library. This collection, roughly 200 modules large at last count, contains platform-independent support for common programming tasks: operating system interfaces, object persistence, text pattern matching, network and Internet scripting, GUI construction, and much more. None of these tools are part of the Python language itself, but you can use them by importing the appropriate modules on any standard Python installation. Because they are standard library modules, you can also be reasonably sure that they will be available and will work portably on most platforms on which you will run Python. You will see a few of the standard library modules in action in this book’s examples, but for a complete look you should browse the standard Python library reference manual, available either with your Python installation (via IDLE or the Python Start button menu on Windows) or online at http://www.python.org. Because there are so many modules, this is really the only way to get a feel for what tools are available. You can also find tutorials on Python library tools in commercial books that cover application-level programming, such as O’Reilly’s Programming Py thon, but the manuals are free, viewable in any web browser (they ship in HTML format), and updated each time Python is rereleased. How Imports Work The prior section talked about importing modules without really explaining what happens when you do so. Because imports are at the heart of program structure in Python, this section goes into more detail on the import operation to make this process less abstract. Some C programmers like to compare the Python module import operation to a C #include, but they really shouldn’t—in Python, imports are not just textual insertions of one file into another. They are really runtime operations that perform three distinct steps the first time a program imports a given file: 1. Find the module’s file. 2. Compile it to byte code (if needed). 3. Run the module’s code to build the objects it defines. How Imports Work | 533 To better understand module imports, we’ll explore these steps in turn. Bear in mind that all three of these steps are carried out only the first time a module is imported during a program’s execution; later imports of the same module bypass all of these steps and simply fetch the already loaded module object in memory. Technically, Python does this by storing loaded modules in a table named sys.modules and checking there at the start of an import operation. If the module is not present, a three-step process begins. 1. Find It First, Python must locate the module file referenced by an import statement. Notice that the import statement in the prior section’s example names the file without a .py suffix and without its directory path: it just says import b, instead of something like import c:\dir1\b.py. In fact, you can only list a simple name; path and suffix details are omitted on purpose and Python uses a standard module search path to locate the module file corresponding to an import statement.* Because this is the main part of the import operation that programmers must know about, we’ll return to this topic in a moment. 2. Compile It (Maybe) After finding a source code file that matches an import statement by traversing the module search path, Python next compiles it to byte code, if necessary. (We discussed byte code in Chapter 2.) Python checks the file timestamps and, if the byte code file is older than the source file (i.e., if you’ve changed the source), automatically regenerates the byte code when the program is run. If, on the other hand, it finds a .pyc byte code file that is not older than the corresponding .py source file, it skips the source-to–byte code compile step. In addition, if Python finds only a byte code file on the search path and no source, it simply loads the byte code directly (this means you can ship a program as just byte code files and avoid sending source). In other words, the compile step is bypassed if possible to speed program startup. Notice that compilation happens when a file is being imported. Because of this, you will not usually see a .pyc byte code file for the top-level file of your program, unless it is also imported elsewhere—only imported files leave behind .pyc files on your * It’s actually syntactically illegal to include path and suffix details in a standard import. Package imports, which we’ll discuss in Chapter 23, allow import statements to include part of the directory path leading to a file as a set of period-separated names; however, package imports still rely on the normal module search path to locate the leftmost directory in a package path (i.e., they are relative to a directory in the search path). They also cannot make use of any platform-specific directory syntax in the import statements; such syntax only works on the search path. Also, note that module file search path issues are not as relevant when you run frozen executables (discussed in Chapter 2); they typically embed byte code in the binary image. 534 | Chapter 21: Modules: The Big Picture machine. The byte code of top-level files is used internally and discarded; byte code of imported files is saved in files to speed future imports. Top-level files are often designed to be executed directly and not imported at all. Later, we’ll see that it is possible to design a file that serves both as the top-level code of a program and as a module of tools to be imported. Such a file may be both executed and imported, and thus does generate a .pyc. To learn how this works, watch for the discussion of the special __name__ attribute and __main__ in Chapter 24. 3. Run It The final step of an import operation executes the byte code of the module. All statements in the file are executed in turn, from top to bottom, and any assignments made to names during this step generate attributes of the resulting module object. This execution step therefore generates all the tools that the module’s code defines. For instance, def statements in a file are run at import time to create functions and assign attributes within the module to those functions. The functions can then be called later in the program by the file’s importers. Because this last import step actually runs the file’s code, if any top-level code in a module file does real work, you’ll see its results at import time. For example, top-level print statements in a module show output when the file is imported. Function def statements simply define objects for later use. As you can see, import operations involve quite a bit of work—they search for files, possibly run a compiler, and run Python code. Because of this, any given module is imported only once per process by default. Future imports skip all three import steps and reuse the already loaded module in memory. If you need to import a file again after it has already been loaded (for example, to support end-user customization), you have to force the issue with an imp.reload call—a tool we’ll meet in the next chapter.† The Module Search Path As mentioned earlier, the part of the import procedure that is most important to programmers is usually the first—locating the file to be imported (the “find it” part). Because you may need to tell Python where to look to find files to import, you need to know how to tap into its search path in order to extend it. † As described earlier, Python keeps already imported modules in the built-in sys.modules dictionary so it can keep track of what’s been loaded. In fact, if you want to see which modules are loaded, you can import sys and print list(sys.modules.keys()). More on other uses for this internal table in Chapter 24. The Module Search Path | 535 In many cases, you can rely on the automatic nature of the module import search path and won’t need to configure this path at all. If you want to be able to import files across directory boundaries, though, you will need to know how the search path works in order to customize it. Roughly, Python’s module search path is composed of the concatenation of these major components, some of which are preset for you and some of which you can tailor to tell Python where to look: 1. 2. 3. 4. The home directory of the program PYTHONPATH directories (if set) Standard library directories The contents of any .pth files (if present) Ultimately, the concatenation of these four components becomes sys.path, a list of directory name strings that I’ll expand upon later in this section. The first and third elements of the search path are defined automatically. Because Python searches the concatenation of these components from first to last, though, the second and fourth elements can be used to extend the path to include your own source code directories. Here is how Python uses each of these path components: Home directory Python first looks for the imported file in the home directory. The meaning of this entry depends on how you are running the code. When you’re running a program, this entry is the directory containing your program’s top-level script file. When you’re working interactively, this entry is the directory in which you are working (i.e., the current working directory). Because this directory is always searched first, if a program is located entirely in a single directory, all of its imports will work automatically with no path configuration required. On the other hand, because this directory is searched first, its files will also override modules of the same name in directories elsewhere on the path; be careful not to accidentally hide library modules this way if you need them in your program. PYTHONPATH directories Next, Python searches all directories listed in your PYTHONPATH environment variable setting, from left to right (assuming you have set this at all). In brief, PYTHONPATH is simply set to a list of user-defined and platform-specific names of directories that contain Python code files. You can add all the directories from which you wish to be able to import, and Python will extend the module search path to include all the directories your PYTHONPATH lists. Because Python searches the home directory first, this setting is only important when importing files across directory boundaries—that is, if you need to import a file that is stored in a different directory from the file that imports it. You’ll probably want to set your PYTHONPATH variable once you start writing substantial programs, but when you’re first starting out, as long as you save all your module files in the 536 | Chapter 21: Modules: The Big Picture directory in which you’re working (i.e., the home directory, described earlier) your imports will work without you needing to worry about this setting at all. Standard library directories Next, Python automatically searches the directories where the standard library modules are installed on your machine. Because these are always searched, they normally do not need to be added to your PYTHONPATH or included in path files (discussed next). .pth path file directories Finally, a lesser-used feature of Python allows users to add directories to the module search path by simply listing them, one per line, in a text file whose name ends with a .pth suffix (for “path”). These path configuration files are a somewhat advanced installation-related feature; we won’t them cover fully here, but they provide an alternative to PYTHONPATH settings. In short, text files of directory names dropped in an appropriate directory can serve roughly the same role as the PYTHONPATH environment variable setting. For instance, if you’re running Windows and Python 3.0, a file named myconfig.pth may be placed at the top level of the Python install directory (C:\Python30) or in the sitepackages subdirectory of the standard library there (C:\Python30\Lib\sitepackages) to extend the module search path. On Unix-like systems, this file might be located in usr/local/lib/python3.0/site-packages or /usr/local/lib/site-python instead. When present, Python will add the directories listed on each line of the file, from first to last, near the end of the module search path list. In fact, Python will collect the directory names in all the path files it finds and will filter out any duplicates and nonexistent directories. Because they are files rather than shell settings, path files can apply to all users of an installation, instead of just one user or shell. Moreover, for some users text files may be simpler to code than environment settings. This feature is more sophisticated than I’ve described here. For more details consult the Python library manual, and especially its documentation for the standard library module site—this module allows the locations of Python libraries and path files to be configured, and its documentation describes the expected locations of path files in general. I recommend that beginners use PYTHONPATH or perhaps a single .pth file, and then only if you must import across directories. Path files are used more often by third-party libraries, which commonly install a path file in Python’s site-packages directory so that user settings are not required (Python’s distutils install system, described in an upcoming sidebar, automates many install steps). Configuring the Search Path The net effect of all of this is that both the PYTHONPATH and path file components of the search path allow you to tailor the places where imports look for files. The way you set environment variables and where you store path files varies per platform. For instance, The Module Search Path | 537 on Windows, you might use your Control Panel’s System icon to set PYTHONPATH to a list of directories separated by semicolons, like this: c:\pycode\utilities;d:\pycode\package1 Or you might instead create a text file called C:\Python30\pydirs.pth, which looks like this: c:\pycode\utilities d:\pycode\package1 These settings are analogous on other platforms, but the details can vary too widely for us to cover in this chapter. See Appendix A for pointers on extending your module search path with PYTHONPATH or .pth files on various platforms. Search Path Variations This description of the module search path is accurate, but generic; the exact configuration of the search path is prone to changing across platforms and Python releases. Depending on your platform, additional directories may automatically be added to the module search path as well. For instance, Python may add an entry for the current working directory—the directory from which you launched your program—in the search path after the PYTHONPATH directories, and before the standard library entries. When you’re launching from a command line, the current working directory may not be the same as the home directory of your top-level file (i.e., the directory where your program file resides). Because the current working directory can vary each time your program runs, you normally shouldn’t depend on its value for import purposes. See Chapter 3 for more on launching programs from command lines.‡ To see how your Python configures the module search path on your platform, you can always inspect sys.path—the topic of the next section. The sys.path List If you want to see how the module search path is truly configured on your machine, you can always inspect the path as Python knows it by printing the built-in sys.path list (that is, the path attribute of the standard library module sys). This list of directory name strings is the actual search path within Python; on imports, Python searches each directory in this list from left to right. ‡ See also Chapter 23’s discussion of the new relative import syntax in Python 3.0; this modifies the search path for from statements in files inside packages when “.” characters are used (e.g., from . import string). By default, a package’s own directory is not automatically searched by imports in Python 3.0, unless relative imports are used by files in the package itself. 538 | Chapter 21: Modules: The Big Picture Really, sys.path is the module search path. Python configures it at program startup, automatically merging the home directory of the top-level file (or an empty string to designate the current working directory), any PYTHONPATH directories, the contents of any .pth file paths you’ve created, and the standard library directories. The result is a list of directory name strings that Python searches on each import of a new file. Python exposes this list for two good reasons. First, it provides a way to verify the search path settings you’ve made—if you don’t see your settings somewhere in this list, you need to recheck your work. For example, here is what my module search path looks like on Windows under Python 3.0, with my PYTHONPATH set to C:\users and a C:\Python30\mypath.py path file that lists C:\users\mark. The empty string at the front means current directory and my two settings are merged in (the rest are standard library directories and files): >>> import sys >>> sys.path ['', 'C:\\users', 'C:\\Windows\\system32\\python30.zip', 'c:\\Python30\\DLLs', 'c:\\Python30\\lib', 'c:\\Python30\\lib\\plat-win', 'c:\\Python30', 'C:\\Users\\Mark', 'c:\\Python30\\lib\\site-packages'] Second, if you know what you’re doing, this list provides a way for scripts to tailor their search paths manually. As you’ll see later in this part of the book, by modifying the sys.path list, you can modify the search path for all future imports. Such changes only last for the duration of the script, however; PYTHONPATH and .pth files offer more permanent ways to modify the path.§ Module File Selection Keep in mind that filename suffixes (e.g., .py) are intentionally omitted from import statements. Python chooses the first file it can find on the search path that matches the imported name. For example, an import statement of the form import b might load: • • • • A source code file named b.py A byte code file named b.pyc A directory named b, for package imports (described in Chapter 23) A compiled extension module, usually coded in C or C++ and dynamically linked when imported (e.g., b.so on Linux, or b.dll or b.pyd on Cygwin and Windows) • A compiled built-in module coded in C and statically linked into Python • A ZIP file component that is automatically extracted when imported • An in-memory image, for frozen executables § Some programs really need to change sys.path, though. Scripts that run on web servers, for example, often run as the user “nobody” to limit machine access. Because such scripts cannot usually depend on “nobody” to have set PYTHONPATH in any particular way, they often set sys.path manually to include required source directories, prior to running any import statements. A sys.path.append(dirname) will often suffice. The Module Search Path | 539 • A Java class, in the Jython version of Python • A .NET component, in the IronPython version of Python C extensions, Jython, and package imports all extend imports beyond simple files. To importers, though, differences in the loaded file type are completely transparent, both when importing and when fetching module attributes. Saying import b gets whatever module b is, according to your module search path, and b.attr fetches an item in the module, be it a Python variable or a linked-in C function. Some standard modules we will use in this book are actually coded in C, not Python; because of this transparency, their clients don’t have to care. If you have both a b.py and a b.so in different directories, Python will always load the one found in the first (leftmost) directory of your module search path during the leftto-right search of sys.path. But what happens if it finds both a b.py and a b.so in the same directory? In this case, Python follows a standard picking order, though this order is not guaranteed to stay the same over time. In general, you should not depend on which type of file Python will choose within a given directory—make your module names distinct, or configure your module search path to make your module selection preferences more obvious. Advanced Module Selection Concepts Normally, imports work as described in this section—they find and load files on your machine. However, it is possible to redefine much of what an import operation does in Python, using what are known as import hooks. These hooks can be used to make imports do various useful things, such as loading files from archives, performing decryption, and so on. In fact, Python itself makes use of these hooks to enable files to be directly imported from ZIP archives: archived files are automatically extracted at import time when a .zip file is selected from the module import search path. One of the standard library directories in the earlier sys.path display, for example, is a .zip file today. For more details, see the Python standard library manual’s description of the built-in __import__ function, the customizable tool that import statements actually run. Python also supports the notion of .pyo optimized byte code files, created and run with the -O Python command-line flag; because these run only slightly faster than normal .pyc files (typically 5 percent faster), however, they are infrequently used. The Psyco system (see Chapter 2) provides more substantial speedups. Third-Party Software: distutils This chapter’s description of module search path settings is targeted mainly at userdefined source code that you write on your own. Third-party extensions for Python typically use the distutils tools in the standard library to automatically install themselves, so no path configuration is required to use their code. 540 | Chapter 21: Modules: The Big Picture Systems that use distutils generally come with a setup.py script, which is run to install them; this script imports and uses distutils modules to place such systems in a directory that is automatically part of the module search path (usually in the Lib\sitepackages subdirectory of the Python install tree, wherever that resides on the target machine). For more details on distributing and installing with distutils, see the Python standard manual set; its use is beyond the scope of this book (for instance, it also provides ways to automatically compile C-coded extensions on the target machine). Also check out the emerging third-party open source eggs system, which adds dependency checking for installed Python software. Chapter Summary In this chapter, we covered the basics of modules, attributes, and imports and explored the operation of import statements. We learned that imports find the designated file on the module search path, compile it to byte code, and execute all of its statements to generate its contents. We also learned how to configure the search path to be able to import from directories other than the home directory and the standard library directories, primarily with PYTHONPATH settings. As this chapter demonstrated, the import operation and modules are at the heart of program architecture in Python. Larger programs are divided into multiple files, which are linked together at runtime by imports. Imports in turn use the module search path to locate files, and modules define attributes for external use. Of course, the whole point of imports and modules is to provide a structure to your program, which divides its logic into self-contained software components. Code in one module is isolated from code in another; in fact, no file can ever see the names defined in another, unless explicit import statements are run. Because of this, modules minimize name collisions between different parts of your program. You’ll see what this all means in terms of actual statements and code in the next chapter. Before we move on, though, let’s run through the chapter quiz. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. How does a module source code file become a module object? Why might you have to set your PYTHONPATH environment variable? Name the four major components of the module import search path. Name four file types that Python might load in response to an import operation. What is a namespace, and what does a module’s namespace contain? Test Your Knowledge: Quiz | 541 Test Your Knowledge: Answers 1. A module’s source code file automatically becomes a module object when that module is imported. Technically, the module’s source code is run during the import, one statement at a time, and all the names assigned in the process become attributes of the module object. 2. You only need to set PYTHONPATH to import from directories other than the one in which you are working (i.e., the current directory when working interactively, or the directory containing your top-level file). 3. The four major components of the module import search path are the top-level script’s home directory (the directory containing it), all directories listed in the PYTHONPATH environment variable, the standard library directories, and all directories listed in .pth path files located in standard places. Of these, programmers can customize PYTHONPATH and .pth files. 4. Python might load a source code (.py) file, a byte code (.pyc) file, a C extension module (e.g., a .so file on Linux or a .dll or .pyd file on Windows), or a directory of the same name for package imports. Imports may also load more exotic things such as ZIP file components, Java classes under the Jython version of Python, .NET components under IronPython, and statically linked C extensions that have no files present at all. With import hooks, imports can load anything. 5. A namespace is a self-contained package of variables, which are known as the attributes of the namespace object. A module’s namespace contains all the names assigned by code at the top level of the module file (i.e., not nested in def or class statements). Technically, a module’s global scope morphs into the module object’s attributes namespace. A module’s namespace may also be altered by assignments from other files that import it, though this is frowned upon (see Chapter 17 for more on this issue). 542 | Chapter 21: Modules: The Big Picture CHAPTER 22 Module Coding Basics Now that we’ve looked at the larger ideas behind modules, let’s turn to a simple example of modules in action. Python modules are easy to create; they’re just files of Python program code created with a text editor. You don’t need to write special syntax to tell Python you’re making a module; almost any text file will do. Because Python handles all the details of finding and loading modules, modules are also easy to use; clients simply import a module, or specific names a module defines, and use the objects they reference. Module Creation To define a module, simply use your text editor to type some Python code into a text file, and save it with a “.py” extension; any such file is automatically considered a Python module. All the names assigned at the top level of the module become its attributes (names associated with the module object) and are exported for clients to use. For instance, if you type the following def into a file called module1.py and import it, you create a module object with one attribute—the name printer, which happens to be a reference to a function object: def printer(x): print(x) # Module attribute Before we go on, I should say a few more words about module filenames. You can call modules just about anything you like, but module filenames should end in a .py suffix if you plan to import them. The .py is technically optional for top-level files that will be run but not imported, but adding it in all cases makes your files’ types more obvious and allows you to import any of your files in the future. Because module names become variable names inside a Python program (without the .py), they should also follow the normal variable name rules outlined in Chapter 11. For instance, you can create a module file named if.py, but you cannot import it because if is a reserved word—when you try to run import if, you’ll get a syntax error. In fact, both the names of module files and the names of directories used in 543 package imports (discussed in the next chapter) must conform to the rules for variable names presented in Chapter 11; they may, for instance, contain only letters, digits, and underscores. Package directories also cannot contain platform-specific syntax such as spaces in their names. When a module is imported, Python maps the internal module name to an external filename by adding a directory path from the module search path to the front, and a .py or other extension at the end. For instance, a module named M ultimately maps to some external file <directory>\M.<extension> that contains the module’s code. As mentioned in the preceding chapter, it is also possible to create a Python module by writing code in an external language such as C or C++ (or Java, in the Jython implementation of the language). Such modules are called extension modules, and they are generally used to wrap up external libraries for use in Python scripts. When imported by Python code, extension modules look and feel the same as modules coded as Python source code files—they are accessed with import statements, and they provide functions and objects as module attributes. Extension modules are beyond the scope of this book; see Python’s standard manuals or advanced texts such as Programming Python for more details. Module Usage Clients can use the simple module file we just wrote by running an import or from statement. Both statements find, compile, and run a module file’s code, if it hasn’t yet been loaded. The chief difference is that import fetches the module as a whole, so you must qualify to fetch its names; in contrast, from fetches (or copies) specific names out of the module. Let’s see what this means in terms of code. All of the following examples wind up calling the printer function defined in the prior section’s module1.py module file, but in different ways. The import Statement In the first example, the name module1 serves two different purposes—it identifies an external file to be loaded, and it becomes a variable in the script, which references the module object after the file is loaded: >>> import module1 >>> module1.printer('Hello world!') Hello world! # Get module as a whole # Qualify to get names Because import gives a name that refers to the whole module object, we must go through the module name to fetch its attributes (e.g., module1.printer). 544 | Chapter 22: Module Coding Basics The from Statement By contrast, because from also copies names from one file over to another scope, it allows us to use the copied names directly in the script without going through the module (e.g., printer): >>> from module1 import printer >>> printer('Hello world!') Hello world! # Copy out one variable # No need to qualify name This has the same effect as the prior example, but because the imported name is copied into the scope where the from statement appears, using that name in the script requires less typing: we can use it directly instead of naming the enclosing module. As you’ll see in more detail later, the from statement is really just a minor extension to the import statement—it imports the module file as usual, but adds an extra step that copies one or more names out of the file. The from * Statement Finally, the next example uses a special form of from: when we use a *, we get copies of all the names assigned at the top level of the referenced module. Here again, we can then use the copied name printer in our script without going through the module name: >>> from module1 import * >>> printer('Hello world!') Hello world! # Copy out all variables Technically, both import and from statements invoke the same import operation; the from * form simply adds an extra step that copies all the names in the module into the importing scope. It essentially collapses one module’s namespace into another; again, the net effect is less typing for us. And that’s it—modules really are simple to use. To give you a better understanding of what really happens when you define and use modules, though, let’s move on to look at some of their properties in more detail. In Python 3.0, the from ...* statement form described here can be used only at the top level of a module file, not within a function. Python 2.6 allows it to be used within a function, but issues a warning. It’s extremely rare to see this statement used inside a function in practice; when present, it makes it impossible for Python to detect variables statically, before the function runs. Module Usage | 545 Imports Happen Only Once One of the most common questions people seem to ask when they start using modules is, “Why won’t my imports keep working?” They often report that the first import works fine, but later imports during an interactive session (or program run) seem to have no effect. In fact, they’re not supposed to. This section explains why. Modules are loaded and run on the first import or from, and only the first. This is on purpose—because importing is an expensive operation, by default Python does it just once per file, per process. Later import operations simply fetch the already loaded module object. As one consequence, because top-level code in a module file is usually executed only once, you can use it to initialize variables. Consider the file simple.py, for example: print('hello') spam = 1 # Initialize variable In this example, the print and = statements run the first time the module is imported, and the variable spam is initialized at import time: % python >>> import simple hello >>> simple.spam 1 # First import: loads and runs file's code # Assignment makes an attribute Second and later imports don’t rerun the module’s code; they just fetch the already created module object from Python’s internal modules table. Thus, the variable spam is not reinitialized: >>> simple.spam = 2 >>> import simple >>> simple.spam 2 # Change attribute in module # Just fetches already loaded module # Code wasn't rerun: attribute unchanged Of course, sometimes you really want a module’s code to be rerun on a subsequent import. We’ll see how to do this with Python’s reload function later in this chapter. import and from Are Assignments Just like def, import and from are executable statements, not compile-time declarations. They may be nested in if tests, appear in function defs, and so on, and they are not resolved or run until Python reaches them while executing your program. In other words, imported modules and names are not available until their associated import or from statements run. Also, like def, import and from are implicit assignments: • import assigns an entire module object to a single name. • from assigns one or more names to objects of the same names in another module. 546 | Chapter 22: Module Coding Basics All the things we’ve already discussed about assignment apply to module access, too. For instance, names copied with a from become references to shared objects; as with function arguments, reassigning a fetched name has no effect on the module from which it was copied, but changing a fetched mutable object can change it in the module from which it was imported. To illustrate, consider the following file, small.py: x = 1 y = [1, 2] % python >>> from small import x, y >>> x = 42 >>> y[0] = 42 # Copy two names out # Changes local x only # Changes shared mutable in-place Here, x is not a shared mutable object, but y is. The name y in the importer and the importee reference the same list object, so changing it from one place changes it in the other: >>> import small >>> small.x 1 >>> small.y [42, 2] # Get module name (from doesn't) # Small's x is not my x # But we share a changed mutable For a graphical picture of what from assignments do with references, flip back to Figure 18-1 (function argument passing), and mentally replace “caller” and “function” with “imported” and “importer.” The effect is the same, except that here we’re dealing with names in modules, not functions. Assignment works the same everywhere in Python. Cross-File Name Changes Recall from the preceding example that the assignment to x in the interactive session changed the name x in that scope only, not the x in the file—there is no link from a name copied with from back to the file it came from. To really change a global name in another file, you must use import: % python >>> from small import x, y >>> x = 42 # Copy two names out # Changes my x only >>> import small >>> small.x = 42 # Get module name # Changes x in other module This phenomenon was introduced in Chapter 17. Because changing variables in other modules like this is a common source of confusion (and often a bad design choice), we’ll revisit this technique again later in this part of the book. Note that the change to y[0] in the prior session is different; it changes an object, not a name. Module Usage | 547 import and from Equivalence Notice in the prior example that we have to execute an import statement after the from to access the small module name at all. from only copies names from one module to another; it does not assign the module name itself. At least conceptually, a from statement like this one: from module import name1, name2 # Copy these two names out (only) is equivalent to this statement sequence: import module name1 = module.name1 name2 = module.name2 del module # Fetch the module object # Copy names out by assignment # Get rid of the module name Like all assignments, the from statement creates new variables in the importer, which initially refer to objects of the same names in the imported file. Only the names are copied out, though, not the module itself. When we use the from * form of this statement (from module import *), the equivalence is the same, but all the top-level names in the module are copied over to the importing scope this way. Notice that the first step of the from runs a normal import operation. Because of this, the from always imports the entire module into memory if it has not yet been imported, regardless of how many names it copies out of the file. There is no way to load just part of a module file (e.g., just one function), but because modules are byte code in Python instead of machine code, the performance implications are generally negligible. Potential Pitfalls of the from Statement Because the from statement makes the location of a variable more implicit and obscure (name is less meaningful to the reader than module.name), some Python users recommend using import instead of from most of the time. I’m not sure this advice is warranted, though; from is commonly and widely used, without too many dire consequences. In practice, in realistic programs, it’s often convenient not to have to type a module’s name every time you wish to use one of its tools. This is especially true for large modules that provide many attributes—the standard library’s tkinter GUI module, for example. It is true that the from statement has the potential to corrupt namespaces, at least in principle—if you use it to import variables that happen to have the same names as existing variables in your scope, your variables will be silently overwritten. This problem doesn’t occur with the simple import statement because you must always go through a module’s name to get to its contents (module.attr will not clash with a variable named attr in your scope). As long as you understand and expect that this can happen when using from, though, this isn’t a major concern in practice, especially if you list the imported names explicitly (e.g., from module import x, y, z). On the other hand, the from statement has more serious issues when used in conjunction with the reload call, as imported names might reference prior versions of objects. 548 | Chapter 22: Module Coding Basics Moreover, the from module import * form really can corrupt namespaces and make names difficult to understand, especially when applied to more than one file—in this case, there is no way to tell which module a name came from, short of searching the external source files. In effect, the from * form collapses one namespace into another, and so defeats the namespace partitioning feature of modules. We will explore these issues in more detail in the section “Module Gotchas” on page 599 at the end of this part of the book (see Chapter 24). Probably the best real-world advice here is to generally prefer import to from for simple modules, to explicitly list the variables you want in most from statements, and to limit the from * form to just one import per file. That way, any undefined names can be assumed to live in the module referenced with the from *. Some care is required when using the from statement, but armed with a little knowledge, most programmers find it to be a convenient way to access modules. When import is required The only time you really must use import instead of from is when you must use the same name defined in two different modules. For example, if two files define the same name differently: # M.py def func(): ...do something... # N.py def func(): ...do something else... and you must use both versions of the name in your program, the from statement will fail—you can only have one assignment to the name in your scope: # O.py from M import func from N import func func() # This overwites the one we got from M # Calls N.func only An import will work here, though, because including the name of the enclosing module makes the two names unique: # O.py import M, N M.func() N.func() # Get the whole modules, not their names # We can call both names now # The module names make them unique This case is unusual enough that you’re unlikely to encounter it very often in practice. If you do, though, import allows you to avoid the name collision. Module Usage | 549 Module Namespaces Modules are probably best understood as simply packages of names—i.e., places to define names you want to make visible to the rest of a system. Technically, modules usually correspond to files, and Python creates a module object to contain all the names assigned in a module file. But in simple terms, modules are just namespaces (places where names are created), and the names that live in a module are called its attributes. We’ll explore how all this works in this section. Files Generate Namespaces So, how do files morph into namespaces? The short story is that every name that is assigned a value at the top level of a module file (i.e., not nested in a function or class body) becomes an attribute of that module. For instance, given an assignment statement such as X = 1 at the top level of a module file M.py, the name X becomes an attribute of M, which we can refer to from outside the module as M.X. The name X also becomes a global variable to other code inside M.py, but we need to explain the notion of module loading and scopes a bit more formally to understand why: • Module statements run on the first import. The first time a module is imported anywhere in a system, Python creates an empty module object and executes the statements in the module file one after another, from the top of the file to the bottom. • Top-level assignments create module attributes. During an import, statements at the top level of the file not nested in a def or class that assign names (e.g., =, def) create attributes of the module object; assigned names are stored in the module’s namespace. • Module namespaces can be accessed via the attribute__dict__ or dir(M). Module namespaces created by imports are dictionaries; they may be accessed through the built-in __dict__ attribute associated with module objects and may be inspected with the dir function. The dir function is roughly equivalent to the sorted keys list of an object’s __dict__ attribute, but it includes inherited names for classes, may not be complete, and is prone to changing from release to release. • Modules are a single scope (local is global). As we saw in Chapter 17, names at the top level of a module follow the same reference/assignment rules as names in a function, but the local and global scopes are the same (more formally, they follow the LEGB scope rule we met in Chapter 17, but without the L and E lookup layers). But, in modules, the module scope becomes an attribute dictionary of a module object after the module has been loaded. Unlike with functions (where the local namespace exists only while the function runs), a module file’s scope becomes a module object’s attribute namespace and lives on after the import. 550 | Chapter 22: Module Coding Basics Here’s a demonstration of these ideas. Suppose we create the following module file in a text editor and call it module2.py: print('starting to load...') import sys name = 42 def func(): pass class klass: pass print('done loading.') The first time this module is imported (or run as a program), Python executes its statements from top to bottom. Some statements create names in the module’s namespace as a side effect, but others do actual work while the import is going on. For instance, the two print statements in this file execute at import time: >>> import module2 starting to load... done loading. Once the module is loaded, its scope becomes an attribute namespace in the module object we get back from import. We can then access attributes in this namespace by qualifying them with the name of the enclosing module: >>> module2.sys <module 'sys' (built-in)> >>> module2.name 42 >>> module2.func <function func at 0x026D3BB8> >>> module2.klass <class 'module2.klass'> Here, sys, name, func, and klass were all assigned while the module’s statements were being run, so they are attributes after the import. We’ll talk about classes in Part VI, but notice the sys attribute—import statements really assign module objects to names, and any type of assignment to a name at the top level of a file generates a module attribute. Internally, module namespaces are stored as dictionary objects. These are just normal dictionary objects with the usual methods. We can access a module’s namespace dictionary through the module’s __dict__ attribute (remember to wrap this in a list call in Python 3.0—it’s a view object): >>> list(module2.__dict__.keys()) ['name', '__builtins__', '__file__', '__package__', 'sys', 'klass', 'func', '__name__', '__doc__'] Module Namespaces | 551 The names we assigned in the module file become dictionary keys internally, so most of the names here reflect top-level assignments in our file. However, Python also adds some names in the module’s namespace for us; for instance, __file__ gives the name of the file the module was loaded from, and __name__ gives its name as known to importers (without the .py extension and directory path). Attribute Name Qualification Now that you’re becoming more familiar with modules, we should look at the notion of name qualification (fetching attributes) in more depth. In Python, you can access the attributes of any object that has attributes using the qualification syntax object.attribute. Qualification is really an expression that returns the value assigned to an attribute name associated with an object. For example, the expression module2.sys in the previous example fetches the value assigned to sys in module2. Similarly, if we have a built-in list object L, L.append returns the append method object associated with that list. So, what does attribute qualification do to the scope rules we studied in Chapter 17? Nothing, really: it’s an independent concept. When you use qualification to access names, you give Python an explicit object from which to fetch the specified names. The LEGB rule applies only to bare, unqualified names. Here are the rules: Simple variables X means search for the name X in the current scopes (following the LEGB rule). Qualification X.Y means find X in the current scopes, then search for the attribute Y in the object X (not in scopes). Qualification paths X.Y.Z means look up the name Y in the object X, then look up Z in the object X.Y. Generality Qualification works on all objects with attributes: modules, classes, C extension types, etc. In Part VI, we’ll see that qualification means a bit more for classes (it’s also the place where something called inheritance happens), but in general, the rules outlined here apply to all names in Python. Imports Versus Scopes As we’ve learned, it is never possible to access names defined in another module file without first importing that file. That is, you never automatically get to see names in another file, regardless of the structure of imports or function calls in your program. A variable’s meaning is always determined by the locations of assignments in your source code, and attributes are always requested of an object explicitly. 552 | Chapter 22: Module Coding Basics For example, consider the following two simple modules. The first, moda.py, defines a variable X global to code in its file only, along with a function that changes the global X in this file: X = 88 def f(): global X X = 99 # My X: global to this file only # Change this file's X # Cannot see names in other modules The second module, modb.py, defines its own global variable X and imports and calls the function in the first module: X = 11 # My X: global to this file only import moda moda.f() print(X, moda.X) # Gain access to names in moda # Sets moda.X, not this file's X When run, moda.f changes the X in moda, not the X in modb. The global scope for moda.f is always the file enclosing it, regardless of which module it is ultimately called from: % python modb.py 11 99 In other words, import operations never give upward visibility to code in imported files—an imported file cannot see names in the importing file. More formally: • Functions can never see names in other functions, unless they are physically enclosing. • Module code can never see names in other modules, unless they are explicitly imported. Such behavior is part of the lexical scoping notion—in Python, the scopes surrounding a piece of code are completely determined by the code’s physical position in your file. Scopes are never influenced by function calls or module imports.* Namespace Nesting In some sense, although imports do not nest namespaces upward, they do nest downward. Using attribute qualification paths, it’s possible to descend into arbitrarily nested modules and access their attributes. For example, consider the next three files. mod3.py defines a single global name and attribute by assignment: X = 3 mod2.py in turn defines its own X, then imports mod3 and uses qualification to access the imported module’s attribute: * Some languages act differently and provide for dynamic scoping, where scopes really may depend on runtime calls. This tends to make code trickier, though, because the meaning of a variable can differ over time. Module Namespaces | 553 X = 2 import mod3 print(X, end=' ') print(mod3.X) # My global X # mod3's X mod1.py also defines its own X, then imports mod2, and fetches attributes in both the first and second files: X = 1 import mod2 print(X, end=' ') print(mod2.X, end=' ') print(mod2.mod3.X) # My global X # mod2's X # Nested mod3's X Really, when mod1 imports mod2 here, it sets up a two-level namespace nesting. By using the path of names mod2.mod3.X, it can descend into mod3, which is nested in the imported mod2. The net effect is that mod1 can see the Xs in all three files, and hence has access to all three global scopes: % python mod1.py 2 3 1 2 3 The reverse, however, is not true: mod3 cannot see names in mod2, and mod2 cannot see names in mod1. This example may be easier to grasp if you don’t think in terms of namespaces and scopes, but instead focus on the objects involved. Within mod1, mod2 is just a name that refers to an object with attributes, some of which may refer to other objects with attributes (import is an assignment). For paths like mod2.mod3.X, Python simply evaluates from left to right, fetching attributes from objects along the way. Note that mod1 can say import mod2, and then mod2.mod3.X, but it cannot say import mod2.mod3—this syntax invokes something called package (directory) imports, described in the next chapter. Package imports also create module namespace nesting, but their import statements are taken to reflect directory trees, not simple import chains. Reloading Modules As we’ve seen, a module’s code is run only once per process by default. To force a module’s code to be reloaded and rerun, you need to ask Python to do so explicitly by calling the reload built-in function. In this section, we’ll explore how to use reloads to make your systems more dynamic. In a nutshell: • Imports (via both import and from statements) load and run a module’s code only the first time the module is imported in a process. • Later imports use the already loaded module object without reloading or rerunning the file’s code. 554 | Chapter 22: Module Coding Basics • The reload function forces an already loaded module’s code to be reloaded and rerun. Assignments in the file’s new code change the existing module object in-place. Why all the fuss about reloading modules? The reload function allows parts of a program to be changed without stopping the whole program. With reload, therefore, the effects of changes in components can be observed immediately. Reloading doesn’t help in every situation, but where it does, it makes for a much shorter development cycle. For instance, imagine a database program that must connect to a server on startup; because program changes or customizations can be tested immediately after reloads, you need to connect only once while debugging. Long-running servers can update themselves this way, too. Because Python is interpreted (more or less), it already gets rid of the compile/link steps you need to go through to get a C program to run: modules are loaded dynamically when imported by a running program. Reloading offers a further performance advantage by allowing you to also change parts of running programs without stopping. Note that reload currently only works on modules written in Python; compiled extension modules coded in a language such as C can be dynamically loaded at runtime, too, but they can’t be reloaded. Version skew note: In Python 2.6, reload is available as a built-in function. In Python 3.0, it has been moved to the imp standard library module—it’s known as imp.reload in 3.0. This simply means that an extra import or from statement is required to load this tool (in 3.0 only). Readers using 2.6 can ignore these imports in this book’s examples, or use them anyhow—2.6 also has a reload in its imp module to ease migration to 3.0. Reloading works the same regardless of its packaging. reload Basics Unlike import and from: • reload is a function in Python, not a statement. • reload is passed an existing module object, not a name. • reload lives in a module in Python 3.0 and must be imported itself. Because reload expects an object, a module must have been previously imported successfully before you can reload it (if the import was unsuccessful, due to a syntax or other error, you may need to repeat it before you can reload the module). Furthermore, the syntax of import statements and reload calls differs: reloads require parentheses, but imports do not. Reloading looks like this: import module ...use module.attributes... ... ... # Initial import # Now, go change the module file Reloading Modules | 555 from imp import reload reload(module) ...use module.attributes... # Get reload itself (in 3.0) # Get updated exports The typical usage pattern is that you import a module, then change its source code in a text editor, and then reload it. When you call reload, Python rereads the module file’s source code and reruns its top-level statements. Perhaps the most important thing to know about reload is that it changes a module object in-place; it does not delete and re-create the module object. Because of that, every reference to a module object anywhere in your program is automatically affected by a reload. Here are the details: • reload runs a module file’s new code in the module’s current namespace. Rerunning a module file’s code overwrites its existing namespace, rather than deleting and re-creating it. • Top-level assignments in the file replace names with new values. For instance, rerunning a def statement replaces the prior version of the function in the module’s namespace by reassigning the function name. • Reloads impact all clients that use import to fetch modules. Because clients that use import qualify to fetch attributes, they’ll find new values in the module object after a reload. • Reloads impact future from clients only. Clients that used from to fetch attributes in the past won’t be affected by a reload; they’ll still have references to the old objects fetched before the reload. reload Example To demonstrate, here’s a more concrete example of reload in action. In the following, we’ll change and reload a module file without stopping the interactive Python session. Reloads are used in many other scenarios, too (see the sidebar “Why You Will Care: Module Reloads” on page 557), but we’ll keep things simple for illustration here. First, in the text editor of your choice, write a module file named changer.py with the following contents: message = "First version" def printer(): print(message) This module creates and exports two names—one bound to a string, and another to a function. Now, start the Python interpreter, import the module, and call the function it exports. The function will print the value of the global message variable: % python >>> import changer >>> changer.printer() First version 556 | Chapter 22: Module Coding Basics Keeping the interpreter active, now edit the module file in another window: ...modify changer.py without stopping Python... % vi changer.py Change the global message variable, as well as the printer function body: message = "After editing" def printer(): print('reloaded:', message) Then, return to the Python window and reload the module to fetch the new code. Notice in the following interaction that importing the module again has no effect; we get the original message, even though the file’s been changed. We have to call reload in order to get the new version: ...back to the Python interpreter/program... >>> import changer >>> changer.printer() First version >>> from imp import reload >>> reload(changer) <module 'changer' from 'changer.py'> >>> changer.printer() reloaded: After editing # No effect: uses loaded module # Forces new code to load/run # Runs the new version now Notice that reload actually returns the module object for us—its result is usually ignored, but because expression results are printed at the interactive prompt, Python shows a default <module 'name'...> representation. Why You Will Care: Module Reloads Besides allowing you to reload (and hence rerun) modules at the interactive prompt, module reloads are also useful in larger systems, especially when the cost of restarting the entire application is prohibitive. For instance, systems that must connect to servers over a network on startup are prime candidates for dynamic reloads. They’re also useful in GUI work (a widget’s callback action can be changed while the GUI remains active), and when Python is used as an embedded language in a C or C++ program (the enclosing program can request a reload of the Python code it runs, without having to stop). See Programming Python for more on reloading GUI callbacks and embedded Python code. More generally, reloads allow programs to provide highly dynamic interfaces. For instance, Python is often used as a customization language for larger systems—users can customize products by coding bits of Python code onsite, without having to recompile the entire product (or even having its source code at all). In such worlds, the Python code already adds a dynamic flavor by itself. Reloading Modules | 557 To be even more dynamic, though, such systems can automatically reload the Python customization code periodically at runtime. That way, users’ changes are picked up while the system is running; there is no need to stop and restart each time the Python code is modified. Not all systems require such a dynamic approach, but for those that do, module reloads provide an easy-to-use dynamic customization tool. Chapter Summary This chapter delved into the basics of module coding tools—the import and from statements, and the reload call. We learned how the from statement simply adds an extra step that copies names out of a file after it has been imported, and how reload forces a file to be imported again without stopping and restarting Python. We also surveyed namespace concepts, saw what happens when imports are nested, explored the way files become module namespaces, and learned about some potential pitfalls of the from statement. Although we’ve already seen enough to handle module files in our programs, the next chapter extends our coverage of the import model by presenting package imports—a way for our import statements to specify part of the directory path leading to the desired module. As we’ll see, package imports give us a hierarchy that is useful in larger systems and allow us to break conflicts between same-named modules. Before we move on, though, here’s a quick quiz on the concepts presented here. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. How do you make a module? How is the from statement related to the import statement? How is the reload function related to imports? When must you use import instead of from? Name three potential pitfalls of the from statement. What...is the airspeed velocity of an unladen swallow? Test Your Knowledge: Answers 1. To create a module, you just write a text file containing Python statements; every source code file is automatically a module, and there is no syntax for declaring one. Import operations load module files into module objects in memory. You can also make a module by writing code in an external language like C or Java, but such extension modules are beyond the scope of this book. 558 | Chapter 22: Module Coding Basics 2. The from statement imports an entire module, like the import statement, but as an extra step it also copies one or more variables from the imported module into the scope where the from appears. This enables you to use the imported names directly (name) instead of having to go through the module (module.name). 3. By default, a module is imported only once per process. The reload function forces a module to be imported again. It is mostly used to pick up new versions of a module’s source code during development, and in dynamic customization scenarios. 4. You must use import instead of from only when you need to access the same name in two different modules; because you’ll have to specify the names of the enclosing modules, the two names will be unique. 5. The from statement can obscure the meaning of a variable (which module it is defined in), can have problems with the reload call (names may reference prior versions of objects), and can corrupt namespaces (it might silently overwrite names you are using in your scope). The from * form is worse in most regards—it can seriously corrupt namespaces and obscure the meaning of variables, so it is probably best used sparingly. 6. What do you mean? An African or European swallow? Test Your Knowledge: Answers | 559 CHAPTER 23 Module Packages So far, when we’ve imported modules, we’ve been loading files. This represents typical module usage, and it’s probably the technique you’ll use for most imports you’ll code early on in your Python career. However, the module import story is a bit richer than I have thus far implied. In addition to a module name, an import can name a directory path. A directory of Python code is said to be a package, so such imports are known as package imports. In effect, a package import turns a directory on your computer into another Python namespace, with attributes corresponding to the subdirectories and module files that the directory contains. This is a somewhat advanced feature, but the hierarchy it provides turns out to be handy for organizing the files in a large system and tends to simplify module search path settings. As we’ll see, package imports are also sometimes required to resolve import ambiguities when multiple program files of the same name are installed on a single machine. Because it is relevant to code in packages only, we’ll also introduce Python’s recent relative imports model and syntax here. As we’ll see, this model modifies search paths and extends the from statement for imports within packages. Package Import Basics So, how do package imports work? In the place where you have been naming a simple file in your import statements, you can instead list a path of names separated by periods: import dir1.dir2.mod The same goes for from statements: from dir1.dir2.mod import x 561 The “dotted” path in these statements is assumed to correspond to a path through the directory hierarchy on your machine, leading to the file mod.py (or similar; the extension may vary). That is, the preceding statements indicate that on your machine there is a directory dir1, which has a subdirectory dir2, which contains a module file mod.py (or similar). Furthermore, these imports imply that dir1 resides within some container directory dir0, which is a component of the Python module search path. In other words, the two import statements imply a directory structure that looks something like this (shown with DOS backslash separators): dir0\dir1\dir2\mod.py # Or mod.pyc, mod.so, etc. The container directory dir0 needs to be added to your module search path (unless it’s the home directory of the top-level file), exactly as if dir1 were a simple module file. More generally, the leftmost component in a package import path is still relative to a directory included in the sys.path module search path list we met in Chapter 21. From there down, though, the import statements in your script give the directory paths leading to the modules explicitly. Packages and Search Path Settings If you use this feature, keep in mind that the directory paths in your import statements can only be variables separated by periods. You cannot use any platform-specific path syntax in your import statements, such as C:\dir1, My Documents.dir2 or ../dir1—these do not work syntactically. Instead, use platform-specific syntax in your module search path settings to name the container directories. For instance, in the prior example, dir0—the directory name you add to your module search path—can be an arbitrarily long and platform-specific directory path leading up to dir1. Instead of using an invalid statement like this: import C:\mycode\dir1\dir2\mod # Error: illegal syntax add C:\mycode to your PYTHONPATH variable or a .pth file (assuming it is not the program’s home directory, in which case this step is not necessary), and say this in your script: import dir1.dir2.mod In effect, entries on the module search path provide platform-specific directory path prefixes, which lead to the leftmost names in import statements. import statements provide directory path tails in a platform-neutral fashion.* * The dot path syntax was chosen partly for platform neutrality, but also because paths in import statements become real nested object paths. This syntax also means that you get odd error messages if you forget to omit the .py in your import statements. For example, import mod.py is assumed to be a directory path import—it loads mod.py, then tries to load a mod\py.py, and ultimately issues a potentially confusing “No module named py” error message. 562 | Chapter 23: Module Packages Package __init__.py Files If you choose to use package imports, there is one more constraint you must follow: each directory named within the path of a package import statement must contain a file named __init__.py, or your package imports will fail. That is, in the example we’ve been using, both dir1 and dir2 must contain a file called __init__.py; the container directory dir0 does not require such a file because it’s not listed in the import statement itself. More formally, for a directory structure such as this: dir0\dir1\dir2\mod.py and an import statement of the form: import dir1.dir2.mod the following rules apply: • dir1 and dir2 both must contain an __init__.py file. • dir0, the container, does not require an __init__.py file; this file will simply be ignored if present. • dir0, not dir0\dir1, must be listed on the module search path (i.e., it must be the home directory, or be listed in your PYTHONPATH, etc.). The net effect is that this example’s directory structure should be as follows, with indentation designating directory nesting: dir0\ dir1\ __init__.py dir2\ __init__.py mod.py # Container on module search path The __init__.py files can contain Python code, just like normal module files. They are partly present as a declaration to Python, however, and can be completely empty. As declarations, these files serve to prevent directories with common names from unintentionally hiding true modules that appear later on the module search path. Without this safeguard, Python might pick a directory that has nothing to do with your code, just because it appears in an earlier directory on the search path. More generally, the __init__.py file serves as a hook for package-initialization-time actions, generates a module namespace for a directory, and implements the behavior of from * (i.e., from .. import *) statements when used with directory imports: Package initialization The first time Python imports through a directory, it automatically runs all the code in the directory’s __init__.py file. Because of that, these files are a natural place to put code to initialize the state required by files in a package. For instance, a package might use its initialization file to create required data files, open connections to Package Import Basics | 563 databases, and so on. Typically, __init__.py files are not meant to be useful if executed directly; they are run automatically when a package is first accessed. Module namespace initialization In the package import model, the directory paths in your script become real nested object paths after an import. For instance, in the preceding example, after the import the expression dir1.dir2 works and returns a module object whose namespace contains all the names assigned by dir2’s __init__.py file. Such files provide a namespace for module objects created for directories, which have no real associated module files. from * statement behavior As an advanced feature, you can use __all__ lists in __init__.py files to define what is exported when a directory is imported with the from * statement form. In an __init__.py file, the __all__ list is taken to be the list of submodule names that should be imported when from * is used on the package (directory) name. If __all__ is not set, the from * statement does not automatically load submodules nested in the directory; instead, it loads just names defined by assignments in the directory’s __init__.py file, including any submodules explicitly imported by code in this file. For instance, the statement from submodule import X in a directory’s __init__.py makes the name X available in that directory’s namespace. (We’ll see additional roles for __all__ in Chapter 24.) You can also simply leave these files empty, if their roles are beyond your needs (and frankly, they are often empty in practice). They must exist, though, for your directory imports to work at all. Don’t confuse package __init__.py files with the class __init__ constructor methods we’ll meet in the next part of the book. The former are files of code run when imports first step through a package directory, while the latter are called when an instance is created. Both have initialization roles, but they are otherwise very different. Package Import Example Let’s actually code the example we’ve been talking about to show how initialization files and paths come into play. The following three files are coded in a directory dir1 and its subdirectory dir2—comments give the path names of these files: # dir1\__init__.py print('dir1 init') x = 1 # dir1\dir2\__init__.py print('dir2 init') y = 2 564 | Chapter 23: Module Packages # dir1\dir2\mod.py print('in mod.py') z = 3 Here, dir1 will be either a subdirectory of the one we’re working in (i.e., the home directory), or a subdirectory of a directory that is listed on the module search path (technically, on sys.path). Either way, dir1’s container does not need an __init__.py file. import statements run each directory’s initialization file the first time that directory is traversed, as Python descends the path; print statements are included here to trace their execution. As with module files, an already imported directory may be passed to reload to force reexecution of that single item. As shown here, reload accepts a dotted pathname to reload nested directories and files: % python >>> import dir1.dir2.mod # First imports run init files dir1 init dir2 init in mod.py >>> >>> import dir1.dir2.mod # Later imports do not >>> >>> from imp import reload # Needed in 3.0 >>> reload(dir1) dir1 init <module 'dir1' from 'dir1\__init__.pyc'> >>> >>> reload(dir1.dir2) dir2 init <module 'dir1.dir2' from 'dir1\dir2\__init__.pyc'> Once imported, the path in your import statement becomes a nested object path in your script. Here, mod is an object nested in the object dir2, which in turn is nested in the object dir1: >>> dir1 <module 'dir1' from 'dir1\__init__.pyc'> >>> dir1.dir2 <module 'dir1.dir2' from 'dir1\dir2\__init__.pyc'> >>> dir1.dir2.mod <module 'dir1.dir2.mod' from 'dir1\dir2\mod.pyc'> In fact, each directory name in the path becomes a variable assigned to a module object whose namespace is initialized by all the assignments in that directory’s __init__.py file. dir1.x refers to the variable x assigned in dir1\__init__.py, much as mod.z refers to the variable z assigned in mod.py: >>> dir1.x 1 >>> dir1.dir2.y 2 >>> dir1.dir2.mod.z 3 Package Import Example | 565 from Versus import with Packages import statements can be somewhat inconvenient to use with packages, because you may have to retype the paths frequently in your program. In the prior section’s example, for instance, you must retype and rerun the full path from dir1 each time you want to reach z. If you try to access dir2 or mod directly, you’ll get an error: >>> dir2.mod NameError: name 'dir2' is not defined >>> mod.z NameError: name 'mod' is not defined It’s often more convenient, therefore, to use the from statement with packages to avoid retyping the paths at each access. Perhaps more importantly, if you ever restructure your directory tree, the from statement requires just one path update in your code, whereas imports may require many. The import as extension, discussed formally in the next chapter, can also help here by providing a shorter synonym for the full path: % python >>> from dir1.dir2 import mod dir1 init dir2 init in mod.py >>> mod.z 3 >>> from dir1.dir2.mod import z >>> z 3 >>> import dir1.dir2.mod as mod >>> mod.z 3 # Code path here only # Don't repeat path # Use shorter name (see Chapter 24) Why Use Package Imports? If you’re new to Python, make sure that you’ve mastered simple modules before stepping up to packages, as they are a somewhat advanced feature. They do serve useful roles, though, especially in larger programs: they make imports more informative, serve as an organizational tool, simplify your module search path, and can resolve ambiguities. First of all, because package imports give some directory information in program files, they both make it easier to locate your files and serve as an organizational tool. Without package paths, you must often resort to consulting the module search path to find files. Moreover, if you organize your files into subdirectories for functional areas, package imports make it more obvious what role a module plays, and so make your code more readable. For example, a normal import of a file in a directory somewhere on the module search path, like this: import utilities 566 | Chapter 23: Module Packages offers much less information than an import that includes the path: import database.client.utilities Package imports can also greatly simplify your PYTHONPATH and .pth file search path settings. In fact, if you use explicit package imports for all your cross-directory imports, and you make those package imports relative to a common root directory where all your Python code is stored, you really only need a single entry on your search path: the common root. Finally, package imports serve to resolve ambiguities by making explicit exactly which files you want to import. The next section explores this role in more detail. A Tale of Three Systems The only time package imports are actually required is to resolve ambiguities that may arise when multiple programs with same-named files are installed on a single machine. This is something of an install issue, but it can also become a concern in general practice. Let’s turn to a hypothetical scenario to illustrate. Suppose that a programmer develops a Python program that contains a file called utilities.py for common utility code and a top-level file named main.py that users launch to start the program. All over this program, its files say import utilities to load and use the common code. When the program is shipped, it arrives as a single .tar or .zip file containing all the program’s files, and when it is installed, it unpacks all its files into a single directory named system1 on the target machine: system1\ utilities.py main.py other.py # Common utility functions, classes # Launch this to start the program # Import utilities to load my tools Now, suppose that a second programmer develops a different program with files also called utilities.py and main.py, and again uses import utilities throughout the program to load the common code file. When this second system is fetched and installed on the same computer as the first system, its files will unpack into a new directory called system2 somewhere on the receiving machine (ensuring that they do not overwrite same-named files from the first system): system2\ utilities.py main.py other.py # Common utilities # Launch this to run # Imports utilities So far, there’s no problem: both systems can coexist and run on the same machine. In fact, you won’t even need to configure the module search path to use these programs on your computer—because Python always searches the home directory first (that is, the directory containing the top-level file), imports in either system’s files will automatically see all the files in that system’s directory. For instance, if you click on system1\main.py, all imports will search system1 first. Similarly, if you launch Why Use Package Imports? | 567 system2\main.py, system2 will be searched first instead. Remember, module search path settings are only needed to import across directory boundaries. However, suppose that after you’ve installed these two programs on your machine, you decide that you’d like to use some of the code in each of the utilities.py files in a system of your own. It’s common utility code, after all, and Python code by nature wants to be reused. In this case, you want to be able to say the following from code that you’re writing in a third directory to load one of the two files: import utilities utilities.func('spam') Now the problem starts to materialize. To make this work at all, you’ll have to set the module search path to include the directories containing the utilities.py files. But which directory do you put first in the path—system1 or system2? The problem is the linear nature of the search path. It is always scanned from left to right, so no matter how long you ponder this dilemma, you will always get utilities.py from the directory listed first (leftmost) on the search path. As is, you’ll never be able to import it from the other directory at all. You could try changing sys.path within your script before each import operation, but that’s both extra work and highly error prone. By default, you’re stuck. This is the issue that packages actually fix. Rather than installing programs as flat lists of files in standalone directories, you can package and install them as subdirectories under a common root. For instance, you might organize all the code in this example as an install hierarchy that looks like this: root\ system1\ __init__.py utilities.py main.py other.py system2\ __init__.py utilities.py main.py other.py system3\ __init__.py myfile.py # Here or elsewhere # Your new code here Now, add just the common root directory to your search path. If your code’s imports are all relative to this common root, you can import either system’s utility file with a package import—the enclosing directory name makes the path (and hence, the module reference) unique. In fact, you can import both utility files in the same module, as long as you use an import statement and repeat the full path each time you reference the utility modules: 568 | Chapter 23: Module Packages import system1.utilities import system2.utilities system1.utilities.function('spam') system2.utilities.function('eggs') The names of the enclosing directories here make the module references unique. Note that you have to use import instead of from with packages only if you need to access the same attribute in two or more paths. If the name of the called function here was different in each path, from statements could be used to avoid repeating the full package path whenever you call one of the functions, as described earlier. Also, notice in the install hierarchy shown earlier that __init__.py files were added to the system1 and system2 directories to make this work, but not to the root directory. Only directories listed within import statements in your code require these files; as you’ll recall, they are run automatically the first time the Python process imports through a package directory. Technically, in this case the system3 directory doesn’t have to be under root—just the packages of code from which you will import. However, because you never know when your own modules might be useful in other programs, you might as well place them under the common root directory as well to avoid similar name-collision problems in the future. Finally, notice that both of the two original systems’ imports will keep working unchanged. Because their home directories are searched first, the addition of the common root on the search path is irrelevant to code in system1 and system2; they can keep saying just import utilities and expect to find their own files. Moreover, if you’re careful to unpack all your Python systems under a common root like this, path configuration becomes simple: you’ll only need to add the common root directory, once. Package Relative Imports The coverage of package imports so far has focused mostly on importing package files from outside the package. Within the package itself, imports of package files can use the same path syntax as outside imports, but they can also make use of special intrapackage search rules to simplify import statements. That is, rather than listing package import paths, imports within the package can be relative to the package. The way this works is version-dependent today: Python 2.6 implicitly searches package directories first on imports, while 3.0 requires explicit relative import syntax. This 3.0 change can enhance code readability, by making same-package imports more obvious. If you’re starting out in Python with version 3.0, your focus in this section will likely be on its new import syntax. If you’ve used other Python packages in the past, though, you’ll probably also be interested in how the 3.0 model differs. Package Relative Imports | 569 Changes in Python 3.0 The way import operations in packages work has changed slightly in Python 3.0. This change applies only to imports within files located in the package directories we’ve been studying in this chapter; imports in other files work as before. For imports in packages, though, Python 3.0 introduces two changes: • It modifies the module import search path semantics to skip the package’s own directory by default. Imports check only other components of the search path. These are known as “absolute” imports. • It extends the syntax of from statements to allow them to explicitly request that imports search the package’s directory only. This is known as “relative” import syntax. These changes are fully present in Python 3.0. The new from statement relative syntax is also available in Python 2.6, but the default search path change must be enabled as an option. It’s currently scheduled to be added in the 2.7 release†—this change is being phased in this way because the search path portion is not backward compatible with earlier Pythons. The impact of this change is that in 3.0 (and optionally in 2.6), you must generally use special from syntax to import modules located in the same package as the importer, unless you spell out a complete path from a package root. Without this syntax, your package is not automatically searched. Relative Import Basics In Python 3.0 and 2.6, from statements can now use leading dots (“.”) to specify that they require modules located within the same package (known as package relative imports), instead of modules located elsewhere on the module import search path (called absolute imports). That is: • In both Python 3.0 and 2.6, you can use leading dots in from statements to indicate that imports should be relative to the containing package—such imports will search for modules inside the package only and will not look for same-named modules located elsewhere on the import search path (sys.path). The net effect is that package modules override outside modules. • In Python 2.6, normal imports in a package’s code (without leading dots) currently default to a relative-then-absolute search path order—that is, they search the package’s own directory first. However, in Python 3.0, imports within a package are absolute by default—in the absence of any special dot syntax, imports skip the containing package itself and look elsewhere on the sys.path search path. † Yes, there will be a 2.7 release, and possibly 2.8 and later releases, in parallel with new releases in the 3.X line. As described in the Preface, both the Python 2 and Python 3 lines are expected to be fully supported for years to come, to accommodate the large existing Python 2 user and code bases. 570 | Chapter 23: Module Packages For example, in both Python 3.0 and 2.6, a statement of the form: # Relative to this package from . import spam instructs Python to import a module named spam located in the same package directory as the file in which this statement appears. Similarly, this statement: from .spam import name means “from a module named spam located in the same package as the file that contains this statement, import the variable name.” The behavior of a statement without the leading dot depends on which version of Python you use. In 2.6, such an import will still default to the current relative-then-absolute search path order (i.e., searching the package’s directory first), unless a statement of the following form is included in the importing file: from __future__ import absolute_import # Required until 2.7? If present, this statement enables the Python 3.0 absolute-by-default default search path change, described in the next paragraph. In 3.0, an import without a leading dot always causes Python to skip the relative components of the module import search path and look instead in the absolute directories that sys.path contains. For instance, in 3.0’s model, a statement of the following form will always find a string module somewhere on sys.path, instead of a module of the same name in the package: import string # Skip this package's version Without the from __future__ statement in 2.6, if there’s a string module in the package, it will be imported instead. To get the same behavior in 3.0 and in 2.6 when the absolute import change is enabled, run a statement of the following form to force a relative import: from . import string # Searches this package only This works in both Python 2.6 and 3.0 today. The only difference in the 3.0 model is that it is required in order to load a module that is located in the same package directory as the file in which this appears, when the module is given with a simple name. Note that leading dots can be used to force relative imports only with the from statement, not with the import statement. In Python 3.0, the import modname statement is always absolute, skipping the containing package’s directory. In 2.6, this statement form still performs relative imports today (i.e., the package’s directory is searched first), but these will become absolute in Python 2.7, too. from statements without leading dots behave the same as import statements—absolute in 3.0 (skipping the package directory), and relative-then-absolute in 2.6 (searching the package directory first). Other dot-based relative reference patterns are possible, too. Within a module file located in a package directory named mypkg, the following alternative import forms work as described: Package Relative Imports | 571 from .string import name1, name2 from . import string from .. import string # Imports names from mypkg.string # Imports mypkg.string # Imports string sibling of mypkg To understand these latter forms better, we need to understand the rationale behind this change. Why Relative Imports? This feature is designed to allow scripts to resolve ambiguities that can arise when a same-named file appears in multiple places on the module search path. Consider the following package directory: mypkg\ __init__.py main.py string.py This defines a package named mypkg containing modules named mypkg.main and mypkg.string. Now, suppose that the main module tries to import a module named string. In Python 2.6 and earlier, Python will first look in the mypkg directory to perform a relative import. It will find and import the string.py file located there, assigning it to the name string in the mypkg.main module’s namespace. It could be, though, that the intent of this import was to load the Python standard library’s string module instead. Unfortunately, in these versions of Python, there’s no straightforward way to ignore mypkg.string and look for the standard library’s string module located on the module search path. Moreover, we cannot resolve this with package import paths, because we cannot depend on any extra package directory structure above the standard library being present on every machine. In other words, imports in packages can be ambiguous—within a package, it’s not clear whether an import spam statement refers to a module within or outside the package. More accurately, a local module or package can hide another hanging directly off of sys.path, whether intentionally or not. In practice, Python users can avoid reusing the names of standard library modules they need for modules of their own (if you need the standard string, don’t name a new module string!). But this doesn’t help if a package accidentally hides a standard module; moreover, Python might add a new standard library module in the future that has the same name as a module of your own. Code that relies on relative imports is also less easy to understand, because the reader may be confused about which module is intended to be used. It’s better if the resolution can be made explicit in code. The relative imports solution in 3.0 To address this dilemma, imports run within packages have changed in Python 3.0 (and as an option in 2.6) to be absolute. Under this model, an import statement of the 572 | Chapter 23: Module Packages following form in our example file mypkg/main.py will always find a string outside the package, via an absolute import search of sys.path: import string # Imports string outside package A from import without leading-dot syntax is considered absolute as well: from string import name # Imports name from string outside package If you really want to import a module from your package without giving its full path from the package root, though, relative imports are still possible by using the dot syntax in the from statement: from . import string # Imports mypkg.string (relative) This form imports the string module relative to the current package only and is the relative equivalent to the prior import example’s absolute form; when this special relative syntax is used, the package’s directory is the only directory searched. We can also copy specific names from a module with relative syntax: from .string import name1, name2 # Imports names from mypkg.string This statement again refers to the string module relative to the current package. If this code appears in our mypkg.main module, for example, it will import name1 and name2 from mypkg.string. In effect, the “.” in a relative import is taken to stand for the package directory containing the file in which the import appears. An additional leading dot performs the relative import starting from the parent of the current package. For example, this statement: from .. import spam # Imports a sibling of mypkg will load a sibling of mypkg—i.e., the spam module located in the package’s own container directory, next to mypkg. More generally, code located in some module A.B.C can do any of these: from . import D from .. import E # Imports A.B.D (. means A.B) # Imports A.E (.. means A) from .D import X from ..E import X # Imports A.B.D.X (. means A.B) # Imports A.E.X (.. means A) Relative imports versus absolute package paths Alternatively, a file can sometimes name its own package explicitly in an absolute import statement. For example, in the following, mypkg will be found in an absolute directory on sys.path: from mypkg import string # Imports mypkg.string (absolute) However, this relies on both the configuration and the order of the module search path settings, while relative import dot syntax does not. In fact, this form requires that the directory immediately containing mypkg be included in the module search path. In Package Relative Imports | 573 general, absolute import statements must list all the directories below the package’s root entry in sys.path when naming packages explicitly like this: from system.section.mypkg import string # system container on sys.path only In large or deep packages, that could be much more work than a dot: from . import string # Relative import syntax With this latter form, the containing package is searched automatically, regardless of the search path settings. The Scope of Relative Imports Relative imports can seem a bit perplexing on first encounter, but it helps if you remember a few key points about them: • Relative imports apply to imports within packages only. Keep in mind that this feature’s module search path change applies only to import statements within module files located in a package. Normal imports coded outside package files still work exactly as described earlier, automatically searching the directory containing the top-level script first. • Relative imports apply to the from statement only. Also remember that this feature’s new syntax applies only to from statements, not import statements. It’s detected by the fact that the module name in a from begins with one or more dots (periods). Module names that contain dots but don’t have a leading dot are package imports, not relative imports. • The terminology is ambiguous. Frankly, the terminology used to describe this feature is probably more confusing than it needs to be. Really, all imports are relative to something. Outside a package, imports are still relative to directories listed on the sys.path module search path. As we learned in Chapter 21, this path includes the program’s container directory, PYTHONPATH settings, path file settings, and standard libraries. When working interactively, the program container directory is simply the current working directory. For imports made inside packages, 2.6 augments this behavior by searching the package itself first. In the 3.0 model, all that really changes is that normal “absolute” import syntax skips the package directory, but special “relative” import syntax causes it to be searched first and only. When we talk about 3.0 imports as being “absolute,” what we really mean is that they are relative to the directories on sys.path, but not the package itself. Conversely, when we speak of “relative” imports, we mean they are relative to the package directory only. Some sys.path entries could, of course, be absolute or relative paths too. (And I could probably make up something more confusing, but it would be a stretch!) 574 | Chapter 23: Module Packages In other words, “package relative imports” in 3.0 really just boil down to a removal of 2.6’s special search path behavior for packages, along with the addition of special from syntax to explicitly request relative behavior. If you wrote your package imports in the past to not depend on 2.6’s special implicit relative lookup (e.g., by always spelling out full paths from a package root), this change is largely a moot point. If you didn’t, you’ll need to update your package files to use the new from syntax for local package files. Module Lookup Rules Summary With packages and relative imports, the module search story in Python 3.0 in its entirety can be summarized as follows: • Simple module names (e.g., A) are looked up by searching each directory on the sys.path list, from left to right. This list is constructed from both system defaults and user-configurable settings. • Packages are simply directories of Python modules with a special __init__.py file, which enables A.B.C directory path syntax in imports. In an import of A.B.C, for example, the directory named A is located relative to the normal module import search of sys.path, B is another package subdirectory within A, and C is a module or other importable item within B. • Within a package’s files, normal import statements use the same sys.path search rule as imports elsewhere. Imports in packages using from statements and leading dots, however, are relative to the package; that is, only the package directory is checked, and the normal sys.path lookup is not used. In from . import A, for example, the module search is restricted to the directory containing the file in which this statement appears. Relative Imports in Action But enough theory: let’s run some quick tests to demonstrate the concepts behind relative imports. Imports outside packages First of all, as mentioned previously, this feature does not impact imports outside a package. Thus, the following finds the standard library string module as expected: C:\test> c:\Python30\python >>> import string >>> string <module 'string' from 'c:\Python30\lib\string.py'> Package Relative Imports | 575 But if we add a module of the same name in the directory we’re working in, it is selected instead, because the first entry on the module search path is the current working directory (CWD): # test\string.py print('string' * 8) C:\test> c:\Python30\python >>> import string stringstringstringstringstringstringstringstring >>> string <module 'string' from 'string.py'> In other words, normal imports are still relative to the “home” directory (the top-level script’s container, or the directory you’re working in). In fact, relative import syntax is not even allowed in code that is not in a file being used as part of a package: >>> from . import string Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: Attempted relative import in non-package In this and all examples in this section, code entered at the interactive prompt behaves the same as it would if run in a top-level script, because the first entry on sys.path is either the interactive working directory or the directory containing the top-level file. The only difference is that the start of sys.path is an absolute directory, not an empty string: # test\main.py import string print(string) C:\test> C:\python30\python main.py stringstringstringstringstringstringstringstring <module 'string' from 'C:\test\string.py'> # Same results in 2.6 Imports within packages Now, let’s get rid of the local string module we coded in the CWD and build a package directory there with two modules, including the required but empty test\pkg \__init__.py file (which I’ll omit here): C:\test> del string* C:\test> mkdir pkg # test\pkg\spam.py import eggs print(eggs.X) # test\pkg\eggs.py X = 99999 import string print(string) 576 | Chapter 23: Module Packages # <== Works in 2.6 but not 3.0! The first file in this package tries to import the second with a normal import statement. Because this is taken to be relative in 2.6 but absolute in 3.0, it fails in the latter. That is, 2.6 searches the containing package first, but 3.0 does not. This is the noncompatible behavior you have to be aware of in 3.0: C:\test> c:\Python26\python >>> import pkg.spam <module 'string' from 'c:\Python26\lib\string.pyc'> 99999 C:\test> c:\Python30\python >>> import pkg.spam Traceback (most recent call last): File "<stdin>", line 1, in <module> File "pkg\spam.py", line 1, in <module> import eggs ImportError: No module named eggs To make this work in both 2.6 and 3.0, change the first file to use the special relative import syntax, so that its import searches the package directory in 3.0, too: # test\pkg\spam.py from . import eggs print(eggs.X) # <== Use package relative import in 2.6 or 3.0 # test\pkg\eggs.py X = 99999 import string print(string) C:\test> c:\Python26\python >>> import pkg.spam <module 'string' from 'c:\Python26\lib\string.pyc'> 99999 C:\test> c:\Python30\python >>> import pkg.spam <module 'string' from 'c:\Python30\lib\string.py'> 99999 Imports are still relative to the CWD Notice in the preceding example that the package modules still have access to standard library modules like string. Really, their imports are still relative to the entries on the module search path, even if those entries are relative themselves. If you add a string module to the CWD again, imports in a package will find it there instead of in the standard library. Although you can skip the package directory with an absolute import in 3.0, you still can’t skip the home directory of the program that imports the package: # test\string.py print('string' * 8) # test\pkg\spam.py from . import eggs Package Relative Imports | 577 print(eggs.X) # test\pkg\eggs.py X = 99999 import string print(string) # <== Gets string in CWD, not Python lib! C:\test> c:\Python30\python # Same result in 2.6 >>> import pkg.spam stringstringstringstringstringstringstringstring <module 'string' from 'string.py'> 99999 Selecting modules with relative and absolute imports To show how this applies to imports of standard library modules, reset the package one more time. Get rid of the local string module, and define a new one inside the package itself: C:\test> del string* # test\pkg\spam.py import string print(string) # <== Relative in 2.6, absolute in 3.0 # test\pkg\string.py print('Ni' * 8) Now, which version of the string module you get depends on which Python you use. As before, 3.0 interprets the import in the first file as absolute and skips the package, but 2.6 does not: C:\test> c:\Python30\python >>> import pkg.spam <module 'string' from 'c:\Python30\lib\string.py'> C:\test> c:\Python26\python >>> import pkg.spam NiNiNiNiNiNiNiNi <module 'pkg.string' from 'pkg\string.py'> Using relative import syntax in 3.0 forces the package to be searched again, as it is in 2.6—by using absolute or relative import syntax in 3.0, you can either skip or select the package directory explicitly. In fact, this is the use case that the 3.0 model addresses: # test\pkg\spam.py from . import string print(string) # test\pkg\string.py print('Ni' * 8) C:\test> c:\Python30\python >>> import pkg.spam NiNiNiNiNiNiNiNi 578 | Chapter 23: Module Packages # <== Relative in both 2.6 and 3.0 <module 'pkg.string' from 'pkg\string.py'> C:\test> c:\Python26\python >>> import pkg.spam NiNiNiNiNiNiNiNi <module 'pkg.string' from 'pkg\string.py'> It’s important to note that relative import syntax is really a binding declaration, not just a preference. If we delete the string.py file in this example, the relative import in spam.py fails in both 3.0 and 2.6, instead of falling back on the standard library’s version of this module (or any other): # test\pkg\spam.py from . import string # <== Fails if no string.py here! C:\test> C:\python30\python >>> import pkg.spam ...text omitted... ImportError: cannot import name string Modules referenced by relative imports must exist in the package directory. Imports are still relative to the CWD (again) Although absolute imports let you skip package modules, they still rely on other components of sys.path. For one last test, let’s define two string modules of our own. In the following, there is one module by that name in the CWD, one in the package, and another in the standard library: # test\string.py print('string' * 8) # test\pkg\spam.py from . import string print(string) # <== Relative in both 2.6 and 3.0 # test\pkg\string.py print('Ni' * 8) When we import the string module with relative import syntax, we get the version in the package, as desired: C:\test> c:\Python30\python # Same result in 2.6 >>> import pkg.spam NiNiNiNiNiNiNiNi <module 'pkg.string' from 'pkg\string.py'> When absolute syntax is used, though, the module we get varies per version again. 2.6 interprets this as relative to the package, but 3.0 makes it “absolute,” which in this case really just means it skips the package and loads the version relative to the CWD (not the version the standard library): # test\string.py print('string' * 8) Package Relative Imports | 579 # test\pkg\spam.py import string print(string) # <== Relative in 2.6, "absolute" in 3.0: CWD! # test\pkg\string.py print('Ni' * 8) C:\test> c:\Python30\python >>> import pkg.spam stringstringstringstringstringstringstringstring <module 'string' from 'string.py'> C:\test> c:\Python26\python >>> import pkg.spam NiNiNiNiNiNiNiNi <module 'pkg.string' from 'pkg\string.pyc'> As you can see, although packages can explicitly request modules within their own directories, their imports are otherwise still relative to the rest of the normal module search path. In this case, a file in the program using the package hides the standard library module the package may want. All that the change in 3.0 really accomplishes is allowing package code to select files either inside or outside the package (i.e., relatively or absolutely). Because import resolution can depend on an enclosing context that may not be foreseen, absolute imports in 3.0 are not a guarantee of finding a module in the standard library. Experiment with these examples on your own for more insight. In practice, this is not usually as ad-hoc as it might seem: you can generally structure your imports, search paths, and module names to work the way you wish during development. You should keep in mind, though, that imports in larger systems may depend upon context of use, and the module import protocol is part of a successful library’s design. Now that you’ve learned about package-relative imports, also keep in mind that they may not always be your best option. Absolute package imports, relative to a directory on sys.path, are still sometimes preferred over both implicit package-relative imports in Python 2, and explicit package-relative import syntax in both Python 2 and 3. Package-relative import syntax and Python 3.0’s new absolute import search rules at least require relative imports from a package to be made explicit, and thus easier to understand and maintain. Files that use imports with dots, though, are implicitly bound to a package directory and cannot be used elsewhere without code changes. Naturally, the extent to which this may impact your modules can vary per package; absolute imports may also require changes when directories are reorganized. 580 | Chapter 23: Module Packages Why You Will Care: Module Packages Now that packages are a standard part of Python, it’s common to see larger third-party extensions shipped as sets of package directories, rather than flat lists of modules. The win32all Windows extensions package for Python, for instance, was one of the first to jump on the package bandwagon. Many of its utility modules reside in packages imported with paths. For instance, to load client-side COM tools, you use a statement like this: from win32com.client import constants, Dispatch This line fetches names from the client module of the win32com package (an install subdirectory). Package imports are also pervasive in code run under the Jython Java-based implementation of Python, because Java libraries are organized into hierarchies as well. In recent Python releases, the email and XML tools are likewise organized into package subdirectories in the standard library, and Python 3.0 groups even more related modules into packages (including tkinter GUI tools, HTTP networking tools, and more). The following imports access various standard library tools in 3.0: from email.message import Message from tkinter.filedialog import askopenfilename from http.server import CGIHTTPRequestHandler Whether you create package directories or not, you will probably import from them eventually. Chapter Summary This chapter introduced Python’s package import model—an optional but useful way to explicitly list part of the directory path leading up to your modules. Package imports are still relative to a directory on your module import search path, but rather than relying on Python to traverse the search path manually, your script gives the rest of the path to the module explicitly. As we’ve seen, packages not only make imports more meaningful in larger systems, but also simplify import search path settings (if all cross-directory imports are relative to a common root directory) and resolve ambiguities when there is more than one module of the same name (including the name of the enclosing directory in a package import helps distinguish between them). Because it’s relevant only to code in packages, we also explored the newer relative import model here—a way for imports in package files to select modules in the same package using leading dots in a from, instead of relying on an older implicit package search rule. Chapter Summary | 581 In the next chapter, we will survey a handful of more advanced module-related topics, such as relative import syntax and the __name__ usage mode variable. As usual, though, we’ll close out this chapter with a short quiz to test what you’ve learned here. Test Your Knowledge: Quiz 1. What is the purpose of an __init__.py file in a module package directory? 2. How can you avoid repeating the full package path every time you reference a package’s content? 3. Which directories require __init__.py files? 4. When must you use import instead of from with packages? 5. What is the difference between from mypkg import spam and from . import spam? Test Your Knowledge: Answers 1. The __init__.py file serves to declare and initialize a module package; Python automatically runs its code the first time you import through a directory in a process. Its assigned variables become the attributes of the module object created in memory to correspond to that directory. It is also not optional—you can’t import through a directory with package syntax unless it contains this file. 2. Use the from statement with a package to copy names out of the package directly, or use the as extension with the import statement to rename the path to a shorter synonym. In both cases, the path is listed in only one place, in the from or import statement. 3. Each directory listed in an import or from statement must contain an __init__.py file. Other directories, including the directory containing the leftmost component of a package path, do not need to include this file. 4. You must use import instead of from with packages only if you need to access the same name defined in more than one path. With import, the path makes the references unique, but from allows only one version of any given name. 5. from mypkg import spam is an absolute import—the search for mypkg skips the package directory and the module is located in an absolute directory in sys.path. A statement from . import spam, on the other hand, is a relative import—spam is looked up relative to the package in which this statement is contained before sys.path is searched. 582 | Chapter 23: Module Packages CHAPTER 24 Advanced Module Topics This chapter concludes this part of the book with a collection of more advanced module-related topics—data hiding, the __future__ module, the __name__ variable, sys.path changes, listing tools, running modules by name string, transitive reloads, and so on—along with the standard set of gotchas and exercises related to what we’ve covered in this part of the book. Along the way, we’ll build some larger and more useful tools than we have so far, that combine functions and modules. Like functions, modules are more effective when their interfaces are well defined, so this chapter also briefly reviews module design concepts, some of which we have explored in prior chapters. Despite the word “advanced” in this chapter’s title, this is also something of a grab bag of additional module topics. Because some of the topics discussed here are widely used (especially the __name__ trick), be sure to take a look before moving on to classes in the next part of the book. Data Hiding in Modules As we’ve seen, a Python module exports all the names assigned at the top level of its file. There is no notion of declaring which names should and shouldn’t be visible outside the module. In fact, there’s no way to prevent a client from changing names inside a module if it wants to. In Python, data hiding in modules is a convention, not a syntactical constraint. If you want to break a module by trashing its names, you can, but fortunately, I’ve yet to meet a programmer who would. Some purists object to this liberal attitude toward data hiding, claiming that it means Python can’t implement encapsulation. However, encapsulation in Python is more about packaging than about restricting. 583 Minimizing from * Damage: _X and __all__ As a special case, you can prefix names with a single underscore (e.g., _X) to prevent them from being copied out when a client imports a module’s names with a from * statement. This really is intended only to minimize namespace pollution; because from * copies out all names, the importer may get more than it’s bargained for (including names that overwrite names in the importer). Underscores aren’t “private” declarations: you can still see and change such names with other import forms, such as the import statement. Alternatively, you can achieve a hiding effect similar to the _X naming convention by assigning a list of variable name strings to the variable __all__ at the top level of the module. For example: __all__ = ["Error", "encode", "decode"] # Export these only When this feature is used, the from * statement will copy out only those names listed in the __all__ list. In effect, this is the converse of the _X convention: __all__ identifies names to be copied, while _X identifies names not to be copied. Python looks for an __all__ list in the module first; if one is not defined, from * copies all names without a single leading underscore. Like the _X convention, the __all__ list has meaning only to the from * statement form and does not amount to a privacy declaration. Module writers can use either trick to implement modules that are well behaved when used with from *. (See also the discussion of __all__ lists in package __init__.py files in Chapter 23; there, these lists declare submodules to be loaded for a from *.) Enabling Future Language Features Changes to the language that may potentially break existing code are introduced gradually. Initially, they appear as optional extensions, which are disabled by default. To turn on such extensions, use a special import statement of this form: from __future__ import featurename This statement should generally appear at the top of a module file (possibly after a docstring), because it enables special compilation of code on a per-module basis. It’s also possible to submit this statement at the interactive prompt to experiment with upcoming language changes; the feature will then be available for the rest of the interactive session. For example, in prior editions of this book, we had to use this statement form to demonstrate generator functions, which required a keyword that was not yet enabled by default (they use a featurename of generators). We also used this statement to activate 3.0 true division in Chapter 5, 3.0 print calls in Chapter 11, and 3.0 absolute imports for packages in Chapter 23. 584 | Chapter 24: Advanced Module Topics All of these changes have the potential to break existing code in Python 2.6, so they are being phased in gradually as optional features enabled with this special import. Mixed Usage Modes: __name__ and __main__ Here’s another module-related trick that lets you both import a file as a module and run it as a standalone program. Each module has a built-in attribute called __name__, which Python sets automatically as follows: • If the file is being run as a top-level program file, __name__ is set to the string "__main__" when it starts. • If the file is being imported instead, __name__ is set to the module’s name as known by its clients. The upshot is that a module can test its own __name__ to determine whether it’s being run or imported. For example, suppose we create the following module file, named runme.py, to export a single function called tester: def tester(): print("It's Christmas in Heaven...") if __name__ == '__main__': tester() # Only when run # Not when imported This module defines a function for clients to import and use as usual: % python >>> import runme >>> runme.tester() It's Christmas in Heaven... But, the module also includes code at the bottom that is set up to call the function when this file is run as a program: % python runme.py It's Christmas in Heaven... In effect, a module’s __name__ variable serves as a usage mode flag, allowing its code to be leveraged as both an importable library and a top-level script. Though simple, you’ll see this hook used in nearly every realistic Python program file you are likely to encounter. Perhaps the most common way you’ll see the __name__ test applied is for self-test code. In short, you can package code that tests a module’s exports in the module itself by wrapping it in a __name__ test at the bottom of the file. This way, you can use the file in clients by importing it, but also test its logic by running it from the system shell or via another launching scheme. In practice, self-test code at the bottom of a file under the __name__ test is probably the most common and simplest unit-testing protocol in Python. (Chapter 35 will discuss other commonly used options for testing Python Mixed Usage Modes: __name__ and __main__ | 585 code—as you’ll see, the unittest and doctest standard library modules provide more advanced testing tools.) The __name__ trick is also commonly used when writing files that can be used both as command-line utilities and as tool libraries. For instance, suppose you write a file-finder script in Python. You can get more mileage out of your code if you package it in functions and add a __name__ test in the file to automatically call those functions when the file is run standalone. That way, the script’s code becomes reusable in other programs. Unit Tests with __name__ In fact, we’ve already seen a prime example in this book of an instance where the __name__ check could be useful. In the section on arguments in Chapter 18, we coded a script that computed the minimum value from the set of arguments sent in: def minmax(test, *args): res = args[0] for arg in args[1:]: if test(arg, res): res = arg return res def lessthan(x, y): return x < y def grtrthan(x, y): return x > y print(minmax(lessthan, 4, 2, 1, 5, 6, 3)) print(minmax(grtrthan, 4, 2, 1, 5, 6, 3)) # Self-test code This script includes self-test code at the bottom, so we can test it without having to retype everything at the interactive command line each time we run it. The problem with the way it is currently coded, however, is that the output of the self-test call will appear every time this file is imported from another file to be used as a tool—not exactly a user-friendly feature! To improve it, we can wrap up the self-test call in a __name__ check, so that it will be launched only when the file is run as a top-level script, not when it is imported: print('I am:', __name__) def minmax(test, *args): res = args[0] for arg in args[1:]: if test(arg, res): res = arg return res def lessthan(x, y): return x < y def grtrthan(x, y): return x > y if __name__ == '__main__': print(minmax(lessthan, 4, 2, 1, 5, 6, 3)) print(minmax(grtrthan, 4, 2, 1, 5, 6, 3)) 586 | Chapter 24: Advanced Module Topics # Self-test code We’re also printing the value of __name__ at the top here to trace its value. Python creates and assigns this usage-mode variable as soon as it starts loading a file. When we run this file as a top-level script, its name is set to __main__, so its self-test code kicks in automatically: % python min.py I am: __main__ 1 6 But, if we import the file, its name is not __main__, so we must explicitly call the function to make it run: >>> import min I am: min >>> min.minmax(min.lessthan, 's', 'p', 'a', 'm') 'a' Again, regardless of whether this is used for testing, the net effect is that we get to use our code in two different roles—as a library module of tools, or as an executable program. Using Command-Line Arguments with __name__ Here’s a more substantial module example that demonstrates another way that the __name__ trick is commonly employed. The following module, formats.py, defines string formatting utilities for importers, but also checks its name to see if it is being run as a top-level script; if so, it tests and uses arguments listed on the system command line to run a canned or passed-in test. In Python, the sys.argv list contains command-line arguments—it is a list of strings reflecting words typed on the command line, where the first item is always the name of the script being run: """ Various specialized string display formatting utilities. Test me with canned self-test or command-line arguments. """ def commas(N): """ format positive integer-like N for display with commas between digit groupings: xxx,yyy,zzz """ digits = str(N) assert(digits.isdigit()) result = '' while digits: digits, last3 = digits[:-3], digits[-3:] result = (last3 + ',' + result) if result else last3 return result def money(N, width=0): """ Mixed Usage Modes: __name__ and __main__ | 587 format number N for display with commas, 2 decimal digits, leading $ and sign, and optional padding: $ -xxx,yyy.zz """ sign = '-' if N < 0 else '' N = abs(N) whole = commas(int(N)) fract = ('%.2f' % N)[-2:] format = '%s%s.%s' % (sign, whole, fract) return '$%*s' % (width, format) if __name__ == '__main__': def selftest(): tests = 0, 1 # fails: −1, 1.23 tests += 12, 123, 1234, 12345, 123456, 1234567 tests += 2 ** 32, 2 ** 100 for test in tests: print(commas(test)) print('') tests = 0, 1, −1, 1.23, 1., 1.2, 3.14159 tests += 12.34, 12.344, 12.345, 12.346 tests += 2 ** 32, (2 ** 32 + .2345) tests += 1.2345, 1.2, 0.2345 tests += −1.2345, −1.2, −0.2345 tests += −(2 ** 32), −(2**32 + .2345) tests += (2 ** 100), −(2 ** 100) for test in tests: print('%s [%s]' % (money(test, 17), test)) import sys if len(sys.argv) == 1: selftest() else: print(money(float(sys.argv[1]), int(sys.argv[2]))) This file works the same in Python 2.6 and 3.0. When run directly, it tests itself as before, but it uses options on the command line to control the test behavior. Run this file directly with no command-line arguments on your own to see what its self-test code prints. To test specific strings, pass them in on the command line along with a minimum field width: C:\misc> python formats.py 999999999 0 $999,999,999.00 C:\misc> python formats.py −999999999 0 $-999,999,999.00 C:\misc> python formats.py 123456789012345 0 $123,456,789,012,345.00 C:\misc> python formats.py −123456789012345 25 $ −123,456,789,012,345.00 C:\misc> python formats.py 123.456 0 $123.46 588 | Chapter 24: Advanced Module Topics C:\misc> python formats.py −123.454 0 $-123.45 C:\misc> python formats.py ...canned tests: try this yourself... As before, because this code is instrumented for dual-mode usage, we can also import its tools normally in other contexts as library components: >>> from formats import money, commas >>> money(123.456) '$123.46' >>> money(-9999999.99, 15) '$ −9,999,999.99' >>> X = 99999999999999999999 >>> '%s (%s)' % (commas(X), X) '99,999,999,999,999,999,999 (99999999999999999999)' Because this file uses the docstring feature introduced in Chapter 15, we can use the help function to explore its tools as well—it serves as a general-purpose tool: >>> import formats >>> help(formats) Help on module formats: NAME formats FILE c:\misc\formats.py DESCRIPTION Various specialized string display formatting utilities. Test me with canned self-test or command-line arguments. FUNCTIONS commas(N) format positive integer-like N for display with commas between digit groupings: xxx,yyy,zzz money(N, width=0) format number N for display with commas, 2 decimal digits, leading $ and sign, and optional padding: $ -xxx,yyy.zz You can use command-line arguments in similar ways to provide general inputs to scripts that may also package their code as functions and classes for reuse by importers. For more advanced command-line processing, be sure to see the getopt and optparse modules in Python’s standard library and manuals. In some scenarios, you might also use the built-in input function introduced in Chapter 3 and used in Chapter 10 to prompt the shell user for test inputs instead of pulling them from the command line. Mixed Usage Modes: __name__ and __main__ | 589 Also see Chapter 7’s discussion of the new {,d} string format method syntax that will be available in Python 3.1 and later; this formatting extension separates thousands groups with commas much like the code here. The module listed here, though, adds money formatting and serves as a manual alternative for comma insertion for Python versions before 3.1. Changing the Module Search Path In Chapter 21, we learned that the module search path is a list of directories that can be customized via the environment variable PYTHONPATH, and possibly via .pth files. What I haven’t shown you until now is how a Python program itself can actually change the search path by changing a built-in list called sys.path (the path attribute in the builtin sys module). sys.path is initialized on startup, but thereafter you can delete, append, and reset its components however you like: >>> import sys >>> sys.path ['', 'C:\\users', 'C:\\Windows\\system32\\python30.zip', ...more deleted...] >>> sys.path.append('C:\\sourcedir') >>> import string # Extend module search path # All imports search the new dir last Once you’ve made such a change, it will impact future imports anywhere in the Python program, as all imports and all files share the single sys.path list. In fact, this list may be changed arbitrarily: >>> sys.path = [r'd:\temp'] >>> sys.path.append('c:\\lp4e\\examples') >>> sys.path ['d:\\temp', 'c:\\lp4e\\examples'] # Change module search path # For this process only >>> import string Traceback (most recent call last): File "<stdin>", line 1, in <module> ImportError: No module named string Thus, you can use this technique to dynamically configure a search path inside a Python program. Be careful, though: if you delete a critical directory from the path, you may lose access to critical utilities. In the prior example, for instance, we no longer have access to the string module because we deleted the Python source library’s directory from the path. Also, remember that such sys.path settings endure for only as long as the Python session or program (technically, process) that made them runs; they are not retained after Python exits. PYTHONPATH and .pth file path configurations live in the operating system instead of a running Python program, and so are more global: they are picked up by every program on your machine and live on after a program completes. 590 | Chapter 24: Advanced Module Topics The as Extension for import and from Both the import and from statements have been extended to allow an imported name to be given a different name in your script. The following import statement: import modulename as name is equivalent to: import modulename name = modulename del modulename # Don't keep original name After such an import, you can (and in fact must) use the name listed after the as to refer to the module. This works in a from statement, too, to assign a name imported from a file to a different name in your script: from modulename import attrname as name This extension is commonly used to provide short synonyms for longer names, and to avoid name clashes when you are already using a name in your script that would otherwise be overwritten by a normal import statement: import reallylongmodulename as name name.func() # Use shorter nickname from module1 import utility as util1 from module2 import utility as util2 util1(); util2() # Can have only 1 "utility" It also comes in handy for providing a short, simple name for an entire directory path when using the package import feature described in Chapter 23: import dir1.dir2.mod as mod mod.func() # Only list full path once Modules Are Objects: Metaprograms Because modules expose most of their interesting properties as built-in attributes, it’s easy to write programs that manage other programs. We usually call such manager programs metaprograms because they work on top of other systems. This is also referred to as introspection, because programs can see and process object internals. Introspection is an advanced feature, but it can be useful for building programming tools. For instance, to get to an attribute called name in a module called M, we can use qualification or index the module’s attribute dictionary, exposed in the built-in __dict__ attribute we met briefly in Chapter 22. Python also exports the list of all loaded modules as the sys.modules dictionary (that is, the modules attribute of the sys module) and provides a built-in called getattr that lets us fetch attributes from their string names (it’s like saying object.attr, but attr is an expression that yields a string at runtime). Because of that, all the following expressions reach the same attribute and object: Modules Are Objects: Metaprograms | 591 M.name M.__dict__['name'] sys.modules['M'].name getattr(M, 'name') # Qualify object # Index namespace dictionary manually # Index loaded-modules table manually # Call built-in fetch function By exposing module internals like this, Python helps you build programs about programs.* For example, here is a module named mydir.py that puts these ideas to work to implement a customized version of the built-in dir function. It defines and exports a function called listing, which takes a module object as an argument and prints a formatted listing of the module’s namespace: """ mydir.py: a module that lists the namespaces of other modules """ seplen = 60 sepchr = '-' def listing(module, verbose=True): sepline = sepchr * seplen if verbose: print(sepline) print('name:', module.__name__, 'file:', module.__file__) print(sepline) count = 0 for attr in module.__dict__: # Scan namespace keys print('%02d) %s' % (count, attr), end = ' ') if attr.startswith('__'): print('<built-in name>') # Skip __file__, etc. else: print(getattr(module, attr)) # Same as .__dict__[attr] count += 1 if verbose: print(sepline) print(module.__name__, 'has %d names' % count) print(sepline) if __name__ == '__main__': import mydir listing(mydir) # Self-test code: list myself Notice the docstring at the top; as in the prior formats.py example, because we may want to use this as a general tool, a docstring is coded to provide functional information accessible via __doc__ attributes or the help function (see Chapter 15 for details): * As we saw in Chapter 17, because a function can access its enclosing module by going through the sys.modules table like this, it’s possible to emulate the effect of the global statement. For instance, the effect of global X; X=0 can be simulated (albeit with much more typing!) by saying this inside a function: import sys; glob=sys.modules[__name__]; glob.X=0. Remember, each module gets a __name__ attribute for free; it’s visible as a global name inside the functions within the module. This trick provides another way to change both local and global variables of the same name inside a function. 592 | Chapter 24: Advanced Module Topics >>> import mydir >>> help(mydir) Help on module mydir: NAME mydir - mydir.py: a module that lists the namespaces of other modules FILE c:\users\veramark\mark\mydir.py FUNCTIONS listing(module, verbose=True) DATA sepchr = '-' seplen = 60 I’ve also provided self-test logic at the bottom of this module, which narcissistically imports and lists itself. Here’s the sort of output produced in Python 3.0 (to use this in 2.6, enable 3.0 print calls with the __future__ import described in Chapter 11—the end keyword is 3.0-only): C:\Users\veramark\Mark> c:\Python30\python mydir.py -----------------------------------------------------------name: mydir file: C:\Users\veramark\Mark\mydir.py -----------------------------------------------------------00) seplen 60 01) __builtins__ <built-in name> 02) __file__ <built-in name> 03) __package__ <built-in name> 04) listing <function listing at 0x026D3B70> 05) __name__ <built-in name> 06) sepchr 07) __doc__ <built-in name> -----------------------------------------------------------mydir has 8 names ------------------------------------------------------------ To use this as a tool for listing other modules, simply pass the modules in as objects to this file’s function. Here it is listing attributes in the tkinter GUI module in the standard library (a.k.a. Tkinter in Python 2.6): >>> import mydir >>> import tkinter >>> mydir.listing(tkinter) -----------------------------------------------------------name: tkinter file: c:\PYTHON30\lib\tkinter\__init__.py -----------------------------------------------------------00) getdouble <class 'float'> 01) MULTIPLE multiple 02) mainloop <function mainloop at 0x02913B70> 03) Canvas <class 'tkinter.Canvas'> 04) AtSelLast <function AtSelLast at 0x028FA7C8> ...many more name omitted... 151) StringVar <class 'tkinter.StringVar'> Modules Are Objects: Metaprograms | 593 152) ARC arc 153) At <function At at 0x028FA738> 154) NSEW nsew 155) SCROLL scroll -----------------------------------------------------------tkinter has 156 names ------------------------------------------------------------ We’ll meet getattr and its relatives again later. The point to notice here is that mydir is a program that lets you browse other programs. Because Python exposes its internals, you can process objects generically.† Importing Modules by Name String The module name in an import or from statement is a hardcoded variable name. Sometimes, though, your program will get the name of a module to be imported as a string at runtime (e.g., if a user selects a module name from within a GUI). Unfortunately, you can’t use import statements directly to load a module given its name as a string— Python expects a variable name, not a string. For instance: >>> import "string" File "<stdin>", line 1 import "string" ^ SyntaxError: invalid syntax It also won’t work to simply assign the string to a variable name: x = "string" import x Here, Python will try to import a file x.py, not the string module—the name in an import statement both becomes a variable assigned to the loaded module and identifies the external file literally. To get around this, you need to use special tools to load a module dynamically from a string that is generated at runtime. The most general approach is to construct an import statement as a string of Python code and pass it to the exec built-in function to run (exec is a statement in Python 2.6, but it can be used exactly as shown here—the parentheses are simply ignored): >>> modname = "string" >>> exec("import " + modname) # Run a string of code >>> string # Imported in this namespace <module 'string' from 'c:\Python30\lib\string.py'> † Tools such as mydir.listing can be preloaded into the interactive namespace by importing them in the file referenced by the PYTHONSTARTUP environment variable. Because code in the startup file runs in the interactive namespace (module __main__), importing common tools in the startup file can save you some typing. See Appendix A for more details. 594 | Chapter 24: Advanced Module Topics The exec function (and its cousin for expressions, eval) compiles a string of code and passes it to the Python interpreter to be executed. In Python, the byte code compiler is available at runtime, so you can write programs that construct and run other programs like this. By default, exec runs the code in the current scope, but you can get more specific by passing in optional namespace dictionaries. The only real drawback to exec is that it must compile the import statement each time it runs; if it runs many times, your code may run quicker if it uses the built-in __import__ function to load from a name string instead. The effect is similar, but __import__ returns the module object, so assign it to a name here to keep it: >>> modname = "string" >>> string = __import__(modname) >>> string <module 'string' from 'c:\Python30\lib\string.py'> Transitive Module Reloads We studied module reloads in Chapter 22, as a way to pick up changes in code without stopping and restarting a program. When you reload a module, though, Python only reloads that particular module’s file; it doesn’t automatically reload modules that the file being reloaded happens to import. For example, if you reload some module A, and A imports modules B and C, the reload applies only to A, not to B and C. The statements inside A that import B and C are rerun during the reload, but they just fetch the already loaded B and C module objects (assuming they’ve been imported before). In actual code, here’s the file A.py: import B import C # Not reloaded when A is # Just an import of an already loaded module % python >>> . . . >>> from imp import reload >>> reload(A) By default, this means that you cannot depend on reloads picking up changes in all the modules in your program transitively—instead, you must use multiple reload calls to update the subcomponents independently. This can require substantial work for large systems you’re testing interactively. You can design your systems to reload their subcomponents automatically by adding reload calls in parent modules like A, but this complicates the modules’ code. A better approach is to write a general tool to do transitive reloads automatically by scanning modules’ __dict__ attributes and checking each item’s type to find nested modules to reload. Such a utility function could call itself recursively to navigate arbitrarily shaped import dependency chains. Module __dict__ attributes were introduced earlier in, the section “Modules Are Objects: Metaprograms” on page 591, and the type call was presented in Chapter 9; we just need to combine the two tools. Transitive Module Reloads | 595 For example, the module reloadall.py listed next has a reload_all function that automatically reloads a module, every module that the module imports, and so on, all the way to the bottom of each import chain. It uses a dictionary to keep track of already reloaded modules, recursion to walk the import chains, and the standard library’s types module, which simply predefines type results for built-in types. The visited dictionary technique works to avoid cycles here when imports are recursive or redundant, because module objects can be dictionary keys (as we learned in Chapter 5, a set would offer similar functionality if we use visited.add(module) to insert): """ reloadall.py: transitively reload nested modules """ import types from imp import reload # from required in 3.0 def status(module): print('reloading ' + module.__name__) def transitive_reload(module, visited): if not module in visited: status(module) reload(module) visited[module] = None for attrobj in module.__dict__.values(): if type(attrobj) == types.ModuleType: transitive_reload(attrobj, visited) # Trap cycles, duplicates # Reload this module # And visit children # For all attrs # Recur if module def reload_all(*args): visited = {} for arg in args: if type(arg) == types.ModuleType: transitive_reload(arg, visited) if __name__ == '__main__': import reloadall reload_all(reloadall) # Test code: reload myself # Should reload this, types To use this utility, import its reload_all function and pass it the name of an already loaded module (like you would the built-in reload function). When the file runs standalone, its self-test code will test itself—it has to import itself because its own name is not defined in the file without an import (this code works in both 3.0 and 2.6 and prints identical output, because we’ve used + instead of a comma in the print): C:\misc> c:\Python30\python reloadall.py reloading reloadall reloading types Here is this module at work in 3.0 on some standard library modules. Notice how os is imported by tkinter, but tkinter reaches sys before os can (if you want to test this on Python 2.6, substitute Tkinter for tkinter): 596 | Chapter 24: Advanced Module Topics >>> from reloadall import reload_all >>> import os, tkinter >>> reload_all(os) reloading os reloading copyreg reloading ntpath reloading genericpath reloading stat reloading sys reloading errno >>> reload_all(tkinter) reloading tkinter reloading _tkinter reloading tkinter._fix reloading sys reloading ctypes reloading os reloading copyreg reloading ntpath reloading genericpath reloading stat reloading errno reloading ctypes._endian reloading tkinter.constants And here is a session that shows the effect of normal versus transitive reloads—changes made to the two nested files are not picked up by reloads, unless the transitive utility is used: import b X = 1 # a.py import c Y = 2 # b.py Z = 3 # c.py C:\misc> C:\Python30\python >>> import a >>> a.X, a.b.Y, a.b.c.Z (1, 2, 3) # Change all three files' assignment values and save >>> from imp import reload >>> reload(a) <module 'a' from 'a.py'> >>> a.X, a.b.Y, a.b.c.Z (111, 2, 3) # Normal reload is top level only >>> from reloadall import reload_all >>> reload_all(a) reloading a Transitive Module Reloads | 597 reloading b reloading c >>> a.X, a.b.Y, a.b.c.Z (111, 222, 333) # Reloads all nested modules too For more insight, study and experiment with this example on your own; it’s another importable tool you might want to add to your own source code library. Module Design Concepts Like functions, modules present design tradeoffs: you have to think about which functions go in which modules, module communication mechanisms, and so on. All of this will become clearer when you start writing bigger Python systems, but here are a few general ideas to keep in mind: • You’re always in a module in Python. There’s no way to write code that doesn’t live in some module. In fact, code typed at the interactive prompt really goes in a built-in module called __main__; the only unique things about the interactive prompt are that code runs and is discarded immediately, and expression results are printed automatically. • Minimize module coupling: global variables. Like functions, modules work best if they’re written to be closed boxes. As a rule of thumb, they should be as independent of global variables used within other modules as possible, except for functions and classes imported from them. • Maximize module cohesion: unified purpose. You can minimize a module’s couplings by maximizing its cohesion; if all the components of a module share a general purpose, you’re less likely to depend on external names. • Modules should rarely change other modules’ variables. We illustrated this with code in Chapter 17, but it’s worth repeating here: it’s perfectly OK to use globals defined in another module (that’s how clients import services, after all), but changing globals in another module is often a symptom of a design problem. There are exceptions, of course, but you should try to communicate results through devices such as function arguments and return values, not cross-module changes. Otherwise, your globals’ values become dependent on the order of arbitrarily remote assignments in other files, and your modules become harder to understand and reuse. As a summary, Figure 24-1 sketches the environment in which modules operate. Modules contain variables, functions, classes, and other modules (if imported). Functions have local variables of their own, as do classes—i.e., objects that live within modules, which we’ll meet next in Chapter 25. 598 | Chapter 24: Advanced Module Topics Figure 24-1. Module execution environment. Modules are imported, but modules also import and use other modules, which may be coded in Python or another language such as C. Modules in turn contain variables, functions, and classes to do their work, and their functions and classes may contain variables and other items of their own. At the top, though, programs are just sets of modules. Module Gotchas In this section, we’ll take a look at the usual collection of boundary cases that make life interesting for Python beginners. Some are so obscure that it was hard to come up with examples, but most illustrate something important about the language. Statement Order Matters in Top-Level Code When a module is first imported (or reloaded), Python executes its statements one by one, from the top of the file to the bottom. This has a few subtle implications regarding forward references that are worth underscoring here: • Code at the top level of a module file (not nested in a function) runs as soon as Python reaches it during an import; because of that, it can’t reference names assigned lower in the file. • Code inside a function body doesn’t run until the function is called; because names in a function aren’t resolved until the function actually runs, they can usually reference names anywhere in the file. Generally, forward references are only a concern in top-level module code that executes immediately; functions can reference names arbitrarily. Here’s an example that illustrates forward reference: Module Gotchas | 599 func1() # Error: "func1" not yet assigned def func1(): print(func2()) # Okay: "func2" looked up later func1() # Error: "func2" not yet assigned def func2(): return "Hello" func1() # Okay: "func1" and "func2" assigned When this file is imported (or run as a standalone program), Python executes its statements from top to bottom. The first call to func1 fails because the func1 def hasn’t run yet. The call to func2 inside func1 works as long as func2’s def has been reached by the time func1 is called (it hasn’t when the second top-level func1 call is run). The last call to func1 at the bottom of the file works because func1 and func2 have both been assigned. Mixing defs with top-level code is not only hard to read, it’s dependent on statement ordering. As a rule of thumb, if you need to mix immediate code with defs, put your defs at the top of the file and your top-level code at the bottom. That way, your functions are guaranteed to be defined and assigned by the time code that uses them runs. from Copies Names but Doesn’t Link Although it’s commonly used, the from statement is the source of a variety of potential gotchas in Python. The from statement is really an assignment to names in the importer’s scope—a name-copy operation, not a name aliasing. The implications of this are the same as for all assignments in Python, but they’re subtle, especially given that the code that shares the objects lives in different files. For instance, suppose we define the following module, nested1.py: # nested1.py X = 99 def printer(): print(X) If we import its two names using from in another module, nested2.py, we get copies of those names, not links to them. Changing a name in the importer resets only the binding of the local version of that name, not the name in nested1.py: # nested2.py from nested1 import X, printer X = 88 printer() % python nested2.py 99 600 | Chapter 24: Advanced Module Topics # Copy names out # Changes my "X" only! # nested1's X is still 99 If we use import to get the whole module and then assign to a qualified name, however, we change the name in nested1.py. Qualification directs Python to a name in the module object, rather than a name in the importer, nested3.py: # nested3.py import nested1 nested1.X = 88 nested1.printer() # Get module as a whole # OK: change nested1's X % python nested3.py 88 from * Can Obscure the Meaning of Variables I mentioned this earlier but saved the details for here. Because you don’t list the variables you want when using the from module import * statement form, it can accidentally overwrite names you’re already using in your scope. Worse, it can make it difficult to determine where a variable comes from. This is especially true if the from * form is used on more than one imported file. For example, if you use from * on three modules, you’ll have no way of knowing what a raw function call really means, short of searching all three external module files (all of which may be in other directories): >>> >>> >>> >>> from module1 import * from module2 import * from module3 import * . . . >>> func() # Bad: may overwrite my names silently # Worse: no way to tell what we get! # Huh??? The solution again is not to do this: try to explicitly list the attributes you want in your from statements, and restrict the from * form to at most one imported module per file. That way, any undefined names must by deduction be in the module named in the single from *. You can avoid the issue altogether if you always use import instead of from, but that advice is too harsh; like much else in programming, from is a convenient tool if used wisely. Even this example isn’t an absolute evil—it’s OK for a program to use this technique to collect names in a single space for convenience, as long as it’s well known. reload May Not Impact from Imports Here’s another from-related gotcha: as discussed previously, because from copies (assigns) names when run, there’s no link back to the modules where the names came from. Names imported with from simply become references to objects, which happen to have been referenced by the same names in the importee when the from ran. Module Gotchas | 601 Because of this behavior, reloading the importee has no effect on clients that import its names using from. That is, the client’s names will still reference the original objects fetched with from, even if the names in the original module are later reset: from module import X . . . from imp import reload reload(module) X # X may not reflect any module reloads! # Changes module, but not my names # Still references old object To make reloads more effective, use import and name qualification instead of from. Because qualifications always go back to the module, they will find the new bindings of module names after reloading: import module . . . from imp import reload reload(module) module.X # Get module, not names # Changes module in-place # Get current X: reflects module reloads reload, from, and Interactive Testing In fact, the prior gotcha is even more subtle than it appears. Chapter 3 warned that it’s usually better not to launch programs with imports and reloads because of the complexities involved. Things get even worse when from is brought into the mix. Python beginners often stumble onto its issues in scenarios like the one outlined next. Say that after opening a module file in a text edit window, you launch an interactive session to load and test your module with from: from module import function function(1, 2, 3) Finding a bug, you jump back to the edit window, make a change, and try to reload the module this way: from imp import reload reload(module) This doesn’t work, because the from statement assigned the name function, not module. To refer to the module in a reload, you have to first load it with an import statement at least once: from imp import reload import module reload(module) function(1, 2, 3) However, this doesn’t quite work either—reload updates the module object, but as discussed in the preceding section, names like function that were copied out of the module in the past still refer to the old objects (in this instance, the original version of the function). To really get the new function, you must refer to it as module.function after the reload, or rerun the from: 602 | Chapter 24: Advanced Module Topics from imp import reload import module reload(module) from module import function function(1, 2, 3) # Or give up and use module.function() Now, the new version of the function will finally run. As you can see, there are problems inherent in using reload with from: not only do you have to remember to reload after imports, but you also have to remember to rerun your from statements after reloads. This is complex enough to trip up even an expert once in a while. (In fact, the situation has gotten even worse in Python 3.0, because you must also remember to import reload itself!) The short story is that you should not expect reload and from to play together nicely. The best policy is not to combine them at all—use reload with import, or launch your programs other ways, as suggested in Chapter 3: using the Run→Run Module menu option in IDLE, file icon clicks, system command lines, or the exec built-in function. Recursive from Imports May Not Work I saved the most bizarre (and, thankfully, obscure) gotcha for last. Because imports execute a file’s statements from top to bottom, you need to be careful when using modules that import each other (known as recursive imports). Because the statements in a module may not all have been run when it imports another module, some of its names may not yet exist. If you use import to fetch the module as a whole, this may or may not matter; the module’s names won’t be accessed until you later use qualification to fetch their values. But if you use from to fetch specific names, you must bear in mind that you will only have access to names in that module that have already been assigned. For instance, take the following modules, recur1 and recur2. recur1 assigns a name X, and then imports recur2 before assigning the name Y. At this point, recur2 can fetch recur1 as a whole with an import (it already exists in Python’s internal modules table), but if it uses from, it will be able to see only the name X; the name Y, which is assigned below the import in recur1, doesn’t yet exist, so you get an error: # recur1.py X = 1 import recur2 Y = 2 # recur2.py from recur1 import X from recur1 import Y # Run recur2 now if it doesn't exist # OK: "X" already assigned # Error: "Y" not yet assigned C:\misc> C:\Python30\python >>> import recur1 Traceback (most recent call last): File "<stdin>", line 1, in <module> Module Gotchas | 603 File "recur1.py", line 2, in <module> import recur2 File "recur2.py", line 2, in <module> from recur1 import Y ImportError: cannot import name Y Python avoids rerunning recur1’s statements when they are imported recursively from recur2 (otherwise the imports would send the script into an infinite loop), but recur1’s namespace is incomplete when it’s imported by recur2. The solution? Don’t use from in recursive imports (no, really!). Python won’t get stuck in a cycle if you do, but your programs will once again be dependent on the order of the statements in the modules. There are two ways out of this gotcha: • You can usually eliminate import cycles like this by careful design—maximizing cohesion and minimizing coupling are good first steps. • If you can’t break the cycles completely, postpone module name accesses by using import and qualification (instead of from), or by running your froms either inside functions (instead of at the top level of the module), or near the bottom of your file to defer their execution. Chapter Summary This chapter surveyed some more advanced module-related concepts. We studied data hiding techniques, enabling new language features with the __future__ module, the __name__ usage mode variable, transitive reloads, importing by name strings, and more. We also explored and summarized module design issues and looked at common mistakes related to modules to help you avoid them in your code. The next chapter begins our look at Python’s object-oriented programming tool, the class. Much of what we’ve covered in the last few chapters will apply there, too—classes live in modules and are namespaces as well, but they add an extra component to attribute lookup called inheritance search. As this is the last chapter in this part of the book, however, before we dive into that topic, be sure to work through this part’s set of lab exercises. And before that, here is this chapter’s quiz to review the topics covered here. Test Your Knowledge: Quiz 1. What is significant about variables at the top level of a module whose names begin with a single underscore? 2. What does it mean when a module’s __name__ variable is the string "__main__"? 604 | Chapter 24: Advanced Module Topics 3. If the user interactively types the name of a module to test, how can you import it? 4. How is changing sys.path different from setting PYTHONPATH to modify the module search path? 5. If the module __future__ allows us to import from the future, can we also import from the past? Test Your Knowledge: Answers 1. Variables at the top level of a module whose names begin with a single underscore are not copied out to the importing scope when the from * statement form is used. They can still be accessed by an import or the normal from statement form, though. 2. If a module’s __name__ variable is the string "__main__", it means that the file is being executed as a top-level script instead of being imported from another file in the program. That is, the file is being used as a program, not a library. 3. User input usually comes into a script as a string; to import the referenced module given its string name, you can build and run an import statement with exec, or pass the string name in a call to the __import__ function. 4. Changing sys.path only affects one running program, and is temporary—the change goes away when the program ends. PYTHONPATH settings live in the operating system—they are picked up globally by all programs on a machine, and changes to these settings endure after programs exit. 5. No, we can’t import from the past in Python. We can install (or stubbornly use) an older version of the language, but the latest Python is generally the best Python. Test Your Knowledge: Part V Exercises See “Part V, Modules” on page 1119 in Appendix B for the solutions. 1. Import basics. Write a program that counts the lines and characters in a file (similar in spirit to wc on Unix). With your text editor, code a Python module called mymod.py that exports three top-level names: • A countLines(name) function that reads an input file and counts the number of lines in it (hint: file.readlines does most of the work for you, and len does the rest). • A countChars(name) function that reads an input file and counts the number of characters in it (hint: file.read returns a single string). • A test(name) function that calls both counting functions with a given input filename. Such a filename generally might be passed in, hardcoded, input with the input built-in function, or pulled from a command line via the sys.argv list shown in this chapter’s formats.py example; for now, you can assume it’s a passed-in function argument. Test Your Knowledge: Part V Exercises | 605 All three mymod functions should expect a filename string to be passed in. If you type more than two or three lines per function, you’re working much too hard— use the hints I just gave! 2. 3. 4. 5. 6. Next, test your module interactively, using import and attribute references to fetch your exports. Does your PYTHONPATH need to include the directory where you created mymod.py? Try running your module on itself: e.g., test("mymod.py"). Note that test opens the file twice; if you’re feeling ambitious, you may be able to improve this by passing an open file object into the two count functions (hint: file.seek(0) is a file rewind). from/from *. Test your mymod module from exercise 1 interactively by using from to load the exports directly, first by name, then using the from * variant to fetch everything. __main__. Add a line in your mymod module that calls the test function automatically only when the module is run as a script, not when it is imported. The line you add will probably test the value of __name__ for the string "__main__", as shown in this chapter. Try running your module from the system command line; then, import the module and test its functions interactively. Does it still work in both modes? Nested imports. Write a second module, myclient.py, that imports mymod and tests its functions; then run myclient from the system command line. If myclient uses from to fetch from mymod, will mymod’s functions be accessible from the top level of myclient? What if it imports with import instead? Try coding both variations in myclient and test interactively by importing myclient and inspecting its __dict__ attribute. Package imports. Import your file from a package. Create a subdirectory called mypkg nested in a directory on your module import search path, move the mymod.py module file you created in exercise 1 or 3 into the new directory, and try to import it with a package import of the form import mypkg.mymod. You’ll need to add an __init__.py file in the directory your module was moved to make this go, but it should work on all major Python platforms (that’s part of the reason Python uses “.” as a path separator). The package directory you create can be simply a subdirectory of the one you’re working in; if it is, it will be found via the home directory component of the search path, and you won’t have to configure your path. Add some code to your __init__.py, and see if it runs on each import. Reloads. Experiment with module reloads: perform the tests in Chapter 22’s changer.py example, changing the called function’s message and/or behavior repeatedly, without stopping the Python interpreter. Depending on your system, you might be able to edit changer in another window, or suspend the Python interpreter and edit in the same window (on Unix, a Ctrl-Z key combination usually suspends the current process, and an fg command later resumes it). 606 | Chapter 24: Advanced Module Topics 7. Circular imports.‡ In the section on recursive import gotchas, importing recur1 raised an error. But if you restart Python and import recur2 interactively, the error doesn’t occur—test this and see for yourself. Why do you think it works to import recur2, but not recur1? (Hint: Python stores new modules in the built-in sys.modules table—a dictionary—before running their code; later imports fetch the module from this table first, whether the module is “complete” yet or not.) Now, try running recur1 as a top-level script file: python recur1.py. Do you get the same error that occurs when recur1 is imported interactively? Why? (Hint: when modules are run as programs, they aren’t imported, so this case has the same effect as importing recur2 interactively; recur2 is the first module imported.) What happens when you run recur2 as a script? ‡ Note that circular imports are extremely rare in practice. On the other hand, if you can understand why they are a potential problem, you know a lot about Python’s import semantics. Test Your Knowledge: Part V Exercises | 607 PART VI Classes and OOP CHAPTER 25 OOP: The Big Picture So far in this book, we’ve been using the term “object” generically. Really, the code written up to this point has been object-based—we’ve passed objects around our scripts, used them in expressions, called their methods, and so on. For our code to qualify as being truly object-oriented (OO), though, our objects will generally need to also participate in something called an inheritance hierarchy. This chapter begins our exploration of the Python class—a device used to implement new kinds of objects in Python that support inheritance. Classes are Python’s main object-oriented programming (OOP) tool, so we’ll also look at OOP basics along the way in this part of the book. OOP offers a different and often more effective way of looking at programming, in which we factor code to minimize redundancy, and write new programs by customizing existing code instead of changing it in-place. In Python, classes are created with a new statement: the class statement. As you’ll see, the objects defined with classes can look a lot like the built-in types we studied earlier in the book. In fact, classes really just apply and extend the ideas we’ve already covered; roughly, they are packages of functions that use and process built-in object types. Classes, though, are designed to create and manage new objects, and they also support inheritance—a mechanism of code customization and reuse above and beyond anything we’ve seen so far. One note up front: in Python, OOP is entirely optional, and you don’t need to use classes just to get started. In fact, you can get plenty of work done with simpler constructs such as functions, or even simple top-level script code. Because using classes well requires some up-front planning, they tend to be of more interest to people who work in strategic mode (doing long-term product development) than to people who work in tactical mode (where time is in very short supply). Still, as you’ll see in this part of the book, classes turn out to be one of the most useful tools Python provides. When used well, classes can actually cut development time radically. They’re also employed in popular Python tools like the tkinter GUI API, so most Python programmers will usually find at least a working knowledge of class basics helpful. 611 Why Use Classes? Remember when I told you that programs “do things with stuff”? In simple terms, classes are just a way to define new sorts of stuff, reflecting real objects in a program’s domain. For instance, suppose we decide to implement that hypothetical pizza-making robot we used as an example in Chapter 16. If we implement it using classes, we can model more of its real-world structure and relationships. Two aspects of OOP prove useful here: Inheritance Pizza-making robots are kinds of robots, so they possess the usual robot-y properties. In OOP terms, we say they “inherit” properties from the general category of all robots. These common properties need to be implemented only once for the general case and can be reused by all types of robots we may build in the future. Composition Pizza-making robots are really collections of components that work together as a team. For instance, for our robot to be successful, it might need arms to roll dough, motors to maneuver to the oven, and so on. In OOP parlance, our robot is an example of composition; it contains other objects that it activates to do its bidding. Each component might be coded as a class, which defines its own behavior and relationships. General OOP ideas like inheritance and composition apply to any application that can be decomposed into a set of objects. For example, in typical GUI systems, interfaces are written as collections of widgets—buttons, labels, and so on—which are all drawn when their container is drawn (composition). Moreover, we may be able to write our own custom widgets—buttons with unique fonts, labels with new color schemes, and the like—which are specialized versions of more general interface devices (inheritance). From a more concrete programming perspective, classes are Python program units, just like functions and modules: they are another compartment for packaging logic and data. In fact, classes also define new namespaces, much like modules. But, compared to other program units we’ve already seen, classes have three critical distinctions that make them more useful when it comes to building new objects: Multiple instances Classes are essentially factories for generating one or more objects. Every time we call a class, we generate a new object with a distinct namespace. Each object generated from a class has access to the class’s attributes and gets a namespace of its own for data that varies per object. Customization via inheritance Classes also support the OOP notion of inheritance; we can extend a class by redefining its attributes outside the class itself. More generally, classes can build up namespace hierarchies, which define names to be used by objects created from classes in the hierarchy. 612 | Chapter 25: OOP: The Big Picture Operator overloading By providing special protocol methods, classes can define objects that respond to the sorts of operations we saw at work on built-in types. For instance, objects made with classes can be sliced, concatenated, indexed, and so on. Python provides hooks that classes can use to intercept and implement any built-in type operation. OOP from 30,000 Feet Before we see what this all means in terms of code, I’d like to say a few words about the general ideas behind OOP. If you’ve never done anything object-oriented in your life before now, some of the terminology in this chapter may seem a bit perplexing on the first pass. Moreover, the motivation for these terms may be elusive until you’ve had a chance to study the ways that programmers apply them in larger systems. OOP is as much an experience as a technology. Attribute Inheritance Search The good news is that OOP is much simpler to understand and use in Python than in other languages, such as C++ or Java. As a dynamically typed scripting language, Python removes much of the syntactic clutter and complexity that clouds OOP in other tools. In fact, most of the OOP story in Python boils down to this expression: object.attribute We’ve been using this expression throughout the book to access module attributes, call methods of objects, and so on. When we say this to an object that is derived from a class statement, however, the expression kicks off a search in Python—it searches a tree of linked objects, looking for the first appearance of attribute that it can find. When classes are involved, the preceding Python expression effectively translates to the following in natural language: Find the first occurrence of attribute by looking in object, then in all classes above it, from bottom to top and left to right. In other words, attribute fetches are simply tree searches. The term inheritance is applied because objects lower in a tree inherit attributes attached to objects higher in that tree. As the search proceeds from the bottom up, in a sense, the objects linked into a tree are the union of all the attributes defined in all their tree parents, all the way up the tree. In Python, this is all very literal: we really do build up trees of linked objects with code, and Python really does climb this tree at runtime searching for attributes every time we use the object.attribute expression. To make this more concrete, Figure 25-1 sketches an example of one of these trees. In this figure, there is a tree of five objects labeled with variables, all of which have attached attributes, ready to be searched. More specifically, this tree links together three OOP from 30,000 Feet | 613 Figure 25-1. A class tree, with two instances at the bottom (I1 and I2), a class above them (C1), and two superclasses at the top (C2 and C3). All of these objects are namespaces (packages of variables), and the inheritance search is simply a search of the tree from bottom to top looking for the lowest occurrence of an attribute name. Code implies the shape of such trees. class objects (the ovals C1, C2, and C3) and two instance objects (the rectangles I1 and I2) into an inheritance search tree. Notice that in the Python object model, classes and the instances you generate from them are two distinct object types: Classes Serve as instance factories. Their attributes provide behavior—data and functions—that is inherited by all the instances generated from them (e.g., a function to compute an employee’s salary from pay and hours). Instances Represent the concrete items in a program’s domain. Their attributes record data that varies per specific object (e.g., an employee’s Social Security number). In terms of search trees, an instance inherits attributes from its class, and a class inherits attributes from all classes above it in the tree. In Figure 25-1, we can further categorize the ovals by their relative positions in the tree. We usually call classes higher in the tree (like C2 and C3) superclasses; classes lower in the tree (like C1) are known as subclasses.* These terms refer to relative tree positions and roles. Superclasses provide behavior shared by all their subclasses, but because the search proceeds from the bottom up, subclasses may override behavior defined in their superclasses by redefining superclass names lower in the tree. As these last few words are really the crux of the matter of software customization in OOP, let’s expand on this concept. Suppose we build up the tree in Figure 25-1, and then say this: I2.w * In other literature, you may also occasionally see the terms base classes and derived classes used to describe superclasses and subclasses, respectively. 614 | Chapter 25: OOP: The Big Picture Right away, this code invokes inheritance. Because this is an object.attribute expression, it triggers a search of the tree in Figure 25-1—Python will search for the attribute w by looking in I2 and above. Specifically, it will search the linked objects in this order: I2, C1, C2, C3 and stop at the first attached w it finds (or raise an error if w isn’t found at all). In this case, w won’t be found until C3 is searched because it appears only in that object. In other words, I2.w resolves to C3.w by virtue of the automatic search. In OOP terminology, I2 “inherits” the attribute w from C3. Ultimately, the two instances inherit four attributes from their classes: w, x, y, and z. Other attribute references will wind up following different paths in the tree. For example: • • • • I1.x and I2.x both find x in C1 and stop because C1 is lower than C2. I1.y and I2.y both find y in C1 because that’s the only place y appears. I1.z and I2.z both find z in C2 because C2 is further to the left than C3. I2.name finds name in I2 without climbing the tree at all. Trace these searches through the tree in Figure 25-1 to get a feel for how inheritance searches work in Python. The first item in the preceding list is perhaps the most important to notice—because C1 redefines the attribute x lower in the tree, it effectively replaces the version above it in C2. As you’ll see in a moment, such redefinitions are at the heart of software customization in OOP—by redefining and replacing the attribute, C1 effectively customizes what it inherits from its superclasses. Classes and Instances Although they are technically two separate object types in the Python model, the classes and instances we put in these trees are almost identical—each type’s main purpose is to serve as another kind of namespace—a package of variables, and a place where we can attach attributes. If classes and instances therefore sound like modules, they should; however, the objects in class trees also have automatically searched links to other namespace objects, and classes correspond to statements, not entire files. The primary difference between classes and instances is that classes are a kind of factory for generating instances. For example, in a realistic application, we might have an Employee class that defines what it means to be an employee; from that class, we generate actual Employee instances. This is another difference between classes and modules: we only ever have one instance of a given module in memory (that’s why we have to reload a module to get its new code), but with classes, we can make as many instances as we need. OOP from 30,000 Feet | 615 Operationally, classes will usually have functions attached to them (e.g., computeSalary), and the instances will have more basic data items used by the class’ functions (e.g., hoursWorked). In fact, the object-oriented model is not that different from the classic data-processing model of programs plus records; in OOP, instances are like records with “data,” and classes are the “programs” for processing those records. In OOP, though, we also have the notion of an inheritance hierarchy, which supports software customization better than earlier models. Class Method Calls In the prior section, we saw how the attribute reference I2.w in our example class tree was translated to C3.w by the inheritance search procedure in Python. Perhaps just as important to understand as the inheritance of attributes, though, is what happens when we try to call methods (i.e., functions attached to classes as attributes). If this I2.w reference is a function call, what it really means is “call the C3.w function to process I2.” That is, Python will automatically map the call I2.w() into the call C3.w(I2), passing in the instance as the first argument to the inherited function. In fact, whenever we call a function attached to a class in this fashion, an instance of the class is always implied. This implied subject or context is part of the reason we refer to this as an object-oriented model—there is always a subject object when an operation is run. In a more realistic example, we might invoke a method called giveRaise attached as an attribute to an Employee class; such a call has no meaning unless qualified with the employee to whom the raise should be given. As we’ll see later, Python passes in the implied instance to a special first argument in the method, called self by convention. As we’ll also learn, methods can be called through either an instance (e.g., bob.giveRaise()) or a class (e.g., Employee.giveRaise(bob)), and both forms serve purposes in our scripts. To see how methods receive their subjects, though, we need to move on to some code. Coding Class Trees Although we are speaking in the abstract here, there is tangible code behind all these ideas. We construct trees, and their objects with class statements and class calls, which we’ll meet in more detail later. In short: • • • • Each class statement generates a new class object. Each time a class is called, it generates a new instance object. Instances are automatically linked to the classes from which they are created. Classes are linked to their superclasses by listing them in parentheses in a class header line; the left-to-right order there gives the order in the tree. 616 | Chapter 25: OOP: The Big Picture To build the tree in Figure 25-1, for example, we would run Python code of this form (I’ve omitted the guts of the class statements here): class C2: ... class C3: ... class C1(C2, C3): ... # Make class objects (ovals) I1 = C1() I2 = C1() # Make instance objects (rectangles) # Linked to their classes # Linked to superclasses Here, we build the three class objects by running three class statements, and make the two instance objects by calling the class C1 twice, as though it were a function. The instances remember the class they were made from, and the class C1 remembers its listed superclasses. Technically, this example is using something called multiple inheritance, which simply means that a class has more than one superclass above it in the class tree. In Python, if there is more than one superclass listed in parentheses in a class statement (like C1’s here), their left-to-right order gives the order in which those superclasses will be searched for attributes. Because of the way inheritance searches proceed, the object to which you attach an attribute turns out to be crucial—it determines the name’s scope. Attributes attached to instances pertain only to those single instances, but attributes attached to classes are shared by all their subclasses and instances. Later, we’ll study the code that hangs attributes on these objects in depth. As we’ll find: • Attributes are usually attached to classes by assignments made within class statements, and not nested inside function def statements. • Attributes are usually attached to instances by assignments to a special argument passed to functions inside classes, called self. For example, classes provide behavior for their instances with functions created by coding def statements inside class statements. Because such nested defs assign names within the class, they wind up attaching attributes to the class object that will be inherited by all instances and subclasses: class C1(C2, C3): def setname(self, who): self.name = who # Make and link class C1 # Assign name: C1.setname # Self is either I1 or I2 I1 = C1() I2 = C1() I1.setname('bob') I2.setname('mel') print(I1.name) # Make two instances # Sets I1.name to 'bob' # Sets I2.name to 'mel' # Prints 'bob' OOP from 30,000 Feet | 617 There’s nothing syntactically unique about def in this context. Operationally, when a def appears inside a class like this, it is usually known as a method, and it automatically receives a special first argument—called self by convention—that provides a handle back to the instance to be processed.† Because classes are factories for multiple instances, their methods usually go through this automatically passed-in self argument whenever they need to fetch or set attributes of the particular instance being processed by a method call. In the preceding code, self is used to store a name in one of two instances. Like simple variables, attributes of classes and instances are not declared ahead of time, but spring into existence the first time they are assigned values. When a method assigns to a self attribute, it creates or changes an attribute in an instance at the bottom of the class tree (i.e., one of the rectangles) because self automatically refers to the instance being processed. In fact, because all the objects in class trees are just namespace objects, we can fetch or set any of their attributes by going through the appropriate names. Saying C1.setname is as valid as saying I1.setname, as long as the names C1 and I1 are in your code’s scopes. As currently coded, our C1 class doesn’t attach a name attribute to an instance until the setname method is called. In fact, referencing I1.name before calling I1.setname would produce an undefined name error. If a class wants to guarantee that an attribute like name is always set in its instances, it more typically will fill out the attribute at construction time, like this: class C1(C2, C3): def __init__(self, who): self.name = who # Set name when constructed # Self is either I1 or I2 I1 = C1('bob') I2 = C1('mel') print(I1.name) # Sets I1.name to 'bob' # Sets I2.name to 'mel' # Prints 'bob' If it’s coded and inherited, Python automatically calls a method named __init__ each time an instance is generated from a class. The new instance is passed in to the self argument of __init__ as usual, and any values listed in parentheses in the class call go to arguments two and beyond. The effect here is to initialize instances when they are made, without requiring extra method calls. The __init__ method is known as the constructor because of when it is run. It’s the most commonly used representative of a larger class of methods called operator overloading methods, which we’ll discuss in more detail in the chapters that follow. Such methods are inherited in class trees as usual and have double underscores at the start and end of their names to make them distinct. Python runs them automatically when instances that support them appear in the corresponding operations, and they are † If you’ve ever used C++ or Java, you’ll recognize that Python’s self is the same as the this pointer, but self is always explicit in Python to make attribute accesses more obvious. 618 | Chapter 25: OOP: The Big Picture mostly an alternative to using simple method calls. They’re also optional: if omitted, the operations are not supported. For example, to implement set intersection, a class might either provide a method named intersect, or overload the & expression operator to dispatch to the required logic by coding a method named __and__. Because the operator scheme makes instances look and feel more like built-in types, it allows some classes to provide a consistent and natural interface, and be compatible with code that expects a built-in type. OOP Is About Code Reuse And that, along with a few syntax details, is most of the OOP story in Python. Of course, there’s a bit more to it than just inheritance. For example, operator overloading is much more general than I’ve described so far—classes may also provide their own implementations of operations such as indexing, fetching attributes, printing, and more. By and large, though, OOP is about looking up attributes in trees. So why would we be interested in building and searching trees of objects? Although it takes some experience to see how, when used well, classes support code reuse in ways that other Python program components cannot. With classes, we code by customizing existing software, instead of either changing existing code in-place or starting from scratch for each new project. At a fundamental level, classes are really just packages of functions and other names, much like modules. However, the automatic attribute inheritance search that we get with classes supports customization of software above and beyond what we can do with modules and functions. Moreover, classes provide a natural structure for code that localizes logic and names, and so aids in debugging. For instance, because methods are simply functions with a special first argument, we can mimic some of their behavior by manually passing objects to be processed to simple functions. The participation of methods in class inheritance, though, allows us to naturally customize existing software by coding subclasses with new method definitions, rather than changing existing code in-place. There is really no such concept with modules and functions. As an example, suppose you’re assigned the task of implementing an employee database application. As a Python OOP programmer, you might begin by coding a general superclass that defines default behavior common to all the kinds of employees in your organization: class Employee: def computeSalary(self): ... def giveRaise(self): ... def promote(self): ... def retire(self): ... # General superclass # Common or default behavior OOP from 30,000 Feet | 619 Once you’ve coded this general behavior, you can specialize it for each specific kind of employee to reflect how the various types differ from the norm. That is, you can code subclasses that customize just the bits of behavior that differ per employee type; the rest of the employee types’ behavior will be inherited from the more general class. For example, if engineers have a unique salary computation rule (i.e., not hours times rate), you can replace just that one method in a subclass: class Engineer(Employee): def computeSalary(self): ... # Specialized subclass # Something custom here Because the computeSalary version here appears lower in the class tree, it will replace (override) the general version in Employee. You then create instances of the kinds of employee classes that the real employees belong to, to get the correct behavior: bob = Employee() mel = Engineer() # Default behavior # Custom salary calculator Notice that you can make instances of any class in a tree, not just the ones at the bottom—the class you make an instance from determines the level at which the attribute search will begin. Ultimately, these two instance objects might wind up embedded in a larger container object (e.g., a list, or an instance of another class) that represents a department or company using the composition idea mentioned at the start of this chapter. When you later ask for these employees’ salaries, they will be computed according to the classes from which the objects were made, due to the principles of the inheritance search:‡ company = [bob, mel] for emp in company: print(emp.computeSalary()) # A composite object # Run this object's version This is yet another instance of the idea of polymorphism introduced in Chapter 4 and revisited in Chapter 16. Recall that polymorphism means that the meaning of an operation depends on the object being operated on. Here, the method computeSalary is located by inheritance search in each object before it is called. In other applications, polymorphism might also be used to hide (i.e., encapsulate) interface differences. For example, a program that processes data streams might be coded to expect objects with input and output methods, without caring what those methods actually do: def processor(reader, converter, writer): while 1: data = reader.read() if not data: break ‡ Note that the company list in this example could be stored in a file with Python object pickling, introduced in Chapter 9 when we met files, to yield a persistent employee database. Python also comes with a module named shelve, which would allow you to store the pickled representation of the class instances in an accessby-key filesystem; the third-party open source ZODB system does the same but has better support for production-quality object-oriented databases. 620 | Chapter 25: OOP: The Big Picture data = converter(data) writer.write(data) By passing in instances of subclasses that specialize the required read and write method interfaces for various data sources, we can reuse the processor function for any data source we need to use, both now and in the future: class Reader: def read(self): ... # Default behavior and tools def other(self): ... class FileReader(Reader): def read(self): ... # Read from a local file class SocketReader(Reader): def read(self): ... # Read from a network socket ... processor(FileReader(...), Converter, FileWriter(...)) processor(SocketReader(...), Converter, TapeWriter(...)) processor(FtpReader(...), Converter, XmlWriter(...)) Moreover, because the internal implementations of those read and write methods have been factored into single locations, they can be changed without impacting code such as this that uses them. In fact, the processor function might itself be a class to allow the conversion logic of converter to be filled in by inheritance, and to allow readers and writers to be embedded by composition (we’ll see how this works later in this part of the book). Once you get used to programming this way (by software customization), you’ll find that when it’s time to write a new program, much of your work may already be done— your task largely becomes one of mixing together existing superclasses that already implement the behavior required by your program. For example, someone else might have written the Employee, Reader, and Writer classes in this example for use in a completely different program. If so, you get all of that person’s code “for free.” In fact, in many application domains, you can fetch or purchase collections of superclasses, known as frameworks, that implement common programming tasks as classes, ready to be mixed into your applications. These frameworks might provide database interfaces, testing protocols, GUI toolkits, and so on. With frameworks, you often simply code a subclass that fills in an expected method or two; the framework classes higher in the tree do most of the work for you. Programming in such an OOP world is just a matter of combining and specializing already debugged code by writing subclasses of your own. Of course, it takes a while to learn how to leverage classes to achieve such OOP utopia. In practice, object-oriented work also entails substantial design work to fully realize the code reuse benefits of classes—to this end, programmers have begun cataloging common OOP structures, known as design patterns, to help with design issues. The actual code you write to do OOP in Python, though, is so simple that it will not in itself pose an additional obstacle to your OOP quest. To see why, you’ll have to move on to Chapter 26. OOP from 30,000 Feet | 621 Chapter Summary We took an abstract look at classes and OOP in this chapter, taking in the big picture before we dive into syntax details. As we’ve seen, OOP is mostly about looking up attributes in trees of linked objects; we call this lookup an inheritance search. Objects at the bottom of the tree inherit attributes from objects higher up in the tree—a feature that enables us to program by customizing code, rather than changing it, or starting from scratch. When used well, this model of programming can cut development time radically. The next chapter will begin to fill in the coding details behind the picture painted here. As we get deeper into Python classes, though, keep in mind that the OOP model in Python is very simple; as I’ve already stated, it’s really just about looking up attributes in object trees. Before we move on, here’s a quick quiz to review what we’ve covered here. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. 7. 8. What is the main point of OOP in Python? Where does an inheritance search look for an attribute? What is the difference between a class object and an instance object? Why is the first argument in a class method function special? What is the __init__ method used for? How do you create a class instance? How do you create a class? How do you specify a class’s superclasses? Test Your Knowledge: Answers 1. OOP is about code reuse—you factor code to minimize redundancy and program by customizing what already exists instead of changing code in-place or starting from scratch. 2. An inheritance search looks for an attribute first in the instance object, then in the class the instance was created from, then in all higher superclasses, progressing from the bottom to the top of the object tree, and from left to right (by default). The search stops at the first place the attribute is found. Because the lowest version of a name found along the way wins, class hierarchies naturally support customization by extension. 622 | Chapter 25: OOP: The Big Picture 3. Both class and instance objects are namespaces (packages of variables that appear as attributes). The main difference between them is that classes are a kind of factory for creating multiple instances. Classes also support operator overloading methods, which instances inherit, and treat any functions nested within them as special methods for processing instances. 4. The first argument in a class method function is special because it always receives the instance object that is the implied subject of the method call. It’s usually called self by convention. Because method functions always have this implied subject object context by default, we say they are “object-oriented”—i.e., designed to process or change objects. 5. If the __init__ method is coded or inherited in a class, Python calls it automatically each time an instance of that class is created. It’s known as the constructor method; it is passed the new instance implicitly, as well as any arguments passed explicitly to the class name. It’s also the most commonly used operator overloading method. If no __init__ method is present, instances simply begin life as empty namespaces. 6. You create a class instance by calling the class name as though it were a function; any arguments passed into the class name show up as arguments two and beyond in the __init__ constructor method. The new instance remembers the class it was created from for inheritance purposes. 7. You create a class by running a class statement; like function definitions, these statements normally run when the enclosing module file is imported (more on this in the next chapter). 8. You specify a class’s superclasses by listing them in parentheses in the class statement, after the new class’s name. The left-to-right order in which the classes are listed in the parentheses gives the left-to-right inheritance search order in the class tree. Test Your Knowledge: Answers | 623 CHAPTER 26 Class Coding Basics Now that we’ve talked about OOP in the abstract, it’s time to see how this translates to actual code. This chapter begins to fill in the syntax details behind the class model in Python. If you’ve never been exposed to OOP in the past, classes can seem somewhat complicated if taken in a single dose. To make class coding easier to absorb, we’ll begin our detailed exploration of OOP by taking a first look at some basic classes in action in this chapter. We’ll expand on the details introduced here in later chapters of this part of the book, but in their basic form, Python classes are easy to understand. In fact, classes have just three primary distinctions. At a base level, they are mostly just namespaces, much like the modules we studied in Part V. Unlike modules, though, classes also have support for generating multiple objects, for namespace inheritance, and for operator overloading. Let’s begin our class statement tour by exploring each of these three distinctions in turn. Classes Generate Multiple Instance Objects To understand how the multiple objects idea works, you have to first understand that there are two kinds of objects in Python’s OOP model: class objects and instance objects. Class objects provide default behavior and serve as factories for instance objects. Instance objects are the real objects your programs process—each is a namespace in its own right, but inherits (i.e., has automatic access to) names in the class from which it was created. Class objects come from statements, and instances come from calls; each time you call a class, you get a new instance of that class. 625 This object-generation concept is very different from any of the other program constructs we’ve seen so far in this book. In effect, classes are essentially factories for generating multiple instances. By contrast, only one copy of each module is ever imported into a single program (in fact, one reason that we have to call imp.reload is to update the single module object so that changes are reflected once they’ve been made). The following is a quick summary of the bare essentials of Python OOP. As you’ll see, Python classes are in some ways similar to both defs and modules, but they may be quite different from what you’re used to in other languages. Class Objects Provide Default Behavior When we run a class statement, we get a class object. Here’s a rundown of the main properties of Python classes: • The class statement creates a class object and assigns it a name. Just like the function def statement, the Python class statement is an executable statement. When reached and run, it generates a new class object and assigns it to the name in the class header. Also, like defs, class statements typically run when the files they are coded in are first imported. • Assignments inside class statements make class attributes. Just like in module files, top-level assignments within a class statement (not nested in a def) generate attributes in a class object. Technically, the class statement scope morphs into the attribute namespace of the class object, just like a module’s global scope. After running a class statement, class attributes are accessed by name qualification: object.name. • Class attributes provide object state and behavior. Attributes of a class object record state information and behavior to be shared by all instances created from the class; function def statements nested inside a class generate methods, which process instances. Instance Objects Are Concrete Items When we call a class object, we get an instance object. Here’s an overview of the key points behind class instances: • Calling a class object like a function makes a new instance object. Each time a class is called, it creates and returns a new instance object. Instances represent concrete items in your program’s domain. • Each instance object inherits class attributes and gets its own namespace. Instance objects created from classes are new namespaces; they start out empty but inherit attributes that live in the class objects from which they were generated. 626 | Chapter 26: Class Coding Basics • Assignments to attributes of self in methods make per-instance attributes. Inside class method functions, the first argument (called self by convention) references the instance object being processed; assignments to attributes of self create or change data in the instance, not the class. A First Example Let’s turn to a real example to show how these ideas work in practice. To begin, let’s define a class named FirstClass by running a Python class statement interactively: >>> class FirstClass: ... def setdata(self, value): ... self.data = value ... def display(self): ... print(self.data) ... # Define a class object # Define class methods # self is the instance # self.data: per instance We’re working interactively here, but typically, such a statement would be run when the module file it is coded in is imported. Like functions created with defs, this class won’t even exist until Python reaches and runs this statement. Like all compound statements, the class starts with a header line that lists the class name, followed by a body of one or more nested and (usually) indented statements. Here, the nested statements are defs; they define functions that implement the behavior the class means to export. As we learned in Part IV, def is really an assignment. Here, it assigns function objects to the names setdata and display in the class statement’s scope, and so generates attributes attached to the class: FirstClass.setdata and FirstClass.display. In fact, any name assigned at the top level of the class’s nested block becomes an attribute of the class. Functions inside a class are usually called methods. They’re coded with normal defs, and they support everything we’ve learned about functions already (they can have defaults, return values, and so on). But in a method function, the first argument automatically receives an implied instance object when called—the subject of the call. We need to create a couple of instances to see how this works: >>> x = FirstClass() >>> y = FirstClass() # Make two instances # Each is a new namespace By calling the class this way (notice the parentheses), we generate instance objects, which are just namespaces that have access to their classes’ attributes. Properly speaking, at this point, we have three objects: two instances and a class. Really, we have three linked namespaces, as sketched in Figure 26-1. In OOP terms, we say that x “is a” FirstClass, as is y. Classes Generate Multiple Instance Objects | 627 Figure 26-1. Classes and instances are linked namespace objects in a class tree that is searched by inheritance. Here, the “data” attribute is found in instances, but “setdata” and “display” are in the class above them. The two instances start out empty but have links back to the class from which they were generated. If we qualify an instance with the name of an attribute that lives in the class object, Python fetches the name from the class by inheritance search (unless it also lives in the instance): >>> x.setdata("King Arthur") >>> y.setdata(3.14159) # Call methods: self is x # Runs: FirstClass.setdata(y, 3.14159) Neither x nor y has a setdata attribute of its own, so to find it, Python follows the link from instance to class. And that’s about all there is to inheritance in Python: it happens at attribute qualification time, and it just involves looking up names in linked objects (e.g., by following the is-a links in Figure 26-1). In the setdata function inside FirstClass, the value passed in is assigned to self.data. Within a method, self—the name given to the leftmost argument by convention—automatically refers to the instance being processed (x or y), so the assignments store values in the instances’ namespaces, not the class’s (that’s how the data names in Figure 26-1 are created). Because classes can generate multiple instances, methods must go through the self argument to get to the instance to be processed. When we call the class’s display method to print self.data, we see that it’s different in each instance; on the other hand, the name display itself is the same in x and y, as it comes (is inherited) from the class: >>> x.display() King Arthur >>> y.display() 3.14159 # self.data differs in each instance Notice that we stored different object types in the data member in each instance (a string, and a floating point). As with everything else in Python, there are no declarations for instance attributes (sometimes called members); they spring into existence the first time they are assigned values, just like simple variables. In fact, if we were to call display on one of our instances before calling setdata, we would trigger an undefined name error—the attribute named data doesn’t even exist in memory until it is assigned within the setdata method. 628 | Chapter 26: Class Coding Basics As another way to appreciate how dynamic this model is, consider that we can change instance attributes in the class itself, by assigning to self in methods, or outside the class, by assigning to an explicit instance object: >>> x.data = "New value" >>> x.display() New value # Can get/set attributes # Outside the class too Although less common, we could even generate a brand new attribute in the instance’s namespace by assigning to its name outside the class’s method functions: >>> x.anothername = "spam" # Can set new attributes here too! This would attach a new attribute called anothername, which may or may not be used by any of the class’s methods, to the instance object x. Classes usually create all of the instance’s attributes by assignment to the self argument, but they don’t have to; programs can fetch, change, or create attributes on any objects to which they have references. Classes Are Customized by Inheritance Besides serving as factories for generating multiple instance objects, classes also allow us to make changes by introducing new components (called subclasses), instead of changing existing components in-place. Instance objects generated from a class inherit the class’s attributes. Python also allows classes to inherit from other classes, opening the door to coding hierarchies of classes that specialize behavior—by redefining attributes in subclasses that appear lower in the hierarchy, we override the more general definitions of those attributes higher in the tree. In effect, the further down the hierarchy we go, the more specific the software becomes. Here, too, there is no parallel with modules: their attributes live in a single, flat namespace that is not as amenable to customization. In Python, instances inherit from classes, and classes inherit from superclasses. Here are the key ideas behind the machinery of attribute inheritance: • Superclasses are listed in parentheses in a class header. To inherit attributes from another class, just list the class in parentheses in a class statement’s header. The class that inherits is usually called a subclass, and the class that is inherited from is its superclass. • Classes inherit attributes from their superclasses. Just as instances inherit the attribute names defined in their classes, classes inherit all the attribute names defined in their superclasses; Python finds them automatically when they’re accessed, if they don’t exist in the subclasses. • Instances inherit attributes from all accessible classes. Each instance gets names from the class it’s generated from, as well as all of that class’s superclasses. When looking for a name, Python checks the instance, then its class, then all superclasses. Classes Are Customized by Inheritance | 629 • Each object.attribute reference invokes a new, independent search. Python performs an independent search of the class tree for each attribute fetch expression. This includes references to instances and classes made outside class statements (e.g., X.attr), as well as references to attributes of the self instance argument in class method functions. Each self.attr expression in a method invokes a new search for attr in self and above. • Logic changes are made by subclassing, not by changing superclasses. By redefining superclass names in subclasses lower in the hierarchy (class tree), subclasses replace and thus customize inherited behavior. The net effect, and the main purpose of all this searching, is that classes support factoring and customization of code better than any other language tool we’ve seen so far. On the one hand, they allow us to minimize code redundancy (and so reduce maintenance costs) by factoring operations into a single, shared implementation; on the other, they allow us to program by customizing what already exists, rather than changing it in-place or starting from scratch. A Second Example To illustrate the role of inheritance, this next example builds on the previous one. First, we’ll define a new class, SecondClass, that inherits all of FirstClass’s names and provides one of its own: >>> class SecondClass(FirstClass): # Inherits setdata ... def display(self): # Changes display ... print('Current value = "%s"' % self.data) ... SecondClass defines the display method to print with a different format. By defining an attribute with the same name as an attribute in FirstClass, SecondClass effectively replaces the display attribute in its superclass. Recall that inheritance searches proceed upward from instances, to subclasses, to superclasses, stopping at the first appearance of the attribute name that it finds. In this case, since the display name in SecondClass will be found before the one in First Class, we say that SecondClass overrides FirstClass’s display. Sometimes we call this act of replacing attributes by redefining them lower in the tree overloading. The net effect here is that SecondClass specializes FirstClass by changing the behavior of the display method. On the other hand, SecondClass (and any instances created from it) still inherits the setdata method in FirstClass verbatim. Let’s make an instance to demonstrate: >>> z = SecondClass() >>> z.setdata(42) >>> z.display() Current value = "42" 630 | Chapter 26: Class Coding Basics # Finds setdata in FirstClass # Finds overridden method in SecondClass As before, we make a SecondClass instance object by calling it. The setdata call still runs the version in FirstClass, but this time the display attribute comes from Second Class and prints a custom message. Figure 26-2 sketches the namespaces involved. Figure 26-2. Specialization by overriding inherited names by redefining them in extensions lower in the class tree. Here, SecondClass redefines and so customizes the “display” method for its instances. Now, here’s a very important thing to notice about OOP: the specialization introduced in SecondClass is completely external to FirstClass. That is, it doesn’t affect existing or future FirstClass objects, like the x from the prior example: >>> x.display() New value # x is still a FirstClass instance (old message) Rather than changing FirstClass, we customized it. Naturally, this is an artificial example, but as a rule, because inheritance allows us to make changes like this in external components (i.e., in subclasses), classes often support extension and reuse better than functions or modules can. Classes Are Attributes in Modules Before we move on, remember that there’s nothing magic about a class name. It’s just a variable assigned to an object when the class statement runs, and the object can be referenced with any normal expression. For instance, if our FirstClass was coded in a module file instead of being typed interactively, we could import it and use its name normally in a class header line: from modulename import FirstClass class SecondClass(FirstClass): def display(self): ... # Copy name into my scope # Use class name directly Or, equivalently: import modulename class SecondClass(modulename.FirstClass): def display(self): ... # Access the whole module # Qualify to reference Classes Are Customized by Inheritance | 631 Like everything else, class names always live within a module, so they must follow all the rules we studied in Part V. For example, more than one class can be coded in a single module file—like other statements in a module, class statements are run during imports to define names, and these names become distinct module attributes. More generally, each module may arbitrarily mix any number of variables, functions, and classes, and all names in a module behave the same way. The file food.py demonstrates: # food.py var = 1 def func(): ... class spam: ... class ham: ... class eggs: ... # food.var # food.func # food.spam # food.ham # food.eggs This holds true even if the module and class happen to have the same name. For example, given the following file, person.py: class person: ... we need to go through the module to fetch the class as usual: import person x = person.person() # Import module # Class within module Although this path may look redundant, it’s required: person.person refers to the person class inside the person module. Saying just person gets the module, not the class, unless the from statement is used: from person import person x = person() # Get class from module # Use class name As with any other variable, we can never see a class in a file without first importing and somehow fetching it from its enclosing file. If this seems confusing, don’t use the same name for a module and a class within it. In fact, common convention in Python dictates that class names should begin with an uppercase letter, to help make them more distinct: import person x = person.Person() # Lowercase for modules # Uppercase for classes Also, keep in mind that although classes and modules are both namespaces for attaching attributes, they correspond to very different source code structures: a module reflects an entire file, but a class is a statement within a file. We’ll say more about such distinctions later in this part of the book. 632 | Chapter 26: Class Coding Basics Classes Can Intercept Python Operators Let’s move on to the third major difference between classes and modules: operator overloading. In simple terms, operator overloading lets objects coded with classes intercept and respond to operations that work on built-in types: addition, slicing, printing, qualification, and so on. It’s mostly just an automatic dispatch mechanism— expressions and other built-in operations route control to implementations in classes. Here, too, there is nothing similar in modules: modules can implement function calls, but not the behavior of expressions. Although we could implement all class behavior as method functions, operator overloading lets objects be more tightly integrated with Python’s object model. Moreover, because operator overloading makes our own objects act like built-ins, it tends to foster object interfaces that are more consistent and easier to learn, and it allows class-based objects to be processed by code written to expect a built-in type’s interface. Here is a quick rundown of the main ideas behind overloading operators: • Methods named with double underscores (__X__) are special hooks. Python operator overloading is implemented by providing specially named methods to intercept operations. The Python language defines a fixed and unchangeable mapping from each of these operations to a specially named method. • Such methods are called automatically when instances appear in built-in operations. For instance, if an instance object inherits an __add__ method, that method is called whenever the object appears in a + expression. The method’s return value becomes the result of the corresponding expression. • Classes may override most built-in type operations. There are dozens of special operator overloading method names for intercepting and implementing nearly every operation available for built-in types. This includes expressions, but also basic operations like printing and object creation. • There are no defaults for operator overloading methods, and none are required. If a class does not define or inherit an operator overloading method, it just means that the corresponding operation is not supported for the class’s instances. If there is no __add__, for example, + expressions raise exceptions. • Operators allow classes to integrate with Python’s object model. By overloading type operations, user-defined objects implemented with classes can act just like built-ins, and so provide consistency as well as compatibility with expected interfaces. Operator overloading is an optional feature; it’s used primarily by people developing tools for other Python programmers, not by application developers. And, candidly, you probably shouldn’t try to use it just because it seems “cool.” Unless a class needs to mimic built-in type interfaces, it should usually stick to simpler named methods. Why would an employee database application support expressions like * and +, for example? Named methods like giveRaise and promote would usually make more sense. Classes Can Intercept Python Operators | 633 Because of this, we won’t go into details on every operator overloading method available in Python in this book. Still, there is one operator overloading method you are likely to see in almost every realistic Python class: the __init__ method, which is known as the constructor method and is used to initialize objects’ state. You should pay special attention to this method, because __init__, along with the self argument, turns out to be a key requirement to understanding most OOP code in Python. A Third Example On to another example. This time, we’ll define a subclass of SecondClass that implements three specially named attributes that Python will call automatically: • __init__ is run when a new instance object is created (self is the new ThirdClass object).* • __add__ is run when a ThirdClass instance appears in a + expression. • __str__ is run when an object is printed (technically, when it’s converted to its print string by the str built-in function or its Python internals equivalent). Our new subclass also defines a normally named method named mul, which changes the instance object in-place. Here’s the new subclass: >>> class ThirdClass(SecondClass): # Inherit from SecondClass ... def __init__(self, value): # On "ThirdClass(value)" ... self.data = value ... def __add__(self, other): # On "self + other" ... return ThirdClass(self.data + other) ... def __str__(self): # On "print(self)", "str()" ... return '[ThirdClass: %s]' % self.data ... def mul(self, other): # In-place change: named ... self.data *= other ... >>> a = ThirdClass('abc') # __init__ called >>> a.display() # Inherited method called Current value = "abc" >>> print(a) # __str__: returns display string [ThirdClass: abc] >>> b = a + 'xyz' >>> b.display() Current value = "abcxyz" >>> print(b) [ThirdClass: abcxyz] # __add__: makes a new instance # b has all ThirdClass methods >>> a.mul(3) >>> print(a) [ThirdClass: abcabcabc] # mul: changes instance in-place # __str__: returns display string * Not to be confused with the __init__.py files in module packages! See Chapter 23 for more details. 634 | Chapter 26: Class Coding Basics ThirdClass “is a” SecondClass, so its instances inherit the customized display method from SecondClass. This time, though, ThirdClass creation calls pass an argument (e.g., “abc”). This argument is passed to the value argument in the __init__ constructor and assigned to self.data there. The net effect is that ThirdClass arranges to set the data attribute automatically at construction time, instead of requiring setdata calls after the fact. Further, ThirdClass objects can now show up in + expressions and print calls. For +, Python passes the instance object on the left to the self argument in __add__ and the value on the right to other, as illustrated in Figure 26-3; whatever __add__ returns becomes the result of the + expression. For print, Python passes the object being printed to self in __str__; whatever string this method returns is taken to be the print string for the object. With __str__ we can use a normal print to display objects of this class, instead of calling the special display method. Figure 26-3. In operator overloading, expression operators and other built-in operations performed on class instances are mapped back to specially named methods in the class. These special methods are optional and may be inherited as usual. Here, a + expression triggers the __add__ method. Specially named methods such as __init__, __add__, and __str__ are inherited by subclasses and instances, just like any other names assigned in a class. If they’re not coded in a class, Python looks for such names in all its superclasses, as usual. Operator overloading method names are also not built-in or reserved words; they are just attributes that Python looks for when objects appear in various contexts. Python usually calls them automatically, but they may occasionally be called by your code as well; the __init__ method, for example, is often called manually to trigger superclass constructors (more on this later). Notice that the __add__ method makes and returns a new instance object of its class, by calling ThirdClass with the result value. By contrast, mul changes the current instance object in-place, by reassigning the self attribute. We could overload the * expression to do the latter, but this would be too different from the behavior of * for built-in types such as numbers and strings, for which it always makes new objects. Common practice dictates that overloaded operators should work the same way that built-in operator implementations do. Because operator overloading is really just an expression-tomethod dispatch mechanism, though, you can interpret operators any way you like in your own class objects. Classes Can Intercept Python Operators | 635 Why Use Operator Overloading? As a class designer, you can choose to use operator overloading or not. Your choice simply depends on how much you want your object to look and feel like built-in types. As mentioned earlier, if you omit an operator overloading method and do not inherit it from a superclass, the corresponding operation will not be supported for your instances; if it’s attempted, an exception will be thrown (or a standard default will be used). Frankly, many operator overloading methods tend to be used only when implementing objects that are mathematical in nature; a vector or matrix class may overload the addition operator, for example, but an employee class likely would not. For simpler classes, you might not use overloading at all, and would rely instead on explicit method calls to implement your objects’ behavior. On the other hand, you might decide to use operator overloading if you need to pass a user-defined object to a function that was coded to expect the operators available on a built-in type like a list or a dictionary. Implementing the same operator set in your class will ensure that your objects support the same expected object interface and so are compatible with the function. Although we won’t cover every operator overloading method in this book, we’ll see some additional operator overloading techniques in action in Chapter 29. One overloading method we will explore here is the __init__ constructor method, which seems to show up in almost every realistic class. Because it allows classes to fill out the attributes in their newly created instances immediately, the constructor is useful for almost every kind of class you might code. In fact, even though instance attributes are not declared in Python, you can usually find out which attributes an instance will have by inspecting its class’s __init__ method. The World’s Simplest Python Class We’ve begun studying class statement syntax in detail in this chapter, but I’d again like to remind you that the basic inheritance model that classes produce is very simple— all it really involves is searching for attributes in trees of linked objects. In fact, we can create a class with nothing in it at all. The following statement makes a class with no attributes attached (an empty namespace object): >>> class rec: pass # Empty namespace object We need the no-operation pass statement (discussed in Chapter 13) here because we don’t have any methods to code. After we make the class by running this statement interactively, we can start attaching attributes to the class by assigning names to it completely outside of the original class statement: >>> rec.name = 'Bob' >>> rec.age = 40 636 | Chapter 26: Class Coding Basics # Just objects with attributes And, after we’ve created these attributes by assignment, we can fetch them with the usual syntax. When used this way, a class is roughly similar to a “struct” in C, or a “record” in Pascal. It’s basically an object with field names attached to it (we can do similar work with dictionary keys, but it requires extra characters): >>> print(rec.name) Bob # Like a C struct or a record Notice that this works even though there are no instances of the class yet; classes are objects in their own right, even without instances. In fact, they are just self-contained namespaces, so as long as we have a reference to a class, we can set or change its attributes anytime we wish. Watch what happens when we do create two instances, though: >>> x = rec() >>> y = rec() # Instances inherit class names These instances begin their lives as completely empty namespace objects. Because they remember the class from which they were made, though, they will obtain the attributes we attached to the class by inheritance: >>> x.name, y.name ('Bob', 'Bob') # name is stored on the class only Really, these instances have no attributes of their own; they simply fetch the name attribute from the class object where it is stored. If we do assign an attribute to an instance, though, it creates (or changes) the attribute in that object, and no other—attribute references kick off inheritance searches, but attribute assignments affect only the objects in which the assignments are made. Here, x gets its own name, but y still inherits the name attached to the class above it: >>> x.name = 'Sue' >>> rec.name, x.name, y.name ('Bob', 'Sue', 'Bob') # But assignment changes x only In fact, as we’ll explore in more detail in Chapter 28, the attributes of a namespace object are usually implemented as dictionaries, and class inheritance trees are (generally speaking) just dictionaries with links to other dictionaries. If you know where to look, you can see this explicitly. For example, the __dict__ attribute is the namespace dictionary for most class-based objects (some classes may also define attributes in __slots__, an advanced and seldomused feature that we’ll study in Chapters 30 and 31). The following was run in Python 3.0; the order of names and set of __X__ internal names present can vary from release to release, but the names we assigned are present in all: >>> rec.__dict__.keys() ['__module__', 'name', 'age', '__dict__', '__weakref__', '__doc__'] >>> list(x.__dict__.keys()) ['name'] The World’s Simplest Python Class | 637 >>> list(y.__dict__.keys()) [] # list() not required in Python 2.6 Here, the class’s namespace dictionary shows the name and age attributes we assigned to it, x has its own name, and y is still empty. Each instance has a link to its class for inheritance, though—it’s called __class__, if you want to inspect it: >>> x.__class__ <class '__main__.rec'> Classes also have a __bases__ attribute, which is a tuple of their superclasses: >>> rec.__bases__ (<class 'object'>,) # () empty tuple in Python 2.6 These two attributes are how class trees are literally represented in memory by Python. The main point to take away from this look under the hood is that Python’s class model is extremely dynamic. Classes and instances are just namespace objects, with attributes created on the fly by assignment. Those assignments usually happen within the class statements you code, but they can occur anywhere you have a reference to one of the objects in the tree. Even methods, normally created by a def nested in a class, can be created completely independently of any class object. The following, for example, defines a simple function outside of any class that takes one argument: >>> def upperName(self): ... return self.name.upper() # Still needs a self There is nothing about a class here yet—it’s a simple function, and it can be called as such at this point, provided we pass in an object with a name attribute (the name self does not make this special in any way): >>> upperName(x) 'SUE' # Call as a simple function If we assign this simple function to an attribute of our class, though, it becomes a method, callable through any instance (as well as through the class name itself, as long as we pass in an instance manually):† >>> rec.method = upperName >>> x.method() 'SUE' # Run method to process x >>> y.method() 'BOB' # Same, but pass y to self † In fact, this is one of the reasons the self argument must always be explicit in Python methods—because methods can be created as simple functions independent of a class, they need to make the implied instance argument explicit. They can be called as either functions or methods, and Python can neither guess nor assume that a simple function might eventually become a class method. The main reason for the explicit self argument, though, is to make the meanings of names more obvious: names not referenced through self are simple variables, while names referenced through self are obviously instance attributes. 638 | Chapter 26: Class Coding Basics >>> rec.method(x) 'SUE' # Can call through instance or class Normally, classes are filled out by class statements, and instance attributes are created by assignments to self attributes in method functions. The point again, though, is that they don’t have to be; OOP in Python really is mostly about looking up attributes in linked namespace objects. Classes Versus Dictionaries Although the simple classes of the prior section are meant to illustrate class model basics, the techniques they employ can also be used for real work. For example, Chapter 8 showed how to use dictionaries to record properties of entities in our programs. It turns out that classes can serve this role, too—they package information like dictionaries, but can also bundle processing logic in the form of methods. For reference, here is the example for dictionary-based records we used earlier in the book: >>> >>> >>> >>> >>> >>> mel rec = {} rec['name'] = 'mel' rec['age'] = 45 rec['job'] = 'trainer/writer' # Dictionary-based record print(rec['name']) This code emulates tools like records in other languages. As we just saw, though, there are also multiple ways to do the same with classes. Perhaps the simplest is this—trading keys for attributes: >>> ... >>> >>> >>> >>> >>> 40 class rec: pass rec.name = 'mel' rec.age = 45 rec.job = 'trainer/writer' # Class-based record print(rec.age) This code has substantially less syntax than the dictionary equivalent. It uses an empty class statement to generate an empty namespace object. Once we make the empty class, we fill it out by assigning class attributes over time, as before. This works, but a new class statement will be required for each distinct record we will need. Perhaps more typically, we can instead generate instances of an empty class to represent each distinct entity: >>> ... >>> >>> >>> class rec: pass pers1 = rec() pers1.name = 'mel' pers1.job = 'trainer' # Instance-based records The World’s Simplest Python Class | 639 >>> pers1.age = 40 >>> >>> pers2 = rec() >>> pers2.name = 'vls' >>> pers2.job = 'developer' >>> >>> pers1.name, pers2.name ('mel', 'vls') Here, we make two records from the same class. Instances start out life empty, just like classes. We then fill in the records by assigning to attributes. This time, though, there are two separate objects, and hence two separate name attributes. In fact, instances of the same class don’t even have to have the same set of attribute names; in this example, one has a unique age name. Instances really are distinct namespaces, so each has a distinct attribute dictionary. Although they are normally filled out consistently by class methods, they are more flexible than you might expect. Finally, we might instead code a more full-blown class to implement the record and its processing: >>> class Person: ... def __init__(self, name, job): ... self.name = name ... self.job = job ... def info(self): ... return (self.name, self.job) ... >>> rec1 = Person('mel', 'trainer') >>> rec2 = Person('vls', 'developer') >>> >>> rec1.job, rec2.info() ('trainer', ('vls', 'developer')) # Class = Data + Logic This scheme also makes multiple instances, but the class is not empty this time: we’ve added logic (methods) to initialize instances at construction time and collect attributes into a tuple. The constructor imposes some consistency on instances here by always setting the name and job attributes. Together, the class’s methods and instance attributes create a package, which combines both data and logic. We could further extend this code by adding logic to compute salaries, parse names, and so on. Ultimately, we might link the class into a larger hierarchy to inherit an existing set of methods via the automatic attribute search of classes, or perhaps even store instances of the class in a file with Python object pickling to make them persistent. In fact, we will—in the next chapter, we’ll expand on this analogy between classes and records with a more realistic running example that demonstrates class basics in action. In the end, although types like dictionaries are flexible, classes allow us to add behavior to objects in ways that built-in types and simple functions do not directly support. Although we can store functions in dictionaries, too, using them to process implied instances is nowhere near as natural as it is in classes. 640 | Chapter 26: Class Coding Basics Chapter Summary This chapter introduced the basics of coding classes in Python. We studied the syntax of the class statement, and we saw how to use it to build up a class inheritance tree. We also studied how Python automatically fills in the first argument in method functions, how attributes are attached to objects in a class tree by simple assignment, and how specially named operator overloading methods intercept and implement built-in operations for our instances (e.g., expressions and printing). Now that we’ve learned all about the mechanics of coding classes in Python, the next chapter turns to a larger and more realistic example that ties together much of what we’ve learned about OOP so far. After that, we’ll continue our look at class coding, taking a second pass over the model to fill in some of the details that were omitted here to keep things simple. First, though, let’s work through a quiz to review the basics we’ve covered so far. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. 7. 8. 9. How are classes related to modules? How are instances and classes created? Where and how are class attributes created? Where and how are instance attributes created? What does self mean in a Python class? How is operator overloading coded in a Python class? When might you want to support operator overloading in your classes? Which operator overloading method is most commonly used? What are the two key concepts required to understand Python OOP code? Test Your Knowledge: Answers 1. Classes are always nested inside a module; they are attributes of a module object. Classes and modules are both namespaces, but classes correspond to statements (not entire files) and support the OOP notions of multiple instances, inheritance, and operator overloading. In a sense, a module is like a single-instance class, without inheritance, which corresponds to an entire file of code. 2. Classes are made by running class statements; instances are created by calling a class as though it were a function. Test Your Knowledge: Answers | 641 3. Class attributes are created by assigning attributes to a class object. They are normally generated by top-level assignments nested in a class statement—each name assigned in the class statement block becomes an attribute of the class object (technically, the class statement scope morphs into the class object’s attribute namespace). Class attributes can also be created, though, by assigning attributes to the class anywhere a reference to the class object exists—i.e., even outside the class statement. 4. Instance attributes are created by assigning attributes to an instance object. They are normally created within class method functions inside the class statement by assigning attributes to the self argument (which is always the implied instance). Again, though, they may be created by assignment anywhere a reference to the instance appears, even outside the class statement. Normally, all instance attributes are initialized in the __init__ constructor method; that way, later method calls can assume the attributes already exist. 5. self is the name commonly given to the first (leftmost) argument in a class method function; Python automatically fills it in with the instance object that is the implied subject of the method call. This argument need not be called self (though this is a very strong convention); its position is what is significant. (Ex-C++ or Java programmers might prefer to call it this because in those languages that name reflects the same idea; in Python, though, this argument must always be explicit.) 6. Operator overloading is coded in a Python class with specially named methods; they all begin and end with double underscores to make them unique. These are not built-in or reserved names; Python just runs them automatically when an instance appears in the corresponding operation. Python itself defines the mappings from operations to special method names. 7. Operator overloading is useful to implement objects that resemble built-in types (e.g., sequences or numeric objects such as matrixes), and to mimic the built-in type interface expected by a piece of code. Mimicking built-in type interfaces enables you to pass in class instances that also have state information—i.e., attributes that remember data between operation calls. You shouldn’t use operator overloading when a simple named method will suffice, though. 8. The __init__ constructor method is the most commonly used; almost every class uses this method to set initial values for instance attributes and perform other startup tasks. 9. The special self argument in method functions and the __init__ constructor method are the two cornerstones of OOP code in Python. 642 | Chapter 26: Class Coding Basics CHAPTER 27 A More Realistic Example We’ll dig into more class syntax details in the next chapter. Before we do, though, I’d like to show you a more realistic example of classes in action that’s more practical than what we’ve seen so far. In this chapter, we’re going to build a set of classes that do something more concrete—recording and processing information about people. As you’ll see, what we call instances and classes in Python programming can often serve the same roles as records and programs in more traditional terms. Specifically, in this chapter we’re going to code two classes: • Person—a class that creates and processes information about people • Manager—a customization of Person that modifies inherited behavior Along the way, we’ll make instances of both classes and test out their functionality. When we’re done, I’ll show you a nice example use case for classes—we’ll store our instances in a shelve object-oriented database, to make them permanent. That way, you can use this code as a template for fleshing out a full-blown personal database written entirely in Python. Besides actual utility, though, our aim here is also educational: this chapter provides a tutorial on object-oriented programming in Python. Often, people grasp the last chapter’s class syntax on paper, but have trouble seeing how to get started when confronted with having to code a new class from scratch. Toward this end, we’ll take it one step at a time here, to help you learn the basics; we’ll build up the classes gradually, so you can see how their features come together in complete programs. In the end, our classes will still be relatively small in terms of code, but they will demonstrate all of the main ideas in Python’s OOP model. Despite its syntax details, Python’s class system really is largely just a matter of searching for an attribute in a tree of objects, along with a special first argument for functions. 643 Step 1: Making Instances OK, so much for the design phase—let’s move on to implementation. Our first task is to start coding the main class, Person. In your favorite text editor, open a new file for the code we’ll be writing. It’s a fairly strong convention in Python to begin module names with a lowercase letter and class names with an uppercase letter; like the name of self arguments in methods, this is not required by the language, but it’s so common that deviating might be confusing to people who later read your code. To conform, we’ll call our new module file person.py and our class within it Person, like this: # File person.py (start) class Person: All our work will be done in this file until later in this chapter. We can code any number of functions and classes in a single module file in Python, and this one’s person.py name might not make much sense if we add unrelated components to it later. For now, we’ll assume everything in it will be Person-related. It probably should be anyhow—as we’ve learned, modules tend to work best when they have a single, cohesive purpose. Coding Constructors Now, the first thing we want to do with our Person class is record basic information about people—to fill out record fields, if you will. Of course, these are known as instance object attributes in Python-speak, and they generally are created by assignment to self attributes in class method functions. The normal way to give instance attributes their first values is to assign them to self in the __init__ constructor method, which contains code run automatically by Python each time an instance is created. Let’s add one to our class: # Add record field initialization class Person: def __init__(self, name, job, pay): self.name = name self.job = job self.pay = pay # Constructor takes 3 arguments # Fill out fields when created # self is the new instance object This is a very common coding pattern: we pass in the data to be attached to an instance as arguments to the constructor method and assign them to self to retain them permanently. In OO terms, self is the newly created instance object, and name, job, and pay become state information—descriptive data saved on an object for later use. Although other techniques (such as enclosing scope references) can save details, too, instance attributes make this very explicit and easy to understand. Notice that the argument names appear twice here. This code might seem a bit redundant at first, but it’s not. The job argument, for example, is a local variable in the scope of the __init__ function, but self.job is an attribute of the instance that’s the implied 644 | Chapter 27: A More Realistic Example subject of the method call. They are two different variables, which happen to have the same name. By assigning the job local to the self.job attribute with self.job=job, we save the passed-in job on the instance for later use. As usual in Python, where a name is assigned (or what object it is assigned to) determines what it means. Speaking of arguments, there’s really nothing magical about __init__, apart from the fact that it’s called automatically when an instance is made and has a special first argument. Despite its weird name, it’s a normal function and supports all the features of functions we’ve already covered. We can, for example, provide defaults for some of its arguments, so they need not be provided in cases where their values aren’t available or useful. To demonstrate, let’s make the job argument optional—it will default to None, meaning the person being created is not (currently) employed. If job defaults to None, we’ll probably want to default pay to 0, too, for consistency (unless some of the people you know manage to get paid without having jobs!). In fact, we have to specify a default for pay because according to Python’s syntax rules, any arguments in a function’s header after the first default must all have defaults, too: # Add defaults for constructor arguments class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay # Normal function args What this code means is that we’ll need to pass in a name when making Persons, but job and pay are now optional; they’ll default to None and 0 if omitted. The self argument, as usual, is filled in by Python automatically to refer to the instance object— assigning values to attributes of self attaches them to the new instance. Testing As You Go This class doesn’t do much yet—it essentially just fills out the fields of a new record— but it’s a real working class. At this point we could add more code to it for more features, but we won’t do that yet. As you’ve probably begun to appreciate already, programming in Python is really a matter of incremental prototyping—you write some code, test it, write more code, test again, and so on. Because Python provides both an interactive session and nearly immediate turnaround after code changes, it’s more natural to test as you go than to write a huge amount of code to test all at once. Before adding more features, then, let’s test what we’ve got so far by making a few instances of our class and displaying their attributes as created by the constructor. We could do this interactively, but as you’ve also probably surmised by now, interactive testing has its limits—it gets tedious to have to reimport modules and retype test cases each time you start a new testing session. More commonly, Python programmers use Step 1: Making Instances | 645 the interactive prompt for simple one-off tests but do more substantial testing by writing code at the bottom of the file that contains the objects to be tested, like this: # Add incremental self-test code class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay bob = Person('Bob Smith') # Test the class sue = Person('Sue Jones', job='dev', pay=100000) # Runs __init__ automatically print(bob.name, bob.pay) # Fetch attached attributes print(sue.name, sue.pay) # sue's and bob's attrs differ Notice here that the bob object accepts the defaults for job and pay, but sue provides values explicitly. Also note how we use keyword arguments when making sue; we could pass by position instead, but the keywords may help remind us later what the data is (and they allow us to pass the arguments in any left-to-right order we like). Again, despite its unusual name, __init__ is a normal function, supporting everything you already know about functions—including both defaults and pass-by-name keyword arguments. When this file runs as a script, the test code at the bottom makes two instances of our class and prints two attributes of each (name and pay): C:\misc> person.py Bob Smith 0 Sue Jones 100000 You can also type this file’s test code at Python’s interactive prompt (assuming you import the Person class there first), but coding canned tests inside the module file like this makes it much easier to rerun them in the future. Although this is fairly simple code, it’s already demonstrating something important. Notice that bob’s name is not sue’s, and sue’s pay is not bob’s. Each is an independent record of information. Technically, bob and sue are both namespace objects—like all class instances, they each have their own independent copy of the state information created by the class. Because each instance of a class has its own set of self attributes, classes are a natural for recording information for multiple objects this way; just like built-in types, classes serve as a sort of object factory. Other Python program structures, such as functions and modules, have no such concept. Using Code Two Ways As is, the test code at the bottom of the file works, but there’s a big catch—its top-level print statements run both when the file is run as a script and when it is imported as a module. This means if we ever decide to import the class in this file in order to use it somewhere else (and we will later in this chapter), we’ll see the output of its test code 646 | Chapter 27: A More Realistic Example every time the file is imported. That’s not very good software citizenship, though: client programs probably don’t care about our internal tests and won’t want to see our output mixed in with their own. Although we could split the test code off into a separate file, it’s often more convenient to code tests in the same file as the items to be tested. It would be better to arrange to run the test statements at the bottom only when the file is run for testing, not when the file is imported. That’s exactly what the module __name__ check is designed for, as you learned in the preceding part of this book. Here’s what this addition looks like: # Allow this file to be imported as well as run/tested class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay if __name__ == '__main__': # When run for testing only # self-test code bob = Person('Bob Smith') sue = Person('Sue Jones', job='dev', pay=100000) print(bob.name, bob.pay) print(sue.name, sue.pay) Now, we get exactly the behavior we’re after—running the file as a top-level script tests it because its __name__ is __main__, but importing it as a library of classes later does not: C:\misc> person.py Bob Smith 0 Sue Jones 100000 c:\misc> python Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) ... >>> import person >>> When imported, the file now defines the class, but does not use it. When run directly, this file creates two instances of our class as before, and prints two attributes of each; again, because each instance is an independent namespace object, the values of their attributes differ. Version Portability Note I’m running all the code in this chapter under Python 3.0, and using the 3.0 print function call syntax. If you run under 2.6 the code will work as-is, but you’ll notice parentheses around some output lines because the extra parentheses in prints turn multiple items into a tuple: c:\misc> c:\python26\python person.py ('Bob Smith', 0) ('Sue Jones', 100000) Step 1: Making Instances | 647 If this difference is the sort of detail that might keep you awake at nights, simply remove the parentheses to use 2.6 print statements. You can also avoid the extra parentheses portably by using formatting to yield a single object to print. Either of the following works in both 2.6 and 3.0, though the method form is newer: print('{0} {1}'.format(bob.name, bob.pay)) print('%s %s' % (bob.name, bob.pay)) # New format method # Format expression Step 2: Adding Behavior Methods Everything looks good so far—at this point, our class is essentially a record factory; it creates and fills out fields of records (attributes of instances, in more Pythonic terms). Even as limited as it is, though, we can still run some operations on its objects. Although classes add an extra layer of structure, they ultimately do most of their work by embedding and processing basic core data types like lists and strings. In other words, if you already know how to use Python’s simple core types, you already know much of the Python class story; classes are really just a minor structural extension. For example, the name field of our objects is a simple string, so we can extract last names from our objects by splitting on spaces and indexing. These are all core data type operations, which work whether their subjects are embedded in class instances or not: >>> name = 'Bob Smith' >>> name.split() ['Bob', 'Smith'] >>> name.split()[-1] 'Smith' # Simple string, outside class # Extract last name # Or [1], if always just two parts Similarly, we can give an object a pay raise by updating its pay field—that is, by changing its state information in-place with an assignment. This task also involves basic operations that work on Python’s core objects, regardless of whether they are standalone or embedded in a class structure: >>> pay = 100000 >>> pay *= 1.10 >>> print(pay) 110000.0 # Simple variable, outside class # Give a 10% raise # Or: pay = pay * 1.10, if you like to type # Or: pay = pay + (pay * .10), if you _really_ do! To apply these operations to the Person objects created by our script, simply do to bob.name and sue.pay what we just did to name and pay. The operations are the same, but the subject objects are attached to attributes in our class structure: # Process embedded built-in types: strings, mutability class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay if __name__ == '__main__': 648 | Chapter 27: A More Realistic Example bob = Person('Bob Smith') sue = Person('Sue Jones', job='dev', pay=100000) print(bob.name, bob.pay) print(sue.name, sue.pay) print(bob.name.split()[-1]) # Extract object's last name sue.pay *= 1.10 # Give this object a raise print(sue.pay) We’ve added the last two lines here; when they’re run, we extract bob’s last name by using basic string and list operations and give sue a pay raise by modifying her pay attribute in-place with basic number operations. In a sense, sue is also a mutable object—her state changes in-place just like a list after an append call: Bob Smith 0 Sue Jones 100000 Smith 110000.0 The preceding code works as planned, but if you show it to a veteran software developer he’ll probably tell you that its general approach is not a great idea in practice. Hardcoding operations like these outside of the class can lead to maintenance problems in the future. For example, what if you’ve hardcoded the last-name-extraction formula at many different places in your program? If you ever need to change the way it works (to support a new name structure, for instance), you’ll need to hunt down and update every occurrence. Similarly, if the pay-raise code ever changes (e.g., to require approval or database updates), you may have multiple copies to modify. Just finding all the appearances of such code may be problematic in larger programs—they may be scattered across many files, split into individual steps, and so on. Coding Methods What we really want to do here is employ a software design concept known as encapsulation. The idea with encapsulation is to wrap up operation logic behind interfaces, such that each operation is coded only once in our program. That way, if our needs change in the future, there is just one copy to update. Moreover, we’re free to change the single copy’s internals almost arbitrarily, without breaking the code that uses it. In Python terms, we want to code operations on objects in class methods, instead of littering them throughout our program. In fact, this is one of the things that classes are very good at—factoring code to remove redundancy and thus optimize maintainability. As an added bonus, turning operations into methods enables them to be applied to any instance of the class, not just those that they’ve been hardcoded to process. This is all simpler in code than it may sound in theory. The following achieves encapsulation by moving the two operations from code outside the class into class methods. While we’re at it, let’s change our self-test code at the bottom to use the new methods we’re creating, instead of hardcoding operations: Step 2: Adding Behavior Methods | 649 # Add methods to encapsulate operations for maintainability class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay def lastName(self): return self.name.split()[-1] def giveRaise(self, percent): self.pay = int(self.pay * (1 + percent)) # Behavior methods # self is implied subject # Must change here only if __name__ == '__main__': bob = Person('Bob Smith') sue = Person('Sue Jones', job='dev', pay=100000) print(bob.name, bob.pay) print(sue.name, sue.pay) print(bob.lastName(), sue.lastName()) # Use the new methods sue.giveRaise(.10) # instead of hardcoding print(sue.pay) As we’ve learned, methods are simply normal functions that are attached to classes and designed to process instances of those classes. The instance is the subject of the method call and is passed to the method’s self argument automatically. The transformation to the methods in this version is straightforward. The new lastName method, for example, simply does to self what the previous version hardcoded for bob, because self is the implied subject when the method is called. lastName also returns the result, because this operation is a called function now; it computes a value for its caller to use, even if it is just to be printed. Similarly, the new giveRaise method just does to self what we did to sue before. When run now, our file’s output is similar to before—we’ve mostly just refactored the code to allow for easier changes in the future, not altered its behavior: Bob Smith 0 Sue Jones 100000 Smith Jones 110000 A few coding details are worth pointing out here. First, notice that sue’s pay is now still an integer after a pay raise—we convert the math result back to an integer by calling the int built-in within the method. Changing the value to either int or float is probably not a significant concern for most purposes (integer and floating-point objects have the same interfaces and can be mixed within expressions), but we may need to address rounding issues in a real system (money probably matters to Persons!). As we learned in Chapter 5, we might handle this by using the round(N, 2) built-in to round and retain cents, using the decimal type to fix precision, or storing monetary values as full floating-point numbers and displaying them with a %.2f or {0:.2f} formatting string to show cents. For this example, we’ll simply truncate any cents with 650 | Chapter 27: A More Realistic Example int. (For another idea, also see the money function in the formats.py module of Chap- ter 24; you can import this tool to show pay with commas, cents, and dollar signs.) Second, notice that we’re also printing sue’s last name this time—because the last-name logic has been encapsulated in a method, we get to use it on any instance of the class. As we’ve seen, Python tells a method which instance to process by automatically passing it in to the first argument, usually called self. Specifically: • In the first call, bob.lastName(), bob is the implied subject passed to self. • In the second call, sue.lastName(), sue goes to self instead. Trace through these calls to see how the instance winds up in self. The net effect is that the method fetches the name of the implied subject each time. The same happens for giveRaise. We could, for example, give bob a raise by calling giveRaise for both instances this way, too; but unfortunately, bob’s zero pay will prevent him from getting a raise as the program is currently coded (something we may want to address in a future 2.0 release of our software). Finally, notice that the giveRaise method assumes that percent is passed in as a floatingpoint number between zero and one. That may be too radical an assumption in the real world (a 1000% raise would probably be a bug for most of us!); we’ll let it pass for this prototype, but we might want to test or at least document this in a future iteration of this code. Stay tuned for a rehash of this idea in a later chapter in this book, where we’ll code something called function decorators and explore Python’s assert statement— alternatives that can do the validity test for us automatically during development. Step 3: Operator Overloading At this point, we have a fairly full-featured class that generates and initializes instances, along with two new bits of behavior for processing instances (in the form of methods). So far, so good. As it stands, though, testing is still a bit less convenient than it needs to be—to trace our objects, we have to manually fetch and print individual attributes (e.g., bob.name, sue.pay). It would be nice if displaying an instance all at once actually gave us some useful information. Unfortunately, the default display format for an instance object isn’t very good—it displays the object’s class name, and its address in memory (which is essentially useless in Python, except as a unique identifier). To see this, change the last line in the script to print(sue) so it displays the object as a whole. Here’s what you’ll get (the output says that sue is an “object” in 3.0 and an “instance” in 2.6): Bob Smith 0 Sue Jones 100000 Smith Jones <__main__.Person object at 0x02614430> Step 3: Operator Overloading | 651 Providing Print Displays Fortunately, it’s easy to do better by employing operator overloading—coding methods in a class that intercept and process built-in operations when run on the class’s instances. Specifically, we can make use of what is probably the second most commonly used operator overloading method in Python, after __init__: the __str__ method introduced in the preceding chapter. __str__ is run automatically every time an instance is converted to its print string. Because that’s what printing an object does, the net transitive effect is that printing an object displays whatever is returned by the object’s __str__ method, if it either defines one itself or inherits one from a superclass (doubleunderscored names are inherited just like any other). Technically speaking, the __init__ constructor method we’ve already coded is operator overloading too—it is run automatically at construction time to initialize a newly created instance. Constructors are so common, though, that they almost seem like a special case. More focused methods like __str__ allow us to tap into specific operations and provide specialized behavior when our objects are used in those contexts. Let’s put this into code. The following extends our class to give a custom display that lists attributes when our class’s instances are displayed as a whole, instead of relying on the less useful default display: # Add __str__ overload method for printing objects class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay def lastName(self): return self.name.split()[-1] def giveRaise(self, percent): self.pay = int(self.pay * (1 + percent)) def __str__(self): return '[Person: %s, %s]' % (self.name, self.pay) # Added method # String to print if __name__ == '__main__': bob = Person('Bob Smith') sue = Person('Sue Jones', job='dev', pay=100000) print(bob) print(sue) print(bob.lastName(), sue.lastName()) sue.giveRaise(.10) print(sue) Notice that we’re doing string % formatting to build the display string in __str__ here; at the bottom, classes use built-in type objects and operations like these to get their work done. Again, everything you’ve already learned about both built-in types and functions applies to class-based code. Classes largely just add an additional layer of structure that packages functions and data together and supports extensions. 652 | Chapter 27: A More Realistic Example We’ve also changed our self-test code to print objects directly, instead of printing individual attributes. When run, the output is more coherent and meaningful now; the “[...]” lines are returned by our new __str__, run automatically by print operations: [Person: Bob Smith, 0] [Person: Sue Jones, 100000] Smith Jones [Person: Sue Jones, 110000] Here’s a subtle point: as we’ll learn in the next chapter, a related overloading method, __repr__, provides an as-code low-level display of an object when present. Sometimes classes provide both a __str__ for user-friendly displays and a __repr__ with extra details for developers to view. Because printing runs __str__ and the interactive prompt echoes results with __repr__, this can provide both target audiences with an appropriate display. Since we’re not interested in displaying an as-code format, __str__ is sufficient for our class. Step 4: Customizing Behavior by Subclassing At this point, our class captures much of the OOP machinery in Python: it makes instances, provides behavior in methods, and even does a bit of operator overloading now to intercept print operations in __str__. It effectively packages our data and logic together into a single, self-contained software component, making it easy to locate code and straightforward to change it in the future. By allowing us to encapsulate behavior, it also allows us to factor that code to avoid redundancy and its associated maintenance headaches. The only major OOP concept it does not yet capture is customization by inheritance. In some sense, we’re already doing inheritance, because instances inherit methods from their classes. To demonstrate the real power of OOP, though, we need to define a superclass/subclass relationship that allows us to extend our software and replace bits of inherited behavior. That’s the main idea behind OOP, after all; by fostering a coding model based upon customization of work already done, it can dramatically cut development time. Coding Subclasses As a next step, then, let’s put OOP’s methodology to use and customize our Person class by extending our software hierarchy. For the purpose of this tutorial, we’ll define a subclass of Person called Manager that replaces the inherited giveRaise method with a more specialized version. Our new class begins as follows: class Manager(Person): # Define a subclass of Person This code means that we’re defining a new class named Manager, which inherits from and may add customizations to the superclass Person. In plain terms, a Manager is almost Step 4: Customizing Behavior by Subclassing | 653 like a Person (admittedly, a very long journey for a very small joke...), but Manager has a custom way to give raises. For the sake of argument, let’s assume that when a Manager gets a raise, it receives the passed-in percentage as usual, but also gets an extra bonus that defaults to 10%. For instance, if a Manager’s raise is specified as 10%, it will really get 20%. (Any relation to Persons living or dead is, of course, strictly coincidental.) Our new method begins as follows; because this redefinition of giveRaise will be closer in the class tree to Manager instances than the original version in Person, it effectively replaces, and thereby customizes, the operation. Recall that according to the inheritance search rules, the lowest version of the name wins: class Manager(Person): def giveRaise(self, percent, bonus=.10): # Inherit Person attrs # Redefine to customize Augmenting Methods: The Bad Way Now, there are two ways we might code this Manager customization: a good way and a bad way. Let’s start with the bad way, since it might be a bit easier to understand. The bad way is to cut and paste the code of giveRaise in Person and modify it for Manager, like this: class Manager(Person): def giveRaise(self, percent, bonus=.10): self.pay = int(self.pay * (1 + percent + bonus)) # Bad: cut-and-paste This works as advertised—when we later call the giveRaise method of a Manager instance, it will run this custom version, which tacks on the extra bonus. So what’s wrong with something that runs correctly? The problem here is a very general one: any time you copy code with cut and paste, you essentially double your maintenance effort in the future. Think about it: because we copied the original version, if we ever have to change the way raises are given (and we probably will), we’ll have to change the code in two places, not one. Although this is a small and artificial example, it’s also representative of a universal issue—any time you’re tempted to program by copying code this way, you probably want to look for a better approach. Augmenting Methods: The Good Way What we really want to do here is somehow augment the original giveRaise, instead of replacing it altogether. The good way to do that in Python is by calling to the original version directly, with augmented arguments, like this: class Manager(Person): def giveRaise(self, percent, bonus=.10): Person.giveRaise(self, percent + bonus) 654 | Chapter 27: A More Realistic Example # Good: augment original This code leverages the fact that a class method can always be called either through an instance (the usual way, where Python sends the instance to the self argument automatically) or through the class (the less common scheme, where you must pass the instance manually). In more symbolic terms, recall that a normal method call of this form: instance.method(args...) is automatically translated by Python into this equivalent form: class.method(instance, args...) where the class containing the method to be run is determined by the inheritance search rule applied to the method’s name. You can code either form in your script, but there is a slight asymmetry between the two—you must remember to pass along the instance manually if you call through the class directly. The method always needs a subject instance one way or another, and Python provides it automatically only for calls made through an instance. For calls through the class name, you need to send an instance to self yourself; for code inside a method like giveRaise, self already is the subject of the call, and hence the instance to pass along. Calling through the class directly effectively subverts inheritance and kicks the call higher up the class tree to run a specific version. In our case, we can use this technique to invoke the default giveRaise in Person, even though it’s been redefined at the Manager level. In some sense, we must call through Person this way, because a self.giveRaise() inside Manager’s giveRaise code would loop—since self already is a Manager, self.giveRaise() would resolve again to Manager.giveRaise, and so on and so forth until available memory is exhausted. This “good” version may seem like a small difference in code, but it can make a huge difference for future code maintenance—because the giveRaise logic lives in just one place now (Person’s method), we have only one version to change in the future as needs evolve. And really, this form captures our intent more directly anyhow—we want to perform the standard giveRaise operation, but simply tack on an extra bonus. Here’s our entire module file with this step applied: # Add customization of one behavior in a subclass class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay def lastName(self): return self.name.split()[-1] def giveRaise(self, percent): self.pay = int(self.pay * (1 + percent)) def __str__(self): return '[Person: %s, %s]' % (self.name, self.pay) class Manager(Person): Step 4: Customizing Behavior by Subclassing | 655 def giveRaise(self, percent, bonus=.10): Person.giveRaise(self, percent + bonus) if __name__ == '__main__': bob = Person('Bob Smith') sue = Person('Sue Jones', job='dev', pay=100000) print(bob) print(sue) print(bob.lastName(), sue.lastName()) sue.giveRaise(.10) print(sue) tom = Manager('Tom Jones', 'mgr', 50000) tom.giveRaise(.10) print(tom.lastName()) print(tom) # Redefine at this level # Call Person's version # Make a Manager: __init__ # Runs custom version # Runs inherited method # Runs inherited __str__ To test our Manager subclass customization, we’ve also added self-test code that makes a Manager, calls its methods, and prints it. Here’s the new version’s output: [Person: Bob [Person: Sue Smith Jones [Person: Sue Jones [Person: Tom Smith, 0] Jones, 100000] Jones, 110000] Jones, 60000] Everything looks good here: bob and sue are as before, and when tom the Manager is given a 10% raise, he really gets 20% (his pay goes from $50K to $60K), because the customized giveRaise in Manager is run for him only. Also notice how printing tom as a whole at the end of the test code displays the nice format defined in Person’s __str__: Manager objects get this, lastName, and the __init__ constructor method’s code “for free” from Person, by inheritance. Polymorphism in Action To make this acquisition of inherited behavior even more striking, we can add the following code at the end of our file: if __name__ == '__main__': ... print('--All three--') for object in (bob, sue, tom): object.giveRaise(.10) print(object) Here’s the resulting output: [Person: Bob Smith, [Person: Sue Jones, Smith Jones [Person: Sue Jones, Jones [Person: Tom Jones, --All three-- 0] 100000] 110000] 60000] 656 | Chapter 27: A More Realistic Example # Process objects generically # Run this object's giveRaise # Run the common __str__ [Person: Bob Smith, 0] [Person: Sue Jones, 121000] [Person: Tom Jones, 72000] In the added code, object is either a Person or a Manager, and Python runs the appropriate giveRaise automatically—our original version in Person for bob and sue, and our customized version in Manager for tom. Trace the method calls yourself to see how Python selects the right giveRaise method for each object. This is just Python’s notion of polymorphism, which we met earlier in the book, at work again—what giveRaise does depends on what you do it to. Here, it’s made all the more obvious when it selects from code we’ve written ourselves in classes. The practical effect in this code is that sue gets another 10% but tom gets another 20%, because giveRaise is dispatched based upon the object’s type. As we’ve learned, polymorphism is at the heart of Python’s flexibility. Passing any of our three objects to a function that calls a giveRaise method, for example, would have the same effect: the appropriate version would be run automatically, depending on which type of object was passed. On the other hand, printing runs the same __str__ for all three objects, because it’s coded just once in Person. Manager both specializes and applies the code we originally wrote in Person. Although this example is small, it’s already leveraging OOP’s talent for code customization and reuse; with classes, this almost seems automatic at times. Inherit, Customize, and Extend In fact, classes can be even more flexible than our example implies. In general, classes can inherit, customize, or extend existing code in superclasses. For example, although we’re focused on customization here, we can also add unique methods to Manager that are not present in Person, if Managers require something completely different (Python namesake reference intended). The following snippet illustrates. Here, giveRaise redefines a superclass method to customize it, but someThingElse defines something new to extend: class Person: def lastName(self): ... def giveRaise(self): ... def __str__(self): ... class Manager(Person): def giveRaise(self, ...): ... def someThingElse(self, ...): ... # Inherit # Customize # Extend tom = Manager() tom.lastName() tom.giveRaise() tom.someThingElse() print(tom) # Inherited verbatim # Customized version # Extension here # Inherited overload method Extra methods like this code’s someThingElse extend the existing software and are available on Manager objects only, not on Persons. For the purposes of this tutorial, however, Step 4: Customizing Behavior by Subclassing | 657 we’ll limit our scope to customizing some of Person’s behavior by redefining it, not adding to it. OOP: The Big Idea As is, our code may be small, but it’s fairly functional. And really, it already illustrates the main point behind OOP in general: in OOP, we program by customizing what has already been done, rather than copying or changing existing code. This isn’t always an obvious win to newcomers at first glance, especially given the extra coding requirements of classes. But overall, the programming style implied by classes can cut development time radically compared to other approaches. For instance, in our example we could theoretically have implemented a custom giveRaise operation without subclassing, but none of the other options yield code as optimal as ours: • Although we could have simply coded Manager from scratch as new, independent code, we would have had to reimplement all the behaviors in Person that are the same for Managers. • Although we could have simply changed the existing Person class in-place for the requirements of Manager’s giveRaise, doing so would probably break the places where we still need the original Person behavior. • Although we could have simply copied the Person class in its entirety, renamed the copy to Manager, and changed its giveRaise, doing so would introduce code redundancy that would double our work in the future—changes made to Person in the future would not be picked up automatically, but would have to be manually propagated to Manager’s code. As usual, the cut-and-paste approach may seem quick now, but it doubles your work in the future. The customizable hierarchies we can build with classes provide a much better solution for software that will evolve over time. No other tools in Python support this development mode. Because we can tailor and extend our prior work by coding new subclasses, we can leverage what we’ve already done, rather than starting from scratch each time, breaking what already works, or introducing multiple copies of code that may all have to be updated in the future. When done right, OOP is a powerful programmer’s ally. Step 5: Customizing Constructors, Too Our code works as it is, but if you study the current version closely, you may be struck by something a bit odd—it seems pointless to have to provide a mgr job name for Manager objects when we create them: this is already implied by the class itself. It would be better if we could somehow fill in this value automatically when a Manager is made. The trick we need to improve on this turns out to be the same as the one we employed in the prior section: we want to customize the constructor logic for Managers in such a 658 | Chapter 27: A More Realistic Example way as to provide a job name automatically. In terms of code, we want to redefine an __init__ method in Manager that provides the mgr string for us. And like with the giveRaise customization, we also want to run the original __init__ in Person by calling through the class name, so it still initializes our objects’ state information attributes. The following extension will do the job—we’ve coded the new Manager constructor and changed the call that creates tom to not pass in the mgr job name: # Add customization of constructor in a subclass class Person: def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay def lastName(self): return self.name.split()[-1] def giveRaise(self, percent): self.pay = int(self.pay * (1 + percent)) def __str__(self): return '[Person: %s, %s]' % (self.name, self.pay) class Manager(Person): def __init__(self, name, pay): Person.__init__(self, name, 'mgr', pay) def giveRaise(self, percent, bonus=.10): Person.giveRaise(self, percent + bonus) # Redefine constructor # Run original with 'mgr' if __name__ == '__main__': bob = Person('Bob Smith') sue = Person('Sue Jones', job='dev', pay=100000) print(bob) print(sue) print(bob.lastName(), sue.lastName()) sue.giveRaise(.10) print(sue) tom = Manager('Tom Jones', 50000) tom.giveRaise(.10) print(tom.lastName()) print(tom) # Job name not needed: # Implied/set by class Again, we’re using the same technique to augment the __init__ constructor here that we used for giveRaise earlier—running the superclass version by calling through the class name directly and passing the self instance along explicitly. Although the constructor has a strange name, the effect is identical. Because we need Person’s construction logic to run too (to initialize instance attributes), we really have to call it this way; otherwise, instances would not have any attributes attached. Calling superclass constructors from redefinitions this way turns out to be a very common coding pattern in Python. By itself, Python uses inheritance to look for and call only one __init__ method at construction time—the lowest one in the class tree. If you need higher __init__ methods to be run at construction time (and you usually do), Step 5: Customizing Constructors, Too | 659 you must call them manually through the superclass’s name. The upside to this is that you can be explicit about which argument to pass up to the superclass’s constructor and can choose to not call it at all: not calling the superclass constructor allows you to replace its logic altogether, rather than augmenting it. The output of this file’s self-test code is the same as before—we haven’t changed what it does, we’ve simply restructured to get rid of some logical redundancy: [Person: Bob [Person: Sue Smith Jones [Person: Sue Jones [Person: Tom Smith, 0] Jones, 100000] Jones, 110000] Jones, 60000] OOP Is Simpler Than You May Think In this complete form, despite their sizes, our classes capture nearly all the important concepts in Python’s OOP machinery: • • • • • Instance creation—filling out instance attributes Behavior methods—encapsulating logic in class methods Operator overloading—providing behavior for built-in operations like printing Customizing behavior—redefining methods in subclasses to specialize them Customizing constructors—adding initialization logic to superclass steps Most of these concepts are based upon just three simple ideas: the inheritance search for attributes in object trees, the special self argument in methods, and operator overloading’s automatic dispatch to methods. Along the way, we’ve also made our code easy to change in the future, by harnessing the class’s propensity for factoring code to reduce redundancy. For example, we wrapped up logic in methods and called back to superclass methods from extensions to avoid having multiple copies of the same code. Most of these steps were a natural outgrowth of the structuring power of classes. By and large, that’s all there is to OOP in Python. Classes certainly can become larger than this, and there are some more advanced class concepts, such as decorators and metaclasses, which we will meet in later chapters. In terms of the basics, though, our classes already do it all. In fact, if you’ve grasped the workings of the classes we’ve written, most OOP Python code should now be within your reach. Other Ways to Combine Classes Having said that, I should also tell you that although the basic mechanics of OOP are simple in Python, some of the art in larger programs lies in the way that classes are put together. We’re focusing on inheritance in this tutorial because that’s the mechanism 660 | Chapter 27: A More Realistic Example the Python language provides, but programmers sometimes combine classes in other ways, too. For example, a common coding pattern involves nesting objects inside each other to build up composites. We’ll explore this pattern in more detail in Chapter 30, which is really more about design than about Python; as a quick example, though, we could use this composition idea to code our Manager extension by embedding a Person, instead of inheriting from it. The following alternative does so by using the __getattr__ operator overloading method we will meet in Chapter 29 to intercept undefined attribute fetches and delegate them to the embedded object with the getattr built-in. The giveRaise method here still achieves customization, by changing the argument passed along to the embedded object. In effect, Manager becomes a controller layer that passes calls down to the embedded object, rather than up to superclass methods: # Embedding-based Manager alternative class Person: ...same... class Manager: def __init__(self, name, pay): self.person = Person(name, 'mgr', pay) def giveRaise(self, percent, bonus=.10): self.person.giveRaise(percent + bonus) def __getattr__(self, attr): return getattr(self.person, attr) def __str__(self): return str(self.person) # Embed a Person object # Intercept and delegate # Delegate all other attrs # Must overload again (in 3.0) if __name__ == '__main__': ...same... In fact, this Manager alternative is representative of a general coding pattern usually known as delegation—a composite-based structure that manages a wrapped object and propagates method calls to it. This pattern works in our example, but it requires about twice as much code and is less well suited than inheritance to the kinds of direct customizations we meant to express (in fact, no reasonable Python programmer would code this example this way in practice, except perhaps those writing general tutorials). Manager isn’t really a Person here, so we need extra code to manually dispatch method calls to the embedded object; operator overloading methods like __str__ must be redefined (in 3.0, at least, as noted in the upcoming sidebar “Catching Built-in Attributes in 3.0” on page 662), and adding new Manager behavior is less straightforward since state information is one level removed. Still, object embedding, and design patterns based upon it, can be a very good fit when embedded objects require more limited interaction with the container than direct customization implies. A controller layer like this alternative Manager, for example, might come in handy if we want to trace or validate calls to another object’s methods (indeed, we will use a nearly identical coding pattern when we study class decorators later in the Step 5: Customizing Constructors, Too | 661 book). Moreover, a hypothetical Department class like the following could aggregate other objects in order to treat them as a set. Add this to the bottom of the person.py file to try this on your own: # Aggregate embedded objects into a composite ... bob = Person(...) sue = Person(...) tom = Manager(...) class Department: def __init__(self, *args): self.members = list(args) def addMember(self, person): self.members.append(person) def giveRaises(self, percent): for person in self.members: person.giveRaise(percent) def showAll(self): for person in self.members: print(person) development = Department(bob, sue) development.addMember(tom) development.giveRaises(.10) development.showAll() # Embed objects in a composite # Runs embedded objects' giveRaise # Runs embedded objects' __str__s Interestingly, this code uses both inheritance and composition—Department is a composite that embeds and controls other objects to aggregate, but the embedded Person and Manager objects themselves use inheritance to customize. As another example, a GUI might similarly use inheritance to customize the behavior or appearance of labels and buttons, but also composition to build up larger packages of embedded widgets, such as input forms, calculators, and text editors. The class structure to use depends on the objects you are trying to model. Design issues like composition are explored in Chapter 30, so we’ll postpone further investigations for now. But again, in terms of the basic mechanics of OOP in Python, our Person and Manager classes already tell the entire story. Having mastered the basics of OOP, though, developing general tools for applying it more easily in your scripts is often a natural next step—and the topic of the next section. Catching Built-in Attributes in 3.0 In Python 3.0 (and 2.6 if new-style classes are used), the alternative delegation-based Manager class we just coded will not be able to intercept and delegate operator overloading method attributes like __str__ without redefining them. Although we know that __str__ is the only such name used in our specific example, this a general issue for delegation-based classes. Recall that built-in operations like printing and indexing implicitly invoke operator overloading methods such as __str__ and __getitem__. In 3.0, built-in operations like 662 | Chapter 27: A More Realistic Example these do not route their implicit attribute fetches through generic attribute managers: neither __getattr__ (run for undefined attributes) nor its cousin __getattribute__ (run for all attributes) is invoked. This is why we have to redefine __str__ redundantly in the alternative Manager, in order to ensure that printing is routed to the embedded Person object when run in Python 3.0. Technically, this happens because classic classes normally look up operator overloading names in instances at runtime, but new-style classes do not—they skip the instance entirely and look up such methods in classes. In 2.6 classic classes, built-ins do route attributes generically—printing, for example, routes __str__ through __getattr__. New-style classes also inherit a default for __str__ that would foil __getattr__, but __getattribute__ doesn’t intercept the name in 3.0 either. This is a change, but isn’t a show-stopper—delegation-based classes can generally redefine operator overloading methods to delegate them to wrapped objects in 3.0, either manually or via tools or superclasses. This topic is too advanced to explore further in this tutorial, though, so don’t sweat the details too much here. Watch for it to be revisited in the attribute management coverage of Chapter 37, and again in the context of Private class decorators in Chapter 38. Step 6: Using Introspection Tools Let’s make one final tweak before we throw our objects onto a database. As they are, our classes are complete and demonstrate most of the basics of OOP in Python. They still have two remaining issues we probably should iron out, though, before we go live with them: • First, if you look at the display of the objects as they are right now, you’ll notice that when you print tom the Manager labels him as a Person. That’s not technically incorrect, since Manager is a kind of customized and specialized Person. Still, it would be more accurate to display objects with the most specific (that is, lowest) classes possible. • Second, and perhaps more importantly, the current display format shows only the attributes we include in our __str__, and that might not account for future goals. For example, we can’t yet verify that tom’s job name has been set to mgr correctly by Manager’s constructor, because the __str__ we coded for Person does not print this field. Worse, if we ever expand or otherwise change the set of attributes assigned to our objects in __init__, we’ll have to remember to also update __str__ for new names to be displayed, or it will become out of sync over time. The last point means that, yet again, we’ve made potential extra work for ourselves in the future by introducing redundancy in our code. Because any disparity in __str__ will be reflected in the program’s output, this redundancy may be more obvious than the other forms we addressed earlier; still, avoiding extra work in the future is generally a good thing. Step 6: Using Introspection Tools | 663 Special Class Attributes We can address both issues with Python’s introspection tools—special attributes and functions that give us access to some of the internals of objects’ implementations. These tools are somewhat advanced and generally used more by people writing tools for other programmers to use than by programmers developing applications. Even so, a basic knowledge of some of these tools is useful because they allow us to write code that processes classes in generic ways. In our code, for example, there are two hooks that can help us out, both of which were introduced near the end of the preceding chapter: • The built-in instance.__class__ attribute provides a link from an instance to the class from which it was created. Classes in turn have a __name__, just like modules, and a __bases__ sequence that provides access to superclasses. We can use these here to print the name of the class from which an instance is made rather than one we’ve hardcoded. • The built-in object.__dict__ attribute provides a dictionary with one key/value pair for every attribute attached to a namespace object (including modules, classes, and instances). Because it is a dictionary, we can fetch its keys list, index by key, iterate over its keys, and so on, to process all attributes generically. We can use this here to print every attribute in any instance, not just those we hardcode in custom displays. Here’s what these tools look like in action at Python’s interactive prompt. Notice how we load Person at the interactive prompt with a from statement here—class names live in and are imported from modules, exactly like function names and other variables: >>> from person import Person >>> bob = Person('Bob Smith') >>> print(bob) [Person: Bob Smith, 0] # Show bob's __str__ >>> bob.__class__ <class 'person.Person'> >>> bob.__class__.__name__ 'Person' # Show bob's class and its name >>> list(bob.__dict__.keys()) ['pay', 'job', 'name'] # Attributes are really dict keys # Use list to force list in 3.0 >>> for key in bob.__dict__: print(key, '=>', bob.__dict__[key]) # Index manually pay => 0 job => None name => Bob Smith >>> for key in bob.__dict__: print(key, '=>', getattr(bob, key)) pay => 0 664 | Chapter 27: A More Realistic Example # obj.attr, but attr is a var job => None name => Bob Smith As noted briefly in the prior chapter, some attributes accessible from an instance might not be stored in the __dict__ dictionary if the instance’s class defines __slots__, an optional and relatively obscure feature of new-style classes (and all classes in Python 3.0) that stores attributes in an array and that we’ll discuss in Chapters 30 and 31. Since slots really belong to classes instead of instances, and since they are very rarely used in any event, we can safely ignore them here and focus on the normal __dict__. A Generic Display Tool We can put these interfaces to work in a superclass that displays accurate class names and formats all attributes of an instance of any class. Open a new file in your text editor to code the following—it’s a new, independent module named classtools.py that implements just such a class. Because its __str__ print overload uses generic introspection tools, it will work on any instance, regardless of its attributes set. And because this is a class, it automatically becomes a general formatting tool: thanks to inheritance, it can be mixed into any class that wishes to use its display format. As an added bonus, if we ever want to change how instances are displayed we need only change this class, as every class that inherits its __str__ will automatically pick up the new format when it’s next run: # File classtools.py (new) "Assorted class utilities and tools" class AttrDisplay: """ Provides an inheritable print overload method that displays instances with their class names and a name=value pair for each attribute stored on the instance itself (but not attrs inherited from its classes). Can be mixed into any class, and will work on any instance. """ def gatherAttrs(self): attrs = [] for key in sorted(self.__dict__): attrs.append('%s=%s' % (key, getattr(self, key))) return ', '.join(attrs) def __str__(self): return '[%s: %s]' % (self.__class__.__name__, self.gatherAttrs()) if __name__ == '__main__': class TopTest(AttrDisplay): count = 0 def __init__(self): self.attr1 = TopTest.count self.attr2 = TopTest.count+1 TopTest.count += 2 Step 6: Using Introspection Tools | 665 class SubTest(TopTest): pass X, Y = TopTest(), SubTest() print(X) print(Y) # Show all instance attrs # Show lowest class name Notice the docstrings here—as a general-purpose tool, we want to add some functional documentation for potential users to read. As we saw in Chapter 15, docstrings can be placed at the top of simple functions and modules, and also at the start of classes and their methods; the help function and the PyDoc tool extracts and displays these automatically (we’ll look at docstrings again in Chapter 28). When run directly, this module’s self-test makes two instances and prints them; the __str__ defined here shows the instance’s class, and all its attributes names and values, in sorted attribute name order: C:\misc> classtools.py [TopTest: attr1=0, attr2=1] [SubTest: attr1=2, attr2=3] Instance Versus Class Attributes If you study the classtools module’s self-test code long enough, you’ll notice that its class displays only instance attributes, attached to the self object at the bottom of the inheritance tree; that’s what self’s __dict__ contains. As an intended consequence, we don’t see attributes inherited by the instance from classes above it in the tree (e.g., count in this file’s self-test code). Inherited class attributes are attached to the class only, not copied down to instances. If you ever do wish to include inherited attributes too, you can climb the __class__ link to the instance’s class, use the __dict__ there to fetch class attributes, and then iterate through the class’s __bases__ attribute to climb to even higher superclasses (repeating as necessary). If you’re a fan of simple code, running a built-in dir call on the instance instead of using __dict__ and climbing would have much the same effect, since dir results include inherited names in the sorted results list: >>> from person import Person >>> bob = Person('Bob Smith') # In Python 2.6: >>> bob.__dict__.keys() ['pay', 'job', 'name'] # Instance attrs only >>> dir(bob) # + inherited attrs in classes ['__doc__', '__init__', '__module__', '__str__', 'giveRaise', 'job', 'lastName', 'name', 'pay'] # In Python 3.0: 666 | Chapter 27: A More Realistic Example # 3.0 keys is a view, not a list >>> list(bob.__dict__.keys()) ['pay', 'job', 'name'] >>> dir(bob) # 3.0 includes class type methods ['__class__', '__delattr__', '__dict__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', ...more lines omitted... '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'giveRaise', 'job', 'lastName', 'name', 'pay'] The output here varies between Python 2.6 and 3.0, because 3.0’s dict.keys is not a list, and 3.0’s dir returns extra class-type implementation attributes. Technically, dir returns more in 3.0 because classes are all “new style” and inherit a large set of operator overloading names from the class type. In fact, you’ll probably want to filter out most of the __X__ names in the 3.0 dir result, since they are internal implementation details and not something you’d normally want to display. In the interest of space, we’ll leave optional display of inherited class attributes with either tree climbs or dir as suggested experiments for now. For more hints on this front, though, watch for the classtree.py inheritance tree climber we will write in Chapter 28, and the lister.py attribute listers and climbers we’ll code in Chapter 30. Name Considerations in Tool Classes One last subtlety here: because our AttrDisplay class in the classtools module is a general tool designed to be mixed into other arbitrary classes, we have to be aware of the potential for unintended name collisions with client classes. As is, I’ve assumed that client subclasses may want to use both its __str__ and gatherAttrs, but the latter of these may be more than a subclass expects—if a subclass innocently defines a gather Attrs name of its own, it will likely break our class, because the lower version in the subclass will be used instead of ours. To see this for yourself, add a gatherAttrs to TopTest in the file’s self-test code; unless the new method is identical, or intentionally customizes the original, our tool class will no longer work as planned: class TopTest(AttrDisplay): .... def gatherAttrs(self): return 'Spam' # Replaces method in AttrDisplay! This isn’t necessarily bad—sometimes we want other methods to be available to subclasses, either for direct calls or for customization. If we really meant to provide a __str__ only, though, this is less than ideal. To minimize the chances of name collisions like this, Python programmers often prefix methods not meant for external use with a single underscore: _gatherAttrs in our case. This isn’t foolproof (what if another class defines _gatherAttrs, too?), but it’s usually sufficient, and it’s a common Python naming convention for methods internal to a class. Step 6: Using Introspection Tools | 667 A better and less commonly used solution would be to use two underscores at the front of the method name only: __gatherAttrs for us. Python automatically expands such names to include the enclosing class’s name, which makes them truly unique. This is a feature usually called pseudoprivate class attributes, which we’ll expand on in Chapter 30. For now, we’ll make both our methods available. Our Classes’ Final Form Now, to use this generic tool in our classes, all we need to do is import it from its module, mix it in by inheritance in our top-level class, and get rid of the more specific __str__ we coded before. The new print overload method will be inherited by instances of Person, as well as Manager; Manager gets __str__ from Person, which now obtains it from the AttrDisplay coded in another module. Here is the final version of our person.py file with these changes applied: # File person.py (final) # Use generic display tool from classtools import AttrDisplay class Person(AttrDisplay): """ Create and process person records """ def __init__(self, name, job=None, pay=0): self.name = name self.job = job self.pay = pay def lastName(self): return self.name.split()[-1] def giveRaise(self, percent): self.pay = int(self.pay * (1 + percent)) # Assumes last is last class Manager(Person): """ A customized Person with special requirements """ def __init__(self, name, pay): Person.__init__(self, name, 'mgr', pay) def giveRaise(self, percent, bonus=.10): Person.giveRaise(self, percent + bonus) # Percent must be 0..1 if __name__ == '__main__': bob = Person('Bob Smith') sue = Person('Sue Jones', job='dev', pay=100000) print(bob) print(sue) print(bob.lastName(), sue.lastName()) sue.giveRaise(.10) print(sue) tom = Manager('Tom Jones', 50000) tom.giveRaise(.10) 668 | Chapter 27: A More Realistic Example print(tom.lastName()) print(tom) As this is the final revision, we’ve added a few comments here to document our work— docstrings for functional descriptions and # for smaller notes, per best-practice conventions. When we run this code now, we see all the attributes of our objects, not just the ones we hardcoded in the original __str__. And our final issue is resolved: because AttrDisplay takes class names off the self instance directly, each object is shown with the name of its closest (lowest) class—tom displays as a Manager now, not a Person, and we can finally verify that his job name has been correctly filled in by the Manager constructor: C:\misc> person.py [Person: job=None, name=Bob Smith, pay=0] [Person: job=dev, name=Sue Jones, pay=100000] Smith Jones [Person: job=dev, name=Sue Jones, pay=110000] Jones [Manager: job=mgr, name=Tom Jones, pay=60000] This is the more useful display we were after. From a larger perspective, though, our attribute display class has become a general tool, which we can mix into any class by inheritance to leverage the display format it defines. Further, all its clients will automatically pick up future changes in our tool. Later in the book, we’ll meet even more powerful class tool concepts, such as decorators and metaclasses; along with Python’s introspection tools, they allow us to write code that augments and manages classes in structured and maintainable ways. Step 7 (Final): Storing Objects in a Database At this point, our work is almost complete. We now have a two-module system that not only implements our original design goals for representing people, but also provides a general attribute display tool we can use in other programs in the future. By coding functions and classes in module files, we’ve ensured that they naturally support reuse. And by coding our software as classes, we’ve ensured that it naturally supports extension. Although our classes work as planned, though, the objects they create are not real database records. That is, if we kill Python, our instances will disappear—they’re transient objects in memory and are not stored in a more permanent medium like a file, so they won’t be available in future program runs. It turns out that it’s easy to make instance objects more permanent, with a Python feature called object persistence— making objects live on after the program that creates them exits. As a final step in this tutorial, let’s make our objects permanent. Step 7 (Final): Storing Objects in a Database | 669 Pickles and Shelves Object persistence is implemented by three standard library modules, available in every Python: pickle Serializes arbitrary Python objects to and from a string of bytes dbm (named anydbm in Python 2.6) Implements an access-by-key filesystem for storing strings shelve Uses the other two modules to store Python objects on a file by key We met these modules very briefly in Chapter 9 when we studied file basics. They provide powerful data storage options. Although we can’t do them complete justice in this tutorial or book, they are simple enough that a brief introduction is enough to get you started. The pickle module is a sort of super-general object formatting and deformatting tool: given a nearly arbitrary Python object in memory, it’s clever enough to convert the object to a string of bytes, which it can use later to reconstruct the original object in memory. The pickle module can handle almost any object you can create—lists, dictionaries, nested combinations thereof, and class instances. The latter are especially useful things to pickle, because they provide both data (attributes) and behavior (methods); in fact, the combination is roughly equivalent to “records” and “programs.” Because pickle is so general, it can replace extra code you might otherwise write to create and parse custom text file representations for your objects. By storing an object’s pickle string on a file, you effectively make it permanent and persistent: simply load and unpickle it later to re-create the original object. Although it’s easy to use pickle by itself to store objects in simple flat files and load them from there later, the shelve module provides an extra layer of structure that allows you to store pickled objects by key. shelve translates an object to its pickled string with pickle and stores that string under a key in a dbm file; when later loading, shelve fetches the pickled string by key and re-creates the original object in memory with pickle. This is all quite a trick, but to your script a shelve* of pickled objects looks just like a dictionary—you index by key to fetch, assign to keys to store, and use dictionary tools such as len, in, and dict.keys to get information. Shelves automatically map dictionary operations to objects stored in a file. In fact, to your script the only coding difference between a shelve and a normal dictionary is that you must open shelves initially and must close them after making changes. The net effect is that a shelve provides a simple database for storing and fetching native Python objects by keys, and thus makes them persistent across program runs. It does * Yes, we use “shelve” as a noun in Python, much to the chagrin of a variety of editors I’ve worked with over the years, both electronic and human. 670 | Chapter 27: A More Realistic Example not support query tools such as SQL, and it lacks some advanced features found in enterprise-level databases (such as true transaction processing), but native Python objects stored on a shelve may be processed with the full power of the Python language once they are fetched back by key. Storing Objects on a Shelve Database Pickling and shelves are somewhat advanced topics, and we won’t go into all their details here; you can read more about them in the standard library manuals, as well as application-focused books such as Programming Python. This is all simpler in Python than in English, though, so let’s jump into some code. Let’s write a new script that throws objects of our classes onto a shelve. In your text editor, open a new file we’ll call makedb.py. Since this is a new file, we’ll need to import our classes in order to create a few instances to store. We used from to load a class at the interactive prompt earlier, but really, as with functions and other variables, there are two ways to load a class from a file (class names are variables like any other, and not at all magic in this context): import person bob = person.Person(...) # Load class with import # Go through module name from person import Person bob = Person(...) # Load class with from # Use name directly We’ll use from to load in our script, just because it’s a bit less to type. Copy or retype this code to make instances of our classes in the new script, so we have something to store (this is a simple demo, so we won’t worry about the test-code redundancy here). Once we have some instances, it’s almost trivial to store them on a shelve. We simply import the shelve module, open a new shelve with an external filename, assign the objects to keys in the shelve, and close the shelve when we’re done because we’ve made changes: # File makedb.py: store Person objects on a shelve database from person import Person, Manager # Load our classes bob = Person('Bob Smith') # Re-create objects to be stored sue = Person('Sue Jones', job='dev', pay=100000) tom = Manager('Tom Jones', 50000) import shelve db = shelve.open('persondb') for object in (bob, sue, tom): db[object.name] = object db.close() # Filename where objects are stored # Use object's name attr as key # Store object on shelve by key # Close after making changes Notice how we assign objects to the shelve using their own names as keys. This is just for convenience; in a shelve, the key can be any string, including one we might create to be unique using tools such as process IDs and timestamps (available in the os and time standard library modules). The only rule is that the keys must be strings and should Step 7 (Final): Storing Objects in a Database | 671 be unique, since we can store just one object per key (though that object can be a list or dictionary containing many objects). The values we store under keys, though, can be Python objects of almost any sort: built-in types like strings, lists, and dictionaries, as well as user-defined class instances, and nested combinations of all of these. That’s all there is to it—if this script has no output when run, it means it probably worked; we’re not printing anything, just creating and storing objects: C:\misc> makedb.py Exploring Shelves Interactively At this point, there are one or more real files in the current directory whose names all start with “persondb”. The actual files created can vary per platform, and just like in the built-in open function, the filename in shelve.open() is relative to the current working directory unless it includes a directory path. Wherever they are stored, these files implement a keyed-access file that contains the pickled representation of our three Python objects. Don’t delete these files—they are your database, and are what you’ll need to copy or transfer when you back up or move your storage. You can look at the shelve’s files if you want to, either from Windows Explorer or the Python shell, but they are binary hash files, and most of their content makes little sense outside the context of the shelve module. With Python 3.0 and no extra software installed, our database is stored in three files (in 2.6, it’s just one file, persondb, because the bsddb extension module is preinstalled with Python for shelves; in 3.0, bsddb is a third-party open source add-on): # Directory listing module: verify files are present >>> import glob >>> glob.glob('person*') ['person.py', 'person.pyc', 'persondb.bak', 'persondb.dat', 'persondb.dir'] # Type the file: text mode for string, binary mode for bytes >>> print(open('persondb.dir').read()) 'Tom Jones', (1024, 91) ...more omitted... >>> print(open('persondb.dat', 'rb').read()) b'\x80\x03cperson\nPerson\nq\x00)\x81q\x01}q\x02(X\x03\x00\x00\x00payq\x03K... ...more omitted... This content isn’t impossible to decipher, but it can vary on different platforms and doesn’t exactly qualify as a user-friendly database interface! To verify our work better, we can write another script, or poke around our shelve at the interactive prompt. Because shelves are Python objects containing Python objects, we can process them with normal Python syntax and development modes. Here, the interactive prompt effectively becomes a database client: 672 | Chapter 27: A More Realistic Example >>> import shelve >>> db = shelve.open('persondb') # Reopen the shelve >>> len(db) 3 >>> list(db.keys()) ['Tom Jones', 'Sue Jones', 'Bob Smith'] # Three 'records' stored >>> bob = db['Bob Smith'] >>> print(bob) [Person: job=None, name=Bob Smith, pay=0] # Fetch bob by key # Runs __str__ from AttrDisplay >>> bob.lastName() 'Smith' # Runs lastName from Person >>> for key in db: print(key, '=>', db[key]) # Iterate, fetch, print # keys is the index # list to make a list in 3.0 Tom Jones => [Manager: job=mgr, name=Tom Jones, pay=50000] Sue Jones => [Person: job=dev, name=Sue Jones, pay=100000] Bob Smith => [Person: job=None, name=Bob Smith, pay=0] >>> for key in sorted(db): print(key, '=>', db[key]) # Iterate by sorted keys Bob Smith => [Person: job=None, name=Bob Smith, pay=0] Sue Jones => [Person: job=dev, name=Sue Jones, pay=100000] Tom Jones => [Manager: job=mgr, name=Tom Jones, pay=50000] Notice that we don’t have to import our Person or Manager classes here in order to load or use our stored objects. For example, we can call bob’s lastName method freely, and get his custom print display format automatically, even though we don’t have his Person class in our scope here. This works because when Python pickles a class instance, it records its self instance attributes, along with the name of the class it was created from and the module where the class lives. When bob is later fetched from the shelve and unpickled, Python will automatically reimport the class and link bob to it. The upshot of this scheme is that class instances automatically acquire all their class behavior when they are loaded in the future. We have to import our classes only to make new instances, not to process existing ones. Although a deliberate feature, this scheme has somewhat mixed consequences: • The downside is that classes and their module’s files must be importable when an instance is later loaded. More formally, pickleable classes must be coded at the top level of a module file accessible from a directory listed on the sys.path module search path (and shouldn’t live in the most script files’ module __main__ unless they’re always in that module when used). Because of this external module file requirement, some applications choose to pickle simpler objects such as dictionaries or lists, especially if they are to be transferred across the Internet. Step 7 (Final): Storing Objects in a Database | 673 • The upside is that changes in a class’s source code file are automatically picked up when instances of the class are loaded again; there is often no need to update stored objects themselves, since updating their class’s code changes their behavior. Shelves also have well-known limitations (the database suggestions at the end of this chapter mention a few of these). For simple object storage, though, shelves and pickles are remarkably easy-to-use tools. Updating Objects on a Shelve Now for one last script: let’s write a program that updates an instance (record) each time it runs, to prove the point that our objects really are persistent (i.e., that their current values are available every time a Python program runs). The following file, updatedb.py, prints the database and gives a raise to one of our stored objects each time. If you trace through what’s going on here, you’ll notice that we’re getting a lot of utility “for free”—printing our objects automatically employs the general __str__ overloading method, and we give raises by calling the giveRaise method we wrote earlier. This all “just works” for objects based on OOP’s inheritance model, even when they live in a file: # File updatedb.py: update Person object on database import shelve db = shelve.open('persondb') # Reopen shelve with same filename for key in sorted(db): print(key, '\t=>', db[key]) # Iterate to display database objects # Prints with custom format sue = db['Sue Jones'] sue.giveRaise(.10) db['Sue Jones'] = sue db.close() # Index by key to fetch # Update in memory using class method # Assign to key to update in shelve # Close after making changes Because this script prints the database when it starts up, we have to run it a few times to see our objects change. Here it is in action, displaying all records and increasing sue’s pay each time it’s run (it’s a pretty good script for sue...): c:\misc> updatedb.py Bob Smith => [Person: job=None, name=Bob Smith, pay=0] Sue Jones => [Person: job=dev, name=Sue Jones, pay=100000] Tom Jones => [Manager: job=mgr, name=Tom Jones, pay=50000] c:\misc> updatedb.py Bob Smith => [Person: job=None, name=Bob Smith, pay=0] Sue Jones => [Person: job=dev, name=Sue Jones, pay=110000] Tom Jones => [Manager: job=mgr, name=Tom Jones, pay=50000] c:\misc> updatedb.py Bob Smith => [Person: job=None, name=Bob Smith, pay=0] Sue Jones => [Person: job=dev, name=Sue Jones, pay=121000] Tom Jones => [Manager: job=mgr, name=Tom Jones, pay=50000] c:\misc> updatedb.py 674 | Chapter 27: A More Realistic Example Bob Smith Sue Jones Tom Jones => [Person: job=None, name=Bob Smith, pay=0] => [Person: job=dev, name=Sue Jones, pay=133100] => [Manager: job=mgr, name=Tom Jones, pay=50000] Again, what we see here is a product of the shelve and pickle tools we get from Python, and of the behavior we coded in our classes ourselves. And once again, we can verify our script’s work at the interactive prompt (the shelve’s equivalent of a database client): c:\misc> python >>> import shelve >>> db = shelve.open('persondb') # Reopen database >>> rec = db['Sue Jones'] # Fetch object by key >>> print(rec) [Person: job=dev, name=Sue Jones, pay=146410] >>> rec.lastName() 'Jones' >>> rec.pay 146410 For another example of object persistence in this book, see the sidebar in Chapter 30 titled “Why You Will Care: Classes and Persistence” on page 744. It stores a somewhat larger composite object in a flat file with pickle instead of shelve, but the effect is similar. For more details on both pickles and shelves, see other books or Python’s manuals. Future Directions And that’s a wrap for this tutorial. At this point, you’ve seen all the basics of Python’s OOP machinery in action, and you’ve learned ways to avoid redundancy and its associated maintenance issues in your code. You’ve built full-featured classes that do real work. As an added bonus, you’ve made them real database records by storing them in a Python shelve, so their information lives on persistently. There is much more we could explore here, of course. For example, we could extend our classes to make them more realistic, add new kinds of behavior to them, and so on. Giving a raise, for instance, should in practice verify that pay increase rates are between zero and one—an extension we’ll add when we meet decorators later in this book. You might also mutate this example into a personal contacts database, by changing the state information stored on objects, as well as the class methods used to process it. We’ll leave this a suggested exercise open to your imagination. We could also expand our scope to use tools that either come with Python or are freely available in the open source world: GUIs As is, we can only process our database with the interactive prompt’s commandbased interface, and scripts. We could also work on expanding our object database’s usability by adding a graphical user interface for browsing and updating its records. GUIs can be built portably with either Python’s tkinter (Tkinter in 2.6) Future Directions | 675 standard library support, or third-party toolkits such as WxPython and PyQt. tkinter ships with Python, lets you build simple GUIs quickly, and is ideal for learning GUI programming techniques; WxPython and PyQt tend to be more complex to use but often produce higher-grade GUIs in the end. Websites Although GUIs are convenient and fast, the Web is hard to beat in terms of accessibility. We might also implement a website for browsing and updating records, instead of or in addition to GUIs and the interactive prompt. Websites can be constructed with either basic CGI scripting tools that come with Python, or fullfeatured third-party web frameworks such as Django, TurboGears, Pylons, web2Py, Zope, or Google’s App Engine. On the Web, your data can still be stored in a shelve, pickle file, or other Python-based medium; the scripts that process it are simply run automatically on a server in response to requests from web browsers and other clients, and they produce HTML to interact with a user, either directly or by interfacing with Framework APIs. Web services Although web clients can often parse information in the replies from websites (a technique colorfully known as “screen scraping”), we might go further and provide a more direct way to fetch records on the Web via a web services interface such as SOAP or XML-RPC calls—APIs supported by either Python itself or the third-party open source domain. Such APIs return data in a more direct form, rather than embedded in the HTML of a reply page. Databases If our database becomes higher-volume or critical, we might eventually move it from shelves to a more full-featured storage mechanism such as the open source ZODB object-oriented database system (OODB), or a more traditional SQL-based relational database system such as MySQL, Oracle, PostgreSQL, or SQLite. Python itself comes with the in-process SQLite database system built-in, but other open source options are freely available on the Web. ZODB, for example, is similar to Python’s shelve but addresses many of its limitations, supporting larger databases, concurrent updates, transaction processing, and automatic write-through on inmemory changes. SQL-based systems like MySQL offer enterprise-level tools for database storage and may be directly used from a within a Python script. ORMs If we do migrate to a relational database system for storage, we don’t have to sacrifice Python’s OOP tools. Object-relational mappers (ORMs) like SQLObject and SQLAlchemy can automatically map relational tables and rows to and from Python classes and instances, such that we can process the stored data using normal Python class syntax. This approach provides an alternative to OODBs like shelve and ZODB and leverages the power of both relational databases and Python’s class model. 676 | Chapter 27: A More Realistic Example While I hope this introduction whets your appetite for future exploration, all of these topics are of course far beyond the scope of this tutorial and this book at large. If you want to explore any of them on your own, see the Web, Python’s standard library manuals, and application-focused books such as Programming Python. In the latter I pick up this example where we’ve stopped here, showing how to add both a GUI and a website on top of the database to allow for browsing and updating instance records. I hope to see you there eventually, but first, let’s return to class fundamentals and finish up the rest of the core Python language story. Chapter Summary In this chapter, we explored all the fundamentals of Python classes and OOP in action, by building upon a simple but real example, step by step. We added constructors, methods, operator overloading, customization with subclasses, and introspection tools, and we met other concepts (such as composition, delegation, and polymorphism) along the way. In the end, we took objects created by our classes and made them persistent by storing them on a shelve object database—an easy-to-use system for saving and retrieving native Python objects by key. While exploring class basics, we also encountered multiple ways to factor our code to reduce redundancy and minimize future maintenance costs. Finally, we briefly previewed ways to extend our code with application-programming tools such as GUIs and databases, covered in follow-up books. In the next chapters of this part of the book we’ll return to our study of the details behind Python’s class model and investigate its application to some of the design concepts used to combine classes in larger programs. Before we move ahead, though, let’s work through this chapter’s quiz to review what we covered here. Since we’ve already done a lot of hands-on work in this chapter, we’ll close with a set of mostly theoryoriented questions designed to make you trace through some of the code and ponder some of the bigger ideas behind it. Test Your Knowledge: Quiz 1. When we fetch a Manager object from the shelve and print it, where does the display format logic come from? 2. When we fetch a Person object from a shelve without importing its module, how does the object know that it has a giveRaise method that we can call? 3. Why is it so important to move processing into methods, instead of hardcoding it outside the class? Test Your Knowledge: Quiz | 677 4. Why is it better to customize by subclassing rather than copying the original and modifying? 5. Why is it better to call back to a superclass method to run default actions, instead of copying and modifying its code in a subclass? 6. Why is it better to use tools like __dict__ that allow objects to be processed generically than to write more custom code for each type of class? 7. In general terms, when might you choose to use object embedding and composition instead of inheritance? 8. How might you modify the classes in this chapter to implement a personal contacts database in Python? Test Your Knowledge: Answers 1. In the final version of our classes, Manager ultimately inherits its __str__ printing method from AttrDisplay in the separate classtools module. Manager doesn’t have one itself, so the inheritance search climbs to its Person superclass; because there is no __str__ there either, the search climbs higher and finds it in AttrDisplay. The class names listed in parentheses in a class statement’s header line provide the links to higher superclasses. 2. Shelves (really, the pickle module they use) automatically relink an instance to the class it was created from when that instance is later loaded back into memory. Python reimports the class from its module internally, creates an instance with its stored attributes, and sets the instance’s __class__ link to point to its original class. This way, loaded instances automatically obtain all their original methods (like lastName, giveRaise, and __str__), even if we have not imported the instance’s class into our scope. 3. It’s important to move processing into methods so that there is only one copy to change in the future, and so that the methods can be run on any instance. This is Python’s notion of encapsulation—wrapping up logic behind interfaces, to better support future code maintenance. If you don’t do so, you create code redundancy that can multiply your work effort as the code evolves in the future. 4. Customizing with subclasses reduces development effort. In OOP, we code by customizing what has already been done, rather than copying or changing existing code. This is the real “big idea” in OOP—because we can easily extend our prior work by coding new subclasses, we can leverage what we’ve already done. This is much better than either starting from scratch each time, or introducing multiple redundant copies of code that may all have to be updated in the future. 678 | Chapter 27: A More Realistic Example 5. Copying and modifying code doubles your potential work effort in the future, regardless of the context. If a subclass needs to perform default actions coded in a superclass method, it’s much better to call back to the original through the superclass’s name than to copy its code. This also holds true for superclass constructors. Again, copying code creates redundancy, which is a major issue as code evolves. 6. Generic tools can avoid hardcoded solutions that must be kept in sync with the rest of the class as it evolves over time. A generic __str__ print method, for example, need not be updated each time a new attribute is added to instances in an __init__ constructor. In addition, a generic print method inherited by all classes only appears, and need only be modified, in one place—changes in the generic version are picked up by all classes that inherit from the generic class. Again, eliminating code redundancy cuts future development effort; that’s one of the primary assets classes bring to the table. 7. Inheritance is best at coding extensions based on direct customization (like our Manager specialization of Person). Composition is well suited to scenarios where multiple objects are aggregated into a whole and directed by a controller layer class. Inheritance passes calls up to reuse, and composition passes down to delegate. Inheritance and composition are not mutually exclusive; often, the objects embedded in a controller are themselves customizations based upon inheritance. 8. The classes in this chapter could be used as boilerplate “template” code to implement a variety of types of databases. Essentially, you can repurpose them by modifying the constructors to record different attributes and providing whatever methods are appropriate for the target application. For instance, you might use attributes such as name, address, birthday, phone, email, and so on for a contacts database, and methods appropriate for this purpose. A method named sendmail, for example, might use Python’s standard library smptlib module to send an email to one of the contacts automatically when called (see Python’s manuals or application-level books for more details on such tools). The AttrDisplay tool we wrote here could be used verbatim to print your objects, because it is intentionally generic. Most of the shelve database code here can be used to store your objects, too, with minor changes. Test Your Knowledge: Answers | 679 CHAPTER 28 Class Coding Details If you haven’t quite gotten all of Python OOP yet, don’t worry; now that we’ve had a quick tour, we’re going to dig a bit deeper and study the concepts introduced earlier in further detail. In this and the following chapter, we’ll take another look at class mechanics. Here, we’re going to study classes, methods, and inheritance, formalizing and expanding on some of the coding ideas introduced in Chapter 26. Because the class is our last namespace tool, we’ll summarize Python’s namespace concepts here as well. The next chapter continues this in-depth second pass over class mechanics by covering one specific aspect: operator overloading. Besides presenting the details, this chapter and the next also give us an opportunity to explore some larger classes than those we have studied so far. The class Statement Although the Python class statement may seem similar to tools in other OOP languages on the surface, on closer inspection, it is quite different from what some programmers are used to. For example, as in C++, the class statement is Python’s main OOP tool, but unlike in C++, Python’s class is not a declaration. Like a def, a class statement is an object builder, and an implicit assignment—when run, it generates a class object and stores a reference to it in the name used in the header. Also like a def, a class statement is true executable code—your class doesn’t exist until Python reaches and runs the class statement that defines it (typically while importing the module it is coded in, but not before). General Form class is a compound statement, with a body of indented statements typically appearing under the header. In the header, superclasses are listed in parentheses after the class name, separated by commas. Listing more than one superclass leads to multiple inheritance (which we’ll discuss more formally in Chapter 30). Here is the statement’s general form: 681 class <name>(superclass,...): data = value def method(self,...): self.member = value # Assign to name # Shared class data # Methods # Per-instance data Within the class statement, any assignments generate class attributes, and specially named methods overload operators; for instance, a function called __init__ is called at instance object construction time, if defined. Example As we’ve seen, classes are mostly just namespaces—that is, tools for defining names (i.e., attributes) that export data and logic to clients. So, how do you get from the class statement to a namespace? Here’s how. Just like in a module file, the statements nested in a class statement body create its attributes. When Python executes a class statement (not a call to a class), it runs all the statements in its body, from top to bottom. Assignments that happen during this process create names in the class’s local scope, which become attributes in the associated class object. Because of this, classes resemble both modules and functions: • Like functions, class statements are local scopes where names created by nested assignments live. • Like names in a module, names assigned in a class statement become attributes in a class object. The main distinction for classes is that their namespaces are also the basis of inheritance in Python; reference attributes that are not found in a class or instance object are fetched from other classes. Because class is a compound statement, any sort of statement can be nested inside its body—print, =, if, def, and so on. All the statements inside the class statement run when the class statement itself runs (not when the class is later called to make an instance). Assigning names inside the class statement makes class attributes, and nested defs make class methods, but other assignments make attributes, too. For example, assignments of simple nonfunction objects to class attributes produce data attributes, shared by all instances: >>> class SharedData: ... spam = 42 ... >>> x = SharedData() >>> y = SharedData() >>> x.spam, y.spam (42, 42) 682 | Chapter 28: Class Coding Details # Generates a class data attribute # Make two instances # They inherit and share 'spam' Here, because the name spam is assigned at the top level of a class statement, it is attached to the class and so will be shared by all instances. We can change it by going through the class name, and we can refer to it through either instances or the class.* >>> SharedData.spam = 99 >>> x.spam, y.spam, SharedData.spam (99, 99, 99) Such class attributes can be used to manage information that spans all the instances— a counter of the number of instances generated, for example (we’ll expand on this idea by example in Chapter 31). Now, watch what happens if we assign the name spam through an instance instead of the class: >>> x.spam = 88 >>> x.spam, y.spam, SharedData.spam (88, 99, 99) Assignments to instance attributes create or change the names in the instance, rather than in the shared class. More generally, inheritance searches occur only on attribute references, not on assignment: assigning to an object’s attribute always changes that object, and no other.† For example, y.spam is looked up in the class by inheritance, but the assignment to x.spam attaches a name to x itself. Here’s a more comprehensive example of this behavior that stores the same name in two places. Suppose we run the following class: class MixedNames: data = 'spam' def __init__(self, value): self.data = value def display(self): print(self.data, MixedNames.data) # Define class # Assign class attr # Assign method name # Assign instance attr # Instance attr, class attr This class contains two defs, which bind class attributes to method functions. It also contains an = assignment statement; because this assignment assigns the name data inside the class, it lives in the class’s local scope and becomes an attribute of the class object. Like all class attributes, this data is inherited and shared by all instances of the class that don’t have data attributes of their own. When we make instances of this class, the name data is attached to those instances by the assignment to self.data in the constructor method: >>> x = MixedNames(1) >>> y = MixedNames(2) # Make two instance objects # Each has its own data * If you’ve used C++ you may recognize this as similar to the notion of C++’s “static” data members—members that are stored in the class, independent of instances. In Python, it’s nothing special: all class attributes are just names assigned in the class statement, whether they happen to reference functions (C++’s “methods”) or something else (C++’s “members”). In Chapter 31, we’ll also meet Python static methods (akin to those in C++), which are just self-less functions that usually process class attributes. † Unless the class has redefined the attribute assignment operation to do something unique with the __setattr__ operator overloading method (discussed in Chapter 29). The class Statement | 683 >>> x.display(); y.display() 1 spam 2 spam # self.data differs, MixedNames.data is the same The net result is that data lives in two places: in the instance objects (created by the self.data assignment in __init__), and in the class from which they inherit names (created by the data assignment in the class). The class’s display method prints both versions, by first qualifying the self instance, and then the class. By using these techniques to store attributes in different objects, we determine their scope of visibility. When attached to classes, names are shared; in instances, names record per-instance data, not shared behavior or data. Although inheritance searches look up names for us, we can always get to an attribute anywhere in a tree by accessing the desired object directly. In the preceding example, for instance, specifying x.data or self.data will return an instance name, which normally hides the same name in the class; however, Mixed Names.data grabs the class name explicitly. We’ll see various roles for such coding patterns later; the next section describes one of the most common. Methods Because you already know about functions, you also know about methods in classes. Methods are just function objects created by def statements nested in a class statement’s body. From an abstract perspective, methods provide behavior for instance objects to inherit. From a programming perspective, methods work in exactly the same way as simple functions, with one crucial exception: a method’s first argument always receives the instance object that is the implied subject of the method call. In other words, Python automatically maps instance method calls to class method functions as follows. Method calls made through an instance, like this: instance.method(args...) are automatically translated to class method function calls of this form: class.method(instance, args...) where the class is determined by locating the method name using Python’s inheritance search procedure. In fact, both call forms are valid in Python. Besides the normal inheritance of method attribute names, the special first argument is the only real magic behind method calls. In a class method, the first argument is usually called self by convention (technically, only its position is significant, not its name). This argument provides methods with a hook back to the instance that is the subject of the call—because classes generate many instance objects, they need to use this argument to manage data that varies per instance. 684 | Chapter 28: Class Coding Details C++ programmers may recognize Python’s self argument as being similar to C++’s this pointer. In Python, though, self is always explicit in your code: methods must always go through self to fetch or change attributes of the instance being processed by the current method call. This explicit nature of self is by design—the presence of this name makes it obvious that you are using instance attribute names in your script, not names in the local or global scope. Method Example To clarify these concepts, let’s turn to an example. Suppose we define the following class: class NextClass: def printer(self, text): self.message = text print(self.message) # Define class # Define method # Change instance # Access instance The name printer references a function object; because it’s assigned in the class statement’s scope, it becomes a class object attribute and is inherited by every instance made from the class. Normally, because methods like printer are designed to process instances, we call them through instances: >>> x = NextClass() # Make instance >>> x.printer('instance call') instance call # Call its method >>> x.message 'instance call' # Instance changed When we call the method by qualifying an instance like this, printer is first located by inheritance, and then its self argument is automatically assigned the instance object (x); the text argument gets the string passed at the call ('instance call'). Notice that because Python automatically passes the first argument to self for us, we only actually have to pass in one argument. Inside printer, the name self is used to access or set per-instance data because it refers back to the instance currently being processed. Methods may be called in one of two ways—through an instance, or through the class itself. For example, we can also call printer by going through the class name, provided we pass an instance to the self argument explicitly: >>> NextClass.printer(x, 'class call') class call # Direct class call >>> x.message 'class call' # Instance changed again Methods | 685 Calls routed through the instance and the class have the exact same effect, as long as we pass the same instance object ourselves in the class form. By default, in fact, you get an error message if you try to call a method without any instance: >>> NextClass.printer('bad call') TypeError: unbound method printer() must be called with NextClass instance... Calling Superclass Constructors Methods are normally called through instances. Calls to methods through a class, though, do show up in a variety of special roles. One common scenario involves the constructor method. The __init__ method, like all attributes, is looked up by inheritance. This means that at construction time, Python locates and calls just one __init__. If subclass constructors need to guarantee that superclass construction-time logic runs, too, they generally must call the superclass’s __init__ method explicitly through the class: class Super: def __init__(self, x): ...default code... class Sub(Super): def __init__(self, x, y): Super.__init__(self, x) ...custom code... # Run superclass __init__ # Do my init actions I = Sub(1, 2) This is one of the few contexts in which your code is likely to call an operator overloading method directly. Naturally, you should only call the superclass constructor this way if you really want it to run—without the call, the subclass replaces it completely. For a more realistic illustration of this technique in action, see the Manager class example in the prior chapter’s tutorial.‡ Other Method Call Possibilities This pattern of calling methods through a class is the general basis of extending (instead of completely replacing) inherited method behavior. In Chapter 31, we’ll also meet a new option added in Python 2.2, static methods, that allow you to code methods that do not expect instance objects in their first arguments. Such methods can act like simple instanceless functions, with names that are local to the classes in which they are coded, and may be used to manage class data. A related concept, the class method, receives a class when called instead of an instance and can be used to manage per-class data. These are advanced and optional extensions, though; normally, you must always pass an instance to a method, whether it is called through an instance or a class. ‡ On a somewhat related note, you can also code multiple __init__ methods within the same class, but only the last definition will be used; see Chapter 30 for more details on multiple method definitions. 686 | Chapter 28: Class Coding Details Inheritance The whole point of a namespace tool like the class statement is to support name inheritance. This section expands on some of the mechanisms and roles of attribute inheritance in Python. In Python, inheritance happens when an object is qualified, and it involves searching an attribute definition tree (one or more namespaces). Every time you use an expression of the form object.attr (where object is an instance or class object), Python searches the namespace tree from bottom to top, beginning with object, looking for the first attr it can find. This includes references to self attributes in your methods. Because lower definitions in the tree override higher ones, inheritance forms the basis of specialization. Attribute Tree Construction Figure 28-1 summarizes the way namespace trees are constructed and populated with names. Generally: • Instance attributes are generated by assignments to self attributes in methods. • Class attributes are created by statements (assignments) in class statements. • Superclass links are made by listing classes in parentheses in a class statement header. The net result is a tree of attribute namespaces that leads from an instance, to the class it was generated from, to all the superclasses listed in the class header. Python searches upward in this tree, from instances to superclasses, each time you use qualification to fetch an attribute name from an instance object.§ Specializing Inherited Methods The tree-searching model of inheritance just described turns out to be a great way to specialize systems. Because inheritance finds names in subclasses before it checks superclasses, subclasses can replace default behavior by redefining their superclasses’ attributes. In fact, you can build entire systems as hierarchies of classes, which are extended by adding new external subclasses rather than changing existing logic in-place. § This description isn’t 100% complete, because we can also create instance and class attributes by assigning to objects outside class statements—but that’s a much less common and sometimes more error-prone approach (changes aren’t isolated to class statements). In Python, all attributes are always accessible by default. We’ll talk more about attribute name privacy in Chapter 29 when we study __setattr__, in Chapter 30 when we meet __X names, and again in Chapter 38, where we’ll implement it with a class decorator. Inheritance | 687 The idea of redefining inherited names leads to a variety of specialization techniques. For instance, subclasses may replace inherited attributes completely, provide attributes that a superclass expects to find, and extend superclass methods by calling back to the superclass from an overridden method. We’ve already seen replacement in action. Here’s an example that shows how extension works: >>> class Super: ... def method(self): ... print('in Super.method') ... >>> class Sub(Super): ... def method(self): ... print('starting Sub.method') ... Super.method(self) ... print('ending Sub.method') ... # Override method # Add actions here # Run default action Figure 28-1. Program code creates a tree of objects in memory to be searched by attribute inheritance. Calling a class creates a new instance that remembers its class, running a class statement creates a new class, and superclasses are listed in parentheses in the class statement header. Each attribute reference triggers a new bottom-up tree search—even references to self attributes within a class’s methods. Direct superclass method calls are the crux of the matter here. The Sub class replaces Super’s method function with its own specialized version, but within the replacement, Sub calls back to the version exported by Super to carry out the default behavior. In other words, Sub.method just extends Super.method’s behavior, rather than replacing it completely: 688 | Chapter 28: Class Coding Details >>> x = Super() >>> x.method() in Super.method # Make a Super instance # Runs Super.method >>> x = Sub() >>> x.method() starting Sub.method in Super.method ending Sub.method # Make a Sub instance # Runs Sub.method, calls Super.method This extension coding pattern is also commonly used with constructors; see the section “Methods” on page 684 for an example. Class Interface Techniques Extension is only one way to interface with a superclass. The file shown in this section, specialize.py, defines multiple classes that illustrate a variety of common techniques: Super Defines a method function and a delegate that expects an action in a subclass. Inheritor Doesn’t provide any new names, so it gets everything defined in Super. Replacer Overrides Super’s method with a version of its own. Extender Customizes Super’s method by overriding and calling back to run the default. Provider Implements the action method expected by Super’s delegate method. Study each of these subclasses to get a feel for the various ways they customize their common superclass. Here’s the file: class Super: def method(self): print('in Super.method') def delegate(self): self.action() # Default behavior # Expected to be defined class Inheritor(Super): pass # Inherit method verbatim class Replacer(Super): def method(self): print('in Replacer.method') # Replace method completely class Extender(Super): def method(self): print('starting Extender.method') Super.method(self) print('ending Extender.method') # Extend method behavior Inheritance | 689 class Provider(Super): def action(self): print('in Provider.action') # Fill in a required method if __name__ == '__main__': for klass in (Inheritor, Replacer, Extender): print('\n' + klass.__name__ + '...') klass().method() print('\nProvider...') x = Provider() x.delegate() A few things are worth pointing out here. First, the self-test code at the end of this example creates instances of three different classes in a for loop. Because classes are objects, you can put them in a tuple and create instances generically (more on this idea later). Classes also have the special __name__ attribute, like modules; it’s preset to a string containing the name in the class header. Here’s what happens when we run the file: % python specialize.py Inheritor... in Super.method Replacer... in Replacer.method Extender... starting Extender.method in Super.method ending Extender.method Provider... in Provider.action Abstract Superclasses Notice how the Provider class in the prior example works. When we call the delegate method through a Provider instance, two independent inheritance searches occur: 1. On the initial x.delegate call, Python finds the delegate method in Super by searching the Provider instance and above. The instance x is passed into the method’s self argument as usual. 2. Inside the Super.delegate method, self.action invokes a new, independent inheritance search of self and above. Because self references a Provider instance, the action method is located in the Provider subclass. This “filling in the blanks” sort of coding structure is typical of OOP frameworks. At least in terms of the delegate method, the superclass in this example is what is sometimes called an abstract superclass—a class that expects parts of its behavior to be 690 | Chapter 28: Class Coding Details provided by its subclasses. If an expected method is not defined in a subclass, Python raises an undefined name exception when the inheritance search fails. Class coders sometimes make such subclass requirements more obvious with assert statements, or by raising the built-in NotImplementedError exception with raise statements (we’ll study statements that may trigger exceptions in depth in the next part of this book). As a quick preview, here’s the assert scheme in action: class Super: def delegate(self): self.action() def action(self): assert False, 'action must be defined!' # If this version is called >>> X = Super() >>> X.delegate() AssertionError: action must be defined! We’ll meet assert in Chapters 32 and 33; in short, if its first expression evaluates to false, it raises an exception with the provided error message. Here, the expression is always false so as to trigger an error message if a method is not redefined, and inheritance locates the version here. Alternatively, some classes simply raise a NotImplementedError exception directly in such method stubs to signal the mistake: class Super: def delegate(self): self.action() def action(self): raise NotImplementedError('action must be defined!') >>> X = Super() >>> X.delegate() NotImplementedError: action must be defined! For instances of subclasses, we still get the exception unless the subclass provides the expected method to replace the default in the superclass: >>> class Sub(Super): pass ... >>> X = Sub() >>> X.delegate() NotImplementedError: action must be defined! >>> class Sub(Super): ... def action(self): print('spam') ... >>> X = Sub() >>> X.delegate() spam For a somewhat more realistic example of this section’s concepts in action, see the “Zoo animal hierarchy” exercise (exercise 8) at the end of Chapter 31, and its solution in “Part VI, Classes and OOP” on page 1122 in Appendix B. Such taxonomies are a Inheritance | 691 traditional way to introduce OOP, but they’re a bit removed from most developers’ job descriptions. Python 2.6 and 3.0 Abstract Superclasses As of Python 2.6 and 3.0, the prior section’s abstract superclasses (a.k.a. “abstract base classes”), which require methods to be filled in by subclasses, may also be implemented with special class syntax. The way we code this varies slightly depending on the version. In Python 3.0, we use a keyword argument in a class header, along with special @ decorator syntax, both of which we’ll study in detail later in this book: from abc import ABCMeta, abstractmethod class Super(metaclass=ABCMeta): @abstractmethod def method(self, ...): pass But in Python 2.6, we use a class attribute instead: class Super: __metaclass__ = ABCMeta @abstractmethod def method(self, ...): pass Either way, the effect is the same—we can’t make an instance unless the method is defined lower in the class tree. In 3.0, for example, here is the special syntax equivalent of the prior section’s example: >>> from abc import ABCMeta, abstractmethod >>> >>> class Super(metaclass=ABCMeta): ... def delegate(self): ... self.action() ... @abstractmethod ... def action(self): ... pass ... >>> X = Super() TypeError: Can't instantiate abstract class Super with abstract methods action >>> class Sub(Super): pass ... >>> X = Sub() TypeError: Can't instantiate abstract class Sub with abstract methods action >>> class Sub(Super): ... def action(self): print('spam') ... >>> X = Sub() >>> X.delegate() spam 692 | Chapter 28: Class Coding Details Coded this way, a class with an abstract method cannot be instantiated (that is, we cannot create an instance by calling it) unless all of its abstract methods have been defined in subclasses. Although this requires more code, the advantage of this approach is that errors for missing methods are issued when we attempt to make an instance of the class, not later when we try to call a missing method. This feature may also be used to define an expected interface, automatically verified in client classes. Unfortunately, this scheme also relies on two advanced language tools we have not met yet—function decorators, introduced in Chapter 31 and covered in depth in Chapter 38, as well as metaclass declarations, mentioned in Chapter 31 and covered in Chapter 39—so we will finesse other facets of this option here. See Python’s standard manuals for more on this, as well as precoded abstract superclasses Python provides. Namespaces: The Whole Story Now that we’ve examined class and instance objects, the Python namespace story is complete. For reference, I’ll quickly summarize all the rules used to resolve names here. The first things you need to remember are that qualified and unqualified names are treated differently, and that some scopes serve to initialize object namespaces: • Unqualified names (e.g., X) deal with scopes. • Qualified attribute names (e.g., object.X) use object namespaces. • Some scopes initialize object namespaces (for modules and classes). Simple Names: Global Unless Assigned Unqualified simple names follow the LEGB lexical scoping rule outlined for functions in Chapter 17: Assignment (X = value) Makes names local: creates or changes the name X in the current local scope, unless declared global. Reference (X) Looks for the name X in the current local scope, then any and all enclosing functions, then the current global scope, then the built-in scope. Attribute Names: Object Namespaces Qualified attribute names refer to attributes of specific objects and obey the rules for modules and classes. For class and instance objects, the reference rules are augmented to include the inheritance search procedure: Namespaces: The Whole Story | 693 Assignment (object.X = value) Creates or alters the attribute name X in the namespace of the object being qualified, and none other. Inheritance-tree climbing happens only on attribute reference, not on attribute assignment. Reference (object.X) For class-based objects, searches for the attribute name X in object, then in all accessible classes above it, using the inheritance search procedure. For nonclass objects such as modules, fetches X from object directly. The “Zen” of Python Namespaces: Assignments Classify Names With distinct search procedures for qualified and unqualified names, and multiple lookup layers for both, it can sometimes be difficult to tell where a name will wind up going. In Python, the place where you assign a name is crucial—it fully determines the scope or object in which a name will reside. The file manynames.py illustrates how this principle translates to code and summarizes the namespace ideas we have seen throughout this book: # manynames.py X = 11 # Global (module) name/attribute (X, or manynames.X) def f(): print(X) # Access global X (11) def g(): X = 22 print(X) class C: X = 33 def m(self): X = 44 self.X = 55 # Local (function) variable (X, hides module X) # Class attribute (C.X) # Local variable in method (X) # Instance attribute (instance.X) This file assigns the same name, X, five times. Because this name is assigned in five different locations, though, all five Xs in this program are completely different variables. From top to bottom, the assignments to X here generate: a module attribute (11), a local variable in a function (22), a class attribute (33), a local variable in a method (44), and an instance attribute (55). Although all five are named X, the fact that they are all assigned at different places in the source code or to different objects makes all of these unique variables. You should take the time to study this example carefully because it collects ideas we’ve been exploring throughout the last few parts of this book. When it makes sense to you, you will have achieved a sort of Python namespace nirvana. Of course, an alternative route to nirvana is to simply run the program and see what happens. Here’s the remainder of this source file, which makes an instance and prints all the Xs that it can fetch: 694 | Chapter 28: Class Coding Details # manynames.py, continued if __name__ == '__main__': print(X) f() g() print(X) # 11: module (a.k.a. manynames.X outside file) # 11: global # 22: local # 11: module name unchanged obj = C() print(obj.X) # Make instance # 33: class name inherited by instance obj.m() print(obj.X) print(C.X) # Attach attribute name X to instance now # 55: instance # 33: class (a.k.a. obj.X if no X in instance) #print(C.m.X) #print(g.X) # FAILS: only visible in method # FAILS: only visible in function The outputs that are printed when the file is run are noted in the comments in the code; trace through them to see which variable named X is being accessed each time. Notice in particular that we can go through the class to fetch its attribute (C.X), but we can never fetch local variables in functions or methods from outside their def statements. Locals are visible only to other code within the def, and in fact only live in memory while a call to the function or method is executing. Some of the names defined by this file are visible outside the file to other modules, but recall that we must always import before we can access names in another file—that is the main point of modules, after all: # otherfile.py import manynames X = 66 print(X) print(manynames.X) # 66: the global here # 11: globals become attributes after imports manynames.f() manynames.g() # 11: manynames's X, not the one here! # 22: local in other file's function print(manynames.C.X) I = manynames.C() print(I.X) I.m() print(I.X) # 33: attribute of class in other module # 33: still from class here # 55: now from instance! Notice here how manynames.f() prints the X in manynames, not the X assigned in this file— scopes are always determined by the position of assignments in your source code (i.e., lexically) and are never influenced by what imports what or who imports whom. Also, notice that the instance’s own X is not created until we call I.m()—attributes, like all variables, spring into existence when assigned, and not before. Normally we create instance attributes by assigning them in class __init__ constructor methods, but this isn’t the only option. Namespaces: The Whole Story | 695 Finally, as we learned in Chapter 17, it’s also possible for a function to change names outside itself, with global and (in Python 3.0) nonlocal statements—these statements provide write access, but also modify assignment’s namespace binding rules: X = 11 # Global in module def g1(): print(X) # Reference global in module def g2(): global X X = 22 # Change global in module def h1(): X = 33 def nested(): print(X) def h2(): X = 33 def nested(): nonlocal X X = 44 # Local in function # Reference local in enclosing scope # Local in function # Python 3.0 statement # Change local in enclosing scope Of course, you generally shouldn’t use the same name for every variable in your script— but as this example demonstrates, even if you do, Python’s namespaces will work to keep names used in one context from accidentally clashing with those used in another. Namespace Dictionaries In Chapter 22, we learned that module namespaces are actually implemented as dictionaries and exposed with the built-in __dict__ attribute. The same holds for class and instance objects: attribute qualification is really a dictionary indexing operation internally, and attribute inheritance is just a matter of searching linked dictionaries. In fact, instance and class objects are mostly just dictionaries with links inside Python. Python exposes these dictionaries, as well as the links between them, for use in advanced roles (e.g., for coding tools). To help you understand how attributes work internally, let’s work through an interactive session that traces the way namespace dictionaries grow when classes are involved. We saw a simpler version of this type of code in Chapter 26, but now that we know more about methods and superclasses, let’s embellish it here. First, let’s define a superclass and a subclass with methods that will store data in their instances: >>> class super: ... def hello(self): ... self.data1 = 'spam' ... >>> class sub(super): ... def hola(self): 696 | Chapter 28: Class Coding Details ... ... self.data2 = 'eggs' When we make an instance of the subclass, the instance starts out with an empty namespace dictionary, but it has links back to the class for the inheritance search to follow. In fact, the inheritance tree is explicitly available in special attributes, which you can inspect. Instances have a __class__ attribute that links to their class, and classes have a __bases__ attribute that is a tuple containing links to higher superclasses (I’m running this on Python 3.0; name formats and some internal attributes vary slightly in 2.6): >>> X = sub() >>> X.__dict__ {} # Instance namespace dict >>> X.__class__ <class '__main__.sub'> # Class of instance >>> sub.__bases__ (<class '__main__.super'>,) # Superclasses of class >>> super.__bases__ (<class 'object'>,) # () empty tuple in Python 2.6 As classes assign to self attributes, they populate the instance objects—that is, attributes wind up in the instances’ attribute namespace dictionaries, not in the classes’. An instance object’s namespace records data that can vary from instance to instance, and self is a hook into that namespace: >>> Y = sub() >>> X.hello() >>> X.__dict__ {'data1': 'spam'} >>> X.hola() >>> X.__dict__ {'data1': 'spam', 'data2': 'eggs'} >>> sub.__dict__.keys() ['__module__', '__doc__', 'hola'] >>> super.__dict__.keys() ['__dict__', '__module__', '__weakref__', 'hello', '__doc__'] >>> Y.__dict__ {} Notice the extra underscore names in the class dictionaries; Python sets these automatically. Most are not used in typical programs, but there are tools that use some of them (e.g., __doc__ holds the docstrings discussed in Chapter 15). Namespaces: The Whole Story | 697 Also, observe that Y, a second instance made at the start of this series, still has an empty namespace dictionary at the end, even though X’s dictionary has been populated by assignments in methods. Again, each instance has an independent namespace dictionary, which starts out empty and can record completely different attributes than those recorded by the namespace dictionaries of other instances of the same class. Because attributes are actually dictionary keys inside Python, there are really two ways to fetch and assign their values—by qualification, or by key indexing: >>> X.data1, X.__dict__['data1'] ('spam', 'spam') >>> X.data3 = 'toast' >>> X.__dict__ {'data1': 'spam', 'data3': 'toast', 'data2': 'eggs'} >>> X.__dict__['data3'] = 'ham' >>> X.data3 'ham' This equivalence applies only to attributes actually attached to the instance, though. Because attribute fetch qualification also performs an inheritance search, it can access attributes that namespace dictionary indexing cannot. The inherited attribute X.hello, for instance, cannot be accessed by X.__dict__['hello']. Finally, here is the built-in dir function we met in Chapters 4 and 15 at work on class and instance objects. This function works on anything with attributes: dir(object) is similar to an object.__dict__.keys() call. Notice, though, that dir sorts its list and includes some system attributes. As of Python 2.2, dir also collects inherited attributes automatically, and in 3.0 it includes names inherited from the object class that is an implied superclass of all classes:‖ >>> X.__dict__, Y.__dict__ ({'data1': 'spam', 'data3': 'ham', 'data2': 'eggs'}, {}) >>> list(X.__dict__.keys()) ['data1', 'data3', 'data2'] # Need list in 3.0 # In Python 2.6: >>>> dir(X) ['__doc__', '__module__', 'data1', 'data2', 'data3', 'hello', 'hola'] >>> dir(sub) ['__doc__', '__module__', 'hello', 'hola'] >>> dir(super) ['__doc__', '__module__', 'hello'] ‖ As you can see, the contents of attribute dictionaries and dir call results may change over time. For example, because Python now allows built-in types to be subclassed like classes, the contents of dir results for builtin types have expanded to include operator overloading methods, just like our dir results here for user-defined classes under Python 3.0. In general, attribute names with leading and trailing double underscores are interpreter-specific. Type subclasses will be discussed further in Chapter 31. 698 | Chapter 28: Class Coding Details # In Python 3.0: >>> dir(X) ['__class__', '__delattr__', '__dict__', '__doc__', '__eq__', '__format__', ...more omitted... 'data1', 'data2', 'data3', 'hello', 'hola'] >>> dir(sub) ['__class__', '__delattr__', '__dict__', '__doc__', '__eq__', '__format__', ...more omitted... 'hello', 'hola'] >>> dir(super) ['__class__', '__delattr__', '__dict__', '__doc__', '__eq__', '__format__', ...more omitted... 'hello' ] Experiment with these special attributes on your own to get a better feel for how namespaces actually do their attribute business. Even if you will never use these in the kinds of programs you write, seeing that they are just normal dictionaries will help demystify the notion of namespaces in general. Namespace Links The prior section introduced the special __class__ and __bases__ instance and class attributes, without really explaining why you might care about them. In short, these attributes allow you to inspect inheritance hierarchies within your own code. For example, they can be used to display a class tree, as in the following example: # classtree.py """ Climb inheritance trees using namespace links, displaying higher superclasses with indentation """ def classtree(cls, indent): print('.' * indent + cls.__name__) for supercls in cls.__bases__: classtree(supercls, indent+3) # Print class name here # Recur to all superclasses # May visit super > once def instancetree(inst): print('Tree of %s' % inst) classtree(inst.__class__, 3) # Show instance # Climb to its class def selftest(): class A: class B(A): class C(A): class D(B,C): class E: class F(D,E): pass pass pass pass pass pass Namespaces: The Whole Story | 699 instancetree(B()) instancetree(F()) if __name__ == '__main__': selftest() The classtree function in this script is recursive—it prints a class’s name using __name__, then climbs up to the superclasses by calling itself. This allows the function to traverse arbitrarily shaped class trees; the recursion climbs to the top, and stops at root superclasses that have empty __bases__ attributes. When using recursion, each active level of a function gets its own copy of the local scope; here, this means that cls and indent are different at each classtree level. Most of this file is self-test code. When run standalone in Python 3.0, it builds an empty class tree, makes two instances from it, and prints their class tree structures: C:\misc> c:\python26\python classtree.py Tree of <__main__.B instance at 0x02557328> ...B ......A Tree of <__main__.F instance at 0x02557328> ...F ......D .........B ............A .........C ............A ......E When run under Python 3.0, the tree includes the implied object superclasses that are automatically added above standalone classes, because all classes are “new style” in 3.0 (more on this change in Chapter 31): C:\misc> c:\python30\python classtree.py Tree of <__main__.B object at 0x02810650> ...B ......A .........object Tree of <__main__.F object at 0x02810650> ...F ......D .........B ............A ...............object .........C ............A ...............object ......E .........object Here, indentation marked by periods is used to denote class tree height. Of course, we could improve on this output format, and perhaps even sketch it in a GUI display. Even as is, though, we can import these functions anywhere we want a quick class tree display: 700 | Chapter 28: Class Coding Details C:\misc> c:\python30\python >>> class Emp: pass ... >>> class Person(Emp): pass >>> bob = Person() >>> import classtree >>> classtree.instancetree(bob) Tree of <__main__.Person object at 0x028203B0> ...Person ......Emp .........object Regardless of whether you will ever code or use such tools, this example demonstrates one of the many ways that you can make use of special attributes that expose interpreter internals. You’ll see another when we code the lister.py general-purpose class display tools in the section “Multiple Inheritance: “Mix-in” Classes” on page 756—there, we will extend this technique to also display attributes in each object in a class tree. And in the last part of this book, we’ll revisit such tools in the context of Python tool building at large, to code tools that implement attribute privacy, argument validation, and more. While not for every Python programmer, access to internals enables powerful development tools. Documentation Strings Revisited The last section’s example includes a docstring for its module, but remember that docstrings can be used for class components as well. Docstrings, which we covered in detail in Chapter 15, are string literals that show up at the top of various structures and are automatically saved by Python in the corresponding objects’ __doc__ attributes. This works for module files, function defs, and classes and methods. Now that we know more about classes and methods, the following file, docstr.py, provides a quick but comprehensive example that summarizes the places where docstrings can show up in your code. All of these can be triple-quoted blocks: "I am: docstr.__doc__" def func(args): "I am: docstr.func.__doc__" pass class spam: "I am: spam.__doc__ or docstr.spam.__doc__" def method(self, arg): "I am: spam.method.__doc__ or self.method.__doc__" pass Documentation Strings Revisited | 701 The main advantage of documentation strings is that they stick around at runtime. Thus, if it’s been coded as a docstring, you can qualify an object with its __doc__ attribute to fetch its documentation: >>> import docstr >>> docstr.__doc__ 'I am: docstr.__doc__' >>> docstr.func.__doc__ 'I am: docstr.func.__doc__' >>> docstr.spam.__doc__ 'I am: spam.__doc__ or docstr.spam.__doc__' >>> docstr.spam.method.__doc__ 'I am: spam.method.__doc__ or self.method.__doc__' A discussion of the PyDoc tool, which knows how to format all these strings in reports, appears in Chapter 15. Here it is running on our code under Python 2.6 (Python 3.0 shows additional attributes inherited from the implied object superclass in the newstyle class model—run this on your own to see the 3.0 extras, and watch for more about this difference in Chapter 31): >>> help(docstr) Help on module docstr: NAME docstr - I am: docstr.__doc__ FILE c:\misc\docstr.py CLASSES spam class spam | I am: spam.__doc__ or docstr.spam.__doc__ | | Methods defined here: | | method(self, arg) | I am: spam.method.__doc__ or self.method.__doc__ FUNCTIONS func(args) I am: docstr.func.__doc__ Documentation strings are available at runtime, but they are less flexible syntactically than # comments (which can appear anywhere in a program). Both forms are useful tools, and any program documentation is good (as long as it’s accurate, of course!). As a best-practice rule of thumb, use docstrings for functional documentation (what your objects do) and hash-mark comments for more micro-level documentation (how arcane expressions work). 702 | Chapter 28: Class Coding Details Classes Versus Modules Let’s wrap up this chapter by briefly comparing the topics of this book’s last two parts: modules and classes. Because they’re both about namespaces, the distinction can be confusing. In short: • Modules — Are data/logic packages — Are created by writing Python files or C extensions — Are used by being imported • Classes — Implement new objects — Are created by class statements — Are used by being called — Always live within a module Classes also support extra features that modules don’t, such as operator overloading, multiple instance generation, and inheritance. Although both classes and modules are namespaces, you should be able to tell by now that they are very different things. Chapter Summary This chapter took us on a second, more in-depth tour of the OOP mechanisms of the Python language. We learned more about classes, methods, and inheritance, and we wrapped up the namespace story in Python by extending it to cover its application to classes. Along the way, we looked at some more advanced concepts, such as abstract superclasses, class data attributes, namespace dictionaries and links, and manual calls to superclass methods and constructors. Now that we’ve learned all about the mechanics of coding classes in Python, Chapter 29 turns to a specific facet of those mechanics: operator overloading. After that we’ll explore common design patterns, looking at some of the ways that classes are commonly used and combined to optimize code reuse. Before you read ahead, though, be sure to work though the usual chapter quiz to review what we’ve covered here. Test Your Knowledge: Quiz 1. What is an abstract superclass? 2. What happens when a simple assignment statement appears at the top level of a class statement? Test Your Knowledge: Quiz | 703 3. Why might a class need to manually call the __init__ method in a superclass? 4. How can you augment, instead of completely replacing, an inherited method? 5. What...was the capital of Assyria? Test Your Knowledge: Answers 1. An abstract superclass is a class that calls a method, but does not inherit or define it—it expects the method to be filled in by a subclass. This is often used as a way to generalize classes when behavior cannot be predicted until a more specific subclass is coded. OOP frameworks also use this as a way to dispatch to client-defined, customizable operations. 2. When a simple assignment statement (X = Y) appears at the top level of a class statement, it attaches a data attribute to the class (Class.X). Like all class attributes, this will be shared by all instances; data attributes are not callable method functions, though. 3. A class must manually call the __init__ method in a superclass if it defines an __init__ constructor of its own, but it also must still kick off the superclass’s construction code. Python itself automatically runs just one constructor—the lowest one in the tree. Superclass constructors are called through the class name, passing in the self instance manually: Superclass.__init__(self, ...). 4. To augment instead of completely replacing an inherited method, redefine it in a subclass, but call back to the superclass’s version of the method manually from the new version of the method in the subclass. That is, pass the self instance to the superclass’s version of the method manually: Superclass.method(self, ...). 5. Ashur (or Qalat Sherqat), Calah (or Nimrud), the short-lived Dur Sharrukin (or Khorsabad), and finally Nineveh. 704 | Chapter 28: Class Coding Details CHAPTER 29 Operator Overloading This chapter continues our in-depth survey of class mechanics by focusing on operator overloading. We looked briefly at operator overloading in prior chapters; here, we’ll fill in more details and look at a handful of commonly used overloading methods. Although we won’t demonstrate each of the many operator overloading methods available, those we will code here are a representative sample large enough to uncover the possibilities of this Python class feature. The Basics Really “operator overloading” simply means intercepting built-in operations in class methods—Python automatically invokes your methods when instances of the class appear in built-in operations, and your method’s return value becomes the result of the corresponding operation. Here’s a review of the key ideas behind overloading: • Operator overloading lets classes intercept normal Python operations. • Classes can overload all Python expression operators. • Classes can also overload built-in operations such as printing, function calls, attribute access, etc. • Overloading makes class instances act more like built-in types. • Overloading is implemented by providing specially named class methods. In other words, when certain specially named methods are provided in a class, Python automatically calls them when instances of the class appear in their associated expressions. As we’ve learned, operator overloading methods are never required and generally don’t have defaults; if you don’t code or inherit one, it just means that your class does not support the corresponding operation. When used, though, these methods allow classes to emulate the interfaces of built-in objects, and so appear more consistent. 705 Constructors and Expressions: __init__ and __sub__ Consider the following simple example: its Number class, coded in the file number.py, provides a method to intercept instance construction (__init__), as well as one for catching subtraction expressions (__sub__). Special methods such as these are the hooks that let you tie into built-in operations: class Number: def __init__(self, start): self.data = start def __sub__(self, other): return Number(self.data - other) >>> >>> >>> >>> 3 # On Number(start) # On instance - other # Result is a new instance # Fetch class from module # Number.__init__(X, 5) # Number.__sub__(X, 2) # Y is new Number instance from number import Number X = Number(5) Y = X – 2 Y.data As discussed previously, the __init__ constructor method seen in this code is the most commonly used operator overloading method in Python; it’s present in most classes. In this chapter, we will tour some of the other tools available in this domain and look at example code that applies them in common use cases. Common Operator Overloading Methods Just about everything you can do to built-in objects such as integers and lists has a corresponding specially named method for overloading in classes. Table 29-1 lists a few of the most common; there are many more. In fact, many overloading methods come in multiple versions (e.g., __add__, __radd__, and __iadd__ for addition), which is one reason there are so many. See other Python books, or the Python language reference manual, for an exhaustive list of the special method names available. Table 29-1. Common operator overloading methods Method Implements Called for __init__ Constructor Object creation: X = Class(args) __del__ Destructor Object reclamation of X __add__ Operator + X + Y, X += Y if no __iadd__ __or__ Operator | (bitwise OR) X | Y, X |= Y if no __ior__ __repr__, __str__ Printing, conversions print(X), repr(X), str(X) __call__ Function calls X(*args, **kargs) __getattr__ Attribute fetch X.undefined __setattr__ Attribute assignment X.any = value __delattr__ Attribute deletion del X.any __getattribute__ Attribute fetch X.any 706 | Chapter 29: Operator Overloading Method Implements Called for __getitem__ Indexing, slicing, iteration X[key], X[i:j], for loops and other iterations if no __iter__ __setitem__ Index and slice assignment X[key] = value, X[i:j] = sequence __delitem__ Index and slice deletion del X[key], del X[i:j] __len__ Length len(X), truth tests if no __bool__ __bool__ Boolean tests bool(X), truth tests (named __nonzero__ in 2.6) __lt__, __gt__, __le__, __ge__, __eq__, __ne__ Comparisons X < Y, X > Y, X <= Y, X >= Y, X == Y, X != Y (or else __cmp__ in 2.6 only) __radd__ Right-side operators Other + X __iadd__ In-place augmented operators X += Y (or else __add__) __iter__, __next__ Iteration contexts I=iter(X), next(I); for loops, in if no __contains__, all comprehensions, map(F,X), others (__next__ is named next in 2.6) __contains__ Membership test item in X (any iterable) __index__ Integer value hex(X), bin(X), oct(X), O[X], O[X:] (replaces Python 2 __oct__, __hex__) __enter__, __exit__ Context manager (Chapter 33) with obj as var: __get__, __set__, __delete__ Descriptor attributes (Chapter 37) X.attr, X.attr = value, del X.attr __new__ Creation (Chapter 39) Object creation, before __init__ All overloading methods have names that start and end with two underscores to keep them distinct from other names you define in your classes. The mappings from special method names to expressions or operations are predefined by the Python language (and documented in the standard language manual). For example, the name __add__ always maps to + expressions by Python language definition, regardless of what an __add__ method’s code actually does. Operator overloading methods may be inherited from superclasses if not defined, just like any other methods. Operator overloading methods are also all optional—if you don’t code or inherit one, that operation is simply unsupported by your class, and attempting it will raise an exception. Some built-in operations, like printing, have defaults (inherited for the implied object class in Python 3.0), but most built-ins fail for class instances if no corresponding operator overloading method is present. Most overloading methods are used only in advanced programs that require objects to behave like built-ins; the __init__ constructor tends to appear in most classes, however, so pay special attention to it. We’ve already met the __init__ initialization-time constructor method, and a few of the others in Table 29-1. Let’s explore some of the additional methods in the table by example. The Basics | 707 Indexing and Slicing: __getitem__ and __setitem__ If defined in a class (or inherited by it), the __getitem__ method is called automatically for instance-indexing operations. When an instance X appears in an indexing expression like X[i], Python calls the __getitem__ method inherited by the instance, passing X to the first argument and the index in brackets to the second argument. For example, the following class returns the square of an index value: >>> class Indexer: ... def __getitem__(self, index): ... return index ** 2 ... >>> X = Indexer() >>> X[2] 4 >>> for i in range(5): ... print(X[i], end=' ') ... 0 1 4 9 16 # X[i] calls X.__getitem__(i) # Runs __getitem__(X, i) each time Intercepting Slices Interestingly, in addition to indexing, __getitem__ is also called for slice expressions. Formally speaking, built-in types handle slicing the same way. Here, for example, is slicing at work on a built-in list, using upper and lower bounds and a stride (see Chapter 7 if you need a refresher on slicing): >>> >>> [7, >>> [6, >>> [5, >>> [5, L = [5, 6, 7, 8, 9] L[2:4] 8] L[1:] 7, 8, 9] L[:-1] 6, 7, 8] L[::2] 7, 9] # Slice with slice syntax Really, though, slicing bounds are bundled up into a slice object and passed to the list’s implementation of indexing. In fact, you can always pass a slice object manually—slice syntax is mostly syntactic sugar for indexing with a slice object: >>> [7, >>> [6, >>> [5, >>> [5, L[slice(2, 4)] 8] L[slice(1, None)] 7, 8, 9] L[slice(None, −1)] 6, 7, 8] L[slice(None, None, 2)] 7, 9] 708 | Chapter 29: Operator Overloading # Slice with slice objects This matters in classes with a __getitem__ method—the method will be called both for basic indexing (with an index) and for slicing (with a slice object). Our previous class won’t handle slicing because its math assumes integer indexes are passed, but the following class will. When called for indexing, the argument is an integer as before: >>> class Indexer: ... data = [5, 6, 7, 8, 9] ... def __getitem__(self, index): ... print('getitem:', index) ... return self.data[index] ... >>> X = Indexer() >>> X[0] getitem: 0 5 >>> X[1] getitem: 1 6 >>> X[-1] getitem: −1 9 # Called for index or slice # Perform index or slice # Indexing sends __getitem__ an integer When called for slicing, though, the method receives a slice object, which is simply passed along to the embedded list indexer in a new index expression: >>> X[2:4] getitem: slice(2, 4, None) [7, 8] >>> X[1:] getitem: slice(1, None, None) [6, 7, 8, 9] >>> X[:-1] getitem: slice(None, −1, None) [5, 6, 7, 8] >>> X[::2] getitem: slice(None, None, 2) [5, 7, 9] # Slicing sends __getitem__ a slice object If used, the __setitem__ index assignment method similarly intercepts both index and slice assignments—it receives a slice object for the latter, which may be passed along in another index assignment in the same way: def __setitem__(self, index, value): ... self.data[index] = value # Intercept index or slice assignment # Assign index or slice In fact, __getitem__ may be called automatically in even more contexts than indexing and slicing, as the next section explains. Slicing and Indexing in Python 2.6 Prior to Python 3.0, classes could also define __getslice__ and __setslice__ methods to intercept slice fetches and assignments specifically; they were passed the bounds of the slice expression and were preferred over __getitem__ and __setitem__ for slices. Indexing and Slicing: __getitem__ and __setitem__ | 709 These slice-specific methods have been removed in 3.0, so you should use __getitem__ and __setitem__ instead and allow for both indexes and slice objects as arguments. In most classes, this works without any special code, because indexing methods can manually pass along the slice object in the square brackets of another index expression (as in our example). See the section “Membership: __contains__, __iter__, and __getitem__” on page 716 for another example of slice interception at work. Also, don’t confuse the (arguably unfortunately named) __index__ method in Python 3.0 for index interception; this method returns an integer value for an instance when needed and is used by built-ins that convert to digit strings: >>> class C: ... def __index__(self): ... return 255 ... >>> X = C() >>> hex(X) # Integer value '0xff' >>> bin(X) '0b11111111' >>> oct(X) '0o377' Although this method does not intercept instance indexing like __getitem__, it is also used in contexts that require an integer—including indexing: >>> ('C' * 256)[255] 'C' >>> ('C' * 256)[X] 'C' >>> ('C' * 256)[X:] 'C' # As index (not X[i]) # As index (not X[i:]) This method works the same way in Python 2.6, except that it is not called for the hex and oct built-in functions (use __hex__ and __oct__ in 2.6 instead to intercept these calls). Index Iteration: __getitem__ Here’s a trick that isn’t always obvious to beginners, but turns out to be surprisingly useful. The for statement works by repeatedly indexing a sequence from zero to higher indexes, until an out-of-bounds exception is detected. Because of that, __getitem__ also turns out to be one way to overload iteration in Python—if this method is defined, for loops call the class’s __getitem__ each time through, with successively higher offsets. It’s a case of “buy one, get one free”—any built-in or user-defined object that responds to indexing also responds to iteration: >>> class stepper: ... def __getitem__(self, i): ... return self.data[i] ... 710 | Chapter 29: Operator Overloading >>> >>> >>> >>> 'p' >>> ... ... S p X = stepper() X.data = "Spam" # X is a stepper object X[1] # Indexing calls __getitem__ for item in X: print(item, end=' ') # for loops call __getitem__ # for indexes items 0..N a m In fact, it’s really a case of “buy one, get a bunch free.” Any class that supports for loops automatically supports all iteration contexts in Python, many of which we’ve seen in earlier chapters (iteration contexts were presented in Chapter 14). For example, the in membership test, list comprehensions, the map built-in, list and tuple assignments, and type constructors will also call __getitem__ automatically, if it’s defined: >>> 'p' in X True # All call __getitem__ too >>> [c for c in X] ['S', 'p', 'a', 'm'] # List comprehension >>> list(map(str.upper, X)) ['S', 'P', 'A', 'M'] # map calls (use list() in 3.0) >>> (a, b, c, d) = X >>> a, c, d ('S', 'a', 'm') # Sequence assignments >>> list(X), tuple(X), ''.join(X) (['S', 'p', 'a', 'm'], ('S', 'p', 'a', 'm'), 'Spam') >>> X <__main__.stepper object at 0x00A8D5D0> In practice, this technique can be used to create objects that provide a sequence interface and to add logic to built-in sequence type operations; we’ll revisit this idea when extending built-in types in Chapter 31. Iterator Objects: __iter__ and __next__ Although the __getitem__ technique of the prior section works, it’s really just a fallback for iteration. Today, all iteration contexts in Python will try the __iter__ method first, before trying __getitem__. That is, they prefer the iteration protocol we learned about in Chapter 14 to repeatedly indexing an object; only if the object does not support the iteration protocol is indexing attempted instead. Generally speaking, you should prefer __iter__ too—it supports general iteration contexts better than __getitem__ can. Technically, iteration contexts work by calling the iter built-in function to try to find an __iter__ method, which is expected to return an iterator object. If it’s provided, Python then repeatedly calls this iterator object’s __next__ method to produce items Iterator Objects: __iter__ and __next__ | 711 until a StopIteration exception is raised. If no such __iter__ method is found, Python falls back on the __getitem__ scheme and repeatedly indexes by offsets as before, until an IndexError exception is raised. A next built-in function is also available as a convenience for manual iterations: next(I) is the same as I.__next__(). Version skew note: As described in Chapter 14, if you are using Python 2.6, the I.__next__() method just described is named I.next() in your Python, and the next(I) built-in is present for portability: it calls I.next() in 2.6 and I.__next__() in 3.0. Iteration works the same in 2.6 in all other respects. User-Defined Iterators In the __iter__ scheme, classes implement user-defined iterators by simply implementing the iteration protocol introduced in Chapters 14 and 20 (refer back to those chapters for more background details on iterators). For example, the following file, iters.py, defines a user-defined iterator class that generates squares: class Squares: def __init__(self, start, stop): self.value = start - 1 self.stop = stop def __iter__(self): return self def __next__(self): if self.value == self.stop: raise StopIteration self.value += 1 return self.value ** 2 % python >>> from iters import Squares >>> for i in Squares(1, 5): ... print(i, end=' ') ... 1 4 9 16 25 # Save state when created # Get iterator object on iter # Return a square on each iteration # Also called by next built-in # for calls iter, which calls __iter__ # Each iteration calls __next__ Here, the iterator object is simply the instance self, because the __next__ method is part of this class. In more complex scenarios, the iterator object may be defined as a separate class and object with its own state information to support multiple active iterations over the same data (we’ll see an example of this in a moment). The end of the iteration is signaled with a Python raise statement (more on raising exceptions in the next part of this book). Manual iterations work as for built-in types as well: >>> >>> >>> 1 >>> 4 X = Squares(1, 5) I = iter(X) next(I) next(I) 712 | Chapter 29: Operator Overloading # Iterate manually: what loops do # iter calls __iter__ # next calls __next__ ...more omitted... >>> next(I) 25 >>> next(I) StopIteration # Can catch this in try statement An equivalent coding of this iterator with __getitem__ might be less natural, because the for would then iterate through all offsets zero and higher; the offsets passed in would be only indirectly related to the range of values produced (0..N would need to map to start..stop). Because __iter__ objects retain explicitly managed state between next calls, they can be more general than __getitem__. On the other hand, using iterators based on __iter__ can sometimes be more complex and less convenient than using __getitem__. They are really designed for iteration, not random indexing—in fact, they don’t overload the indexing expression at all: >>> X = Squares(1, 5) >>> X[1] AttributeError: Squares instance has no attribute '__getitem__' The __iter__ scheme is also the implementation for all the other iteration contexts we saw in action for __getitem__ (membership tests, type constructors, sequence assignment, and so on). However, unlike our prior __getitem__ example, we also need to be aware that a class’s __iter__ may be designed for a single traversal, not many. For example, the Squares class is a one-shot iteration; once you’ve iterated over an instance of that class, it’s empty. You need to make a new iterator object for each new iteration: >>> >>> [1, >>> [] >>> [1, >>> [1, X = Squares(1, 5) [n for n in X] 4, 9, 16, 25] [n for n in X] [n for n in Squares(1, 5)] 4, 9, 16, 25] list(Squares(1, 3)) 4, 9] # Exhausts items # Now it's empty # Make a new iterator object Notice that this example would probably be simpler if it were coded with generator functions (topics or expressions introduced in Chapter 20 and related to iterators): >>> def gsquares(start, stop): ... for i in range(start, stop+1): ... yield i ** 2 ... >>> for i in gsquares(1, 5): ... print(i, end=' ') ... 1 4 9 16 25 # or: (x ** 2 for x in range(1, 5)) Unlike the class, the function automatically saves its state between iterations. Of course, for this artificial example, you could in fact skip both techniques and simply use a for loop, map, or a list comprehension to build the list all at once. The best and fastest way to accomplish a task in Python is often also the simplest: Iterator Objects: __iter__ and __next__ | 713 >>> [x ** 2 for x in range(1, 6)] [1, 4, 9, 16, 25] However, classes may be better at modeling more complex iterations, especially when they can benefit from state information and inheritance hierarchies. The next section explores one such use case. Multiple Iterators on One Object Earlier, I mentioned that the iterator object may be defined as a separate class with its own state information to support multiple active iterations over the same data. Consider what happens when we step across a built-in type like a string: >>> S = 'ace' >>> for x in S: ... for y in S: ... print(x + y, end=' ') ... aa ac ae ca cc ce ea ec ee Here, the outer loop grabs an iterator from the string by calling iter, and each nested loop does the same to get an independent iterator. Because each active iterator has its own state information, each loop can maintain its own position in the string, regardless of any other active loops. We saw related examples earlier, in Chapters 14 and 20. For instance, generator functions and expressions, as well as built-ins like map and zip, proved to be single-iterator objects; by contrast, the range built-in and other built-in types, like lists, support multiple active iterators with independent positions. When we code user-defined iterators with classes, it’s up to us to decide whether we will support a single active iteration or many. To achieve the multiple-iterator effect, __iter__ simply needs to define a new stateful object for the iterator, instead of returning self. The following, for example, defines an iterator class that skips every other item on iterations. Because the iterator object is created anew for each iteration, it supports multiple active loops: class SkipIterator: def __init__(self, wrapped): self.wrapped = wrapped self.offset = 0 def __next__(self): if self.offset >= len(self.wrapped): raise StopIteration else: item = self.wrapped[self.offset] self.offset += 2 return item class SkipObject: 714 | Chapter 29: Operator Overloading # Iterator state information # Terminate iterations # else return and skip def __init__(self, wrapped): self.wrapped = wrapped def __iter__(self): return SkipIterator(self.wrapped) if __name__ == '__main__': alpha = 'abcdef' skipper = SkipObject(alpha) I = iter(skipper) print(next(I), next(I), next(I)) for x in skipper: for y in skipper: print(x + y, end=' ') # Save item to be used # New iterator each time # Make container object # Make an iterator on it # Visit offsets 0, 2, 4 # for calls __iter__ automatically # Nested fors call __iter__ again each time # Each iterator has its own state, offset When run, this example works like the nested loops with built-in strings. Each active loop has its own position in the string because each obtains an independent iterator object that records its own state information: % python skipper.py a c e aa ac ae ca cc ce ea ec ee By contrast, our earlier Squares example supports just one active iteration, unless we call Squares again in nested loops to obtain new objects. Here, there is just one SkipObject, with multiple iterator objects created from it. As before, we could achieve similar results with built-in tools—for example, slicing with a third bound to skip items: >>> S = 'abcdef' >>> for x in S[::2]: ... for y in S[::2]: ... print(x + y, end=' ') ... aa ac ae ca cc ce ea ec ee # New objects on each iteration This isn’t quite the same, though, for two reasons. First, each slice expression here will physically store the result list all at once in memory; iterators, on the other hand, produce just one value at a time, which can save substantial space for large result lists. Second, slices produce new objects, so we’re not really iterating over the same object in multiple places here. To be closer to the class, we would need to make a single object to step across by slicing ahead of time: >>> S = 'abcdef' >>> S = S[::2] >>> S 'ace' >>> for x in S: ... for y in S: ... print(x + y, end=' ') ... aa ac ae ca cc ce ea ec ee # Same object, new iterators Iterator Objects: __iter__ and __next__ | 715 This is more similar to our class-based solution, but it still stores the slice result in memory all at once (there is no generator form of built-in slicing today), and it’s only equivalent for this particular case of skipping every other item. Because iterators can do anything a class can do, they are much more general than this example may imply. Regardless of whether our applications require such generality, user-defined iterators are a powerful tool—they allow us to make arbitrary objects look and feel like the other sequences and iterables we have met in this book. We could use this technique with a database object, for example, to support iterations over database fetches, with multiple cursors into the same query result. Membership: __contains__, __iter__, and __getitem__ The iteration story is even richer than we’ve seen thus far. Operator overloading is often layered: classes may provide specific methods, or more general alternatives used as fallback options. For example: • Comparisons in Python 2.6 use specific methods such as __lt__ for less than if present, or else the general __cmp__. Python 3.0 uses only specific methods, not __cmp__, as discussed later in this chapter. • Boolean tests similarly try a specific __bool__ first (to give an explicit True/False result), and if it’s absent fall back on the more general __len__ (a nonzero length means True). As we’ll also see later in this chapter, Python 2.6 works the same but uses the name __nonzero__ instead of __bool__. In the iterations domain, classes normally implement the in membership operator as an iteration, using either the __iter__ method or the __getitem__ method. To support more specific membership, though, classes may code a __contains__ method—when present, this method is preferred over __iter__, which is preferred over __getitem__. The __contains__ method should define membership as applying to keys for a mapping (and can use quick lookups), and as a search for sequences. Consider the following class, which codes all three methods and tests membership and various iteration contexts applied to an instance. Its methods print trace messages when called: class Iters: def __init__(self, value): self.data = value def __getitem__(self, i): print('get[%s]:' % i, end='') return self.data[i] def __iter__(self): print('iter=> ', end='') self.ix = 0 return self def __next__(self): print('next:', end='') 716 | Chapter 29: Operator Overloading # Fallback for iteration # Also for index, slice # Preferred for iteration # Allows only 1 active iterator if self.ix == len(self.data): raise StopIteration item = self.data[self.ix] self.ix += 1 return item def __contains__(self, x): # Preferred for 'in' print('contains: ', end='') return x in self.data X = Iters([1, 2, 3, 4, 5]) print(3 in X) for i in X: print(i, end=' | ') print() print([i ** 2 for i in X]) print( list(map(bin, X)) ) I = iter(X) while True: try: print(next(I), end=' @ ') except StopIteration: break # Make instance # Membership # For loops # Other iteration contexts # Manual iteration (what other contexts do) When run as it is, this script’s output is as follows—the specific __contains__ intercepts membership, the general __iter__ catches other iteration contexts such that __next__ is called repeatedly, and __getitem__ is never called: contains: True iter=> next:1 | next:2 | next:3 | next:4 | next:5 | next: iter=> next:next:next:next:next:next:[1, 4, 9, 16, 25] iter=> next:next:next:next:next:next:['0b1', '0b10', '0b11', '0b100', '0b101'] iter=> next:1 @ next:2 @ next:3 @ next:4 @ next:5 @ next: Watch what happens to this code’s output if we comment out its __contains__ method, though—membership is now routed to the general __iter__ instead: iter=> iter=> iter=> iter=> iter=> next:next:next:True next:1 | next:2 | next:3 | next:4 | next:5 | next: next:next:next:next:next:next:[1, 4, 9, 16, 25] next:next:next:next:next:next:['0b1', '0b10', '0b11', '0b100', '0b101'] next:1 @ next:2 @ next:3 @ next:4 @ next:5 @ next: And finally, here is the output if both __contains__ and __iter__ are commented out— the indexing __getitem__ fallback is called with successively higher indexes for membership and other iteration contexts: get[0]:get[1]:get[2]:True get[0]:1 | get[1]:2 | get[2]:3 | get[3]:4 | get[4]:5 | get[5]: get[0]:get[1]:get[2]:get[3]:get[4]:get[5]:[1, 4, 9, 16, 25] get[0]:get[1]:get[2]:get[3]:get[4]:get[5]:['0b1', '0b10', '0b11', '0b100','0b101'] get[0]:1 @ get[1]:2 @ get[2]:3 @ get[3]:4 @ get[4]:5 @ get[5]: Membership: __contains__, __iter__, and __getitem__ | 717 As we’ve seen, the __getitem__ method is even more general: besides iterations, it also intercepts explicit indexing as well as slicing. Slice expressions trigger __getitem__ with a slice object containing bounds, both for built-in types and user-defined classes, so slicing is automatic in our class: >>> X = Iters('spam') >>> X[0] get[0]:'s' # Indexing # __getitem__(0) >>> 'spam'[1:] 'pam' >>> 'spam'[slice(1, None)] 'pam' # Slice syntax >>> X[1:] get[slice(1, None, None)]:'pam' >>> X[:-1] get[slice(None, −1, None)]:'spa' # __getitem__(slice(..)) # Slice object In more realistic iteration use cases that are not sequence-oriented, though, the __iter__ method may be easier to write since it must not manage an integer index, and __contains__ allows for membership optimization as a special case. Attribute Reference: __getattr__ and __setattr__ The __getattr__ method intercepts attribute qualifications. More specifically, it’s called with the attribute name as a string whenever you try to qualify an instance with an undefined (nonexistent) attribute name. It is not called if Python can find the attribute using its inheritance tree search procedure. Because of its behavior, __getattr__ is useful as a hook for responding to attribute requests in a generic fashion. For example: >>> class empty: ... def __getattr__(self, attrname): ... if attrname == "age": ... return 40 ... else: ... raise AttributeError, attrname ... >>> X = empty() >>> X.age 40 >>> X.name ...error text omitted... AttributeError: name Here, the empty class and its instance X have no real attributes of their own, so the access to X.age gets routed to the __getattr__ method; self is assigned the instance (X), and attrname is assigned the undefined attribute name string ("age"). The class makes age look like a real attribute by returning a real value as the result of the X.age qualification expression (40). In effect, age becomes a dynamically computed attribute. 718 | Chapter 29: Operator Overloading For attributes that the class doesn’t know how to handle, __getattr__ raises the builtin AttributeError exception to tell Python that these are bona fide undefined names; asking for X.name triggers the error. You’ll see __getattr__ again when we see delegation and properties at work in the next two chapters, and I’ll say more about exceptions in Part VII. A related overloading method, __setattr__, intercepts all attribute assignments. If this method is defined, self.attr = value becomes self.__setattr__('attr', value). This is a bit trickier to use because assigning to any self attributes within __setattr__ calls __setattr__ again, causing an infinite recursion loop (and eventually, a stack overflow exception!). If you want to use this method, be sure that it assigns any instance attributes by indexing the attribute dictionary, discussed in the next section. That is, use self.__dict__['name'] = x, not self.name = x: >>> class accesscontrol: ... def __setattr__(self, attr, value): ... if attr == 'age': ... self.__dict__[attr] = value ... else: ... raise AttributeError, attr + ' not allowed' ... >>> X = accesscontrol() >>> X.age = 40 # Calls __setattr__ >>> X.age 40 >>> X.name = 'mel' ...text omitted... AttributeError: name not allowed These two attribute-access overloading methods allow you to control or specialize access to attributes in your objects. They tend to play highly specialized roles, some of which we’ll explore later in this book. Other Attribute Management Tools For future reference, also note that there are other ways to manage attribute access in Python: • The __getattribute__ method intercepts all attribute fetches, not just those that are undefined, but when using it you must be more cautious than with __getattr__ to avoid loops. • The property built-in function allows us to associate methods with fetch and set operations on a specific class attribute. • Descriptors provide a protocol for associating __get__ and __set__ methods of a class with accesses to a specific class attribute. Because these are somewhat advanced tools not of interest to every Python programmer, we’ll defer a look at properties until Chapter 31 and detailed coverage of all the attribute management techniques until Chapter 37. Attribute Reference: __getattr__ and __setattr__ | 719 Emulating Privacy for Instance Attributes: Part 1 The following code generalizes the previous example, to allow each subclass to have its own list of private names that cannot be assigned to its instances: class PrivateExc(Exception): pass class Privacy: def __setattr__(self, attrname, value): if attrname in self.privates: raise PrivateExc(attrname, self) else: self.__dict__[attrname] = value # More on exceptions later # On self.attrname = value # self.attrname = value loops! class Test1(Privacy): privates = ['age'] class Test2(Privacy): privates = ['name', 'pay'] def __init__(self): self.__dict__['name'] = 'Tom' x = Test1() y = Test2() x.name = 'Bob' y.name = 'Sue' # Fails y.age x.age # Fails = 30 = 40 In fact, this is a first-cut solution for an implementation of attribute privacy in Python (i.e., disallowing changes to attribute names outside a class). Although Python doesn’t support private declarations per se, techniques like this can emulate much of their purpose. This is a partial solution, though; to make it more effective, it must be augmented to allow subclasses to set private attributes more naturally, too, and to use __getattr__ and a wrapper (sometimes called a proxy) class to check for private attribute fetches. We’ll postpone a more complete solution to attribute privacy until Chapter 38, where we’ll use class decorators to intercept and validate attributes more generally. Even though privacy can be emulated this way, though, it almost never is in practice. Python programmers are able to write large OOP frameworks and applications without private declarations—an interesting finding about access controls in general that is beyond the scope of our purposes here. Catching attribute references and assignments is generally a useful technique; it supports delegation, a design technique that allows controller objects to wrap up embedded objects, add new behaviors, and route other operations back to the wrapped objects (more on delegation and wrapper classes in Chapter 30). 720 | Chapter 29: Operator Overloading String Representation: __repr__ and __str__ The next example exercises the __init__ constructor and the __add__ overload method, both of which we’ve already seen, as well as defining a __repr__ method that returns a string representation for instances. String formatting is used to convert the managed self.data object to a string. If defined, __repr__ (or its sibling, __str__) is called automatically when class instances are printed or converted to strings. These methods allow you to define a better display format for your objects than the default instance display. The default display of instance objects is neither useful nor pretty: >>> class adder: ... def __init__(self, value=0): ... self.data = value ... def __add__(self, other): ... self.data += other ... >>> x = adder() >>> print(x) <__main__.adder object at 0x025D66B0> >>> x <__main__.adder object at 0x025D66B0> # Initialize data # Add other in-place (bad!) # Default displays But coding or inheriting string representation methods allows us to customize the display: >>> class addrepr(adder): ... def __repr__(self): ... return 'addrepr(%s)' % self.data ... >>> x = addrepr(2) >>> x + 1 >>> x addrepr(3) >>> print(x) addrepr(3) >>> str(x), repr(x) ('addrepr(3)', 'addrepr(3)') # Inherit __init__, __add__ # Add string representation # Convert to as-code string # Runs __init__ # Runs __add__ # Runs __repr__ # Runs __repr__ # Runs __repr__ for both So why two display methods? Mostly, to support different audiences. In full detail: • __str__ is tried first for the print operation and the str built-in function (the internal equivalent of which print runs). It generally should return a user-friendly display. • __repr__ is used in all other contexts: for interactive echoes, the repr function, and nested appearances, as well as by print and str if no __str__ is present. It should generally return an as-code string that could be used to re-create the object, or a detailed display for developers. In a nutshell, __repr__ is used everywhere, except by print and str when a __str__ is defined. Note, however, that while printing falls back on __repr__ if no __str__ is String Representation: __repr__ and __str__ | 721 defined, the inverse is not true—other contexts, such as interactive echoes, use __repr__ only and don’t try __str__ at all: >>> class addstr(adder): ... def __str__(self): # __str__ but no __repr__ ... return '[Value: %s]' % self.data # Convert to nice string ... >>> x = addstr(3) >>> x + 1 >>> x # Default __repr__ <__main__.addstr object at 0x00B35EF0> >>> print(x) # Runs __str__ [Value: 4] >>> str(x), repr(x) ('[Value: 4]', '<__main__.addstr object at 0x00B35EF0>') Because of this, __repr__ may be best if you want a single display for all contexts. By defining both methods, though, you can support different displays in different contexts—for example, an end-user display with __str__, and a low-level display for programmers to use during development with __repr__. In effect, __str__ simply overrides __repr__ for user-friendly display contexts: >>> class addboth(adder): ... def __str__(self): ... return '[Value: %s]' % self.data ... def __repr__(self): ... return 'addboth(%s)' % self.data ... >>> x = addboth(4) >>> x + 1 >>> x addboth(5) >>> print(x) [Value: 5] >>> str(x), repr(x) ('[Value: 5]', 'addboth(5)') # User-friendly string # As-code string # Runs __repr__ # Runs __str__ I should mention two usage notes here. First, keep in mind that __str__ and __repr__ must both return strings; other result types are not converted and raise errors, so be sure to run them through a converter if needed. Second, depending on a container’s string-conversion logic, the user-friendly display of __str__ might only apply when objects appear at the top level of a print operation; objects nested in larger objects might still print with their __repr__ or its default. The following illustrates both of these points: >>> class Printer: ... def __init__(self, val): ... self.val = val ... def __str__(self): ... return str(self.val) ... >>> objs = [Printer(2), Printer(3)] >>> for x in objs: print(x) ... 722 | Chapter 29: Operator Overloading # Used for instance itself # Convert to a string result # __str__ run when instance printed # But not when instance in a list! 2 3 >>> print(objs) [<__main__.Printer object at 0x025D06F0>, <__main__.Printer object at ...more... >>> objs [<__main__.Printer object at 0x025D06F0>, <__main__.Printer object at ...more... To ensure that a custom display is run in all contexts regardless of the container, code __repr__, not __str__; the former is run in all cases if the latter doesn’t apply: >>> ... ... ... ... ... >>> >>> ... 2 3 >>> [2, >>> [2, class Printer: def __init__(self, val): self.val = val def __repr__(self): return str(self.val) # __repr__ used by print if no __str__ # __repr__ used if echoed or nested objs = [Printer(2), Printer(3)] for x in objs: print(x) # No __str__: runs __repr__ # Runs __repr__, not ___str__ print(objs) 3] objs 3] In practice, __str__ (or its low-level relative, __repr__) seems to be the second most commonly used operator overloading method in Python scripts, behind __init__. Any time you can print an object and see a custom display, one of these two tools is probably in use. Right-Side and In-Place Addition: __radd__ and __iadd__ Technically, the __add__ method that appeared in the prior example does not support the use of instance objects on the right side of the + operator. To implement such expressions, and hence support commutative-style operators, code the __radd__ method as well. Python calls __radd__ only when the object on the right side of the + is your class instance, but the object on the left is not an instance of your class. The __add__ method for the object on the left is called instead in all other cases: >>> class Commuter: ... def __init__(self, val): ... self.val = val ... def __add__(self, other): ... print('add', self.val, other) ... return self.val + other ... def __radd__(self, other): ... print('radd', self.val, other) ... return other + self.val ... >>> x = Commuter(88) >>> y = Commuter(99) Right-Side and In-Place Addition: __radd__ and __iadd__ | 723 >>> x + 1 # __add__: instance + noninstance add 88 1 89 >>> 1 + y # __radd__: noninstance + instance radd 99 1 100 >>> x + y # __add__: instance + instance, triggers __radd__ add 88 <__main__.Commuter object at 0x02630910> radd 99 88 187 Notice how the order is reversed in __radd__: self is really on the right of the +, and other is on the left. Also note that x and y are instances of the same class here; when instances of different classes appear mixed in an expression, Python prefers the class of the one on the left. When we add the two instances together, Python runs __add__, which in turn triggers __radd__ by simplifying the left operand. In more realistic classes where the class type may need to be propagated in results, things can become trickier: type testing may be required to tell whether it’s safe to convert and thus avoid nesting. For instance, without the isinstance test in the following, we could wind up with a Commuter whose val is another Commuter when two instances are added and __add__ triggers __radd__: >>> class Commuter: # Propagate class type in results ... def __init__(self, val): ... self.val = val ... def __add__(self, other): ... if isinstance(other, Commuter): other = other.val ... return Commuter(self.val + other) ... def __radd__(self, other): ... return Commuter(other + self.val) ... def __str__(self): ... return '<Commuter: %s>' % self.val ... >>> x = Commuter(88) >>> y = Commuter(99) >>> print(x + 10) # Result is another Commuter instance <Commuter: 98> >>> print(10 + y) <Commuter: 109> >>> z = x + y >>> print(z) <Commuter: 187> >>> print(z + 10) <Commuter: 197> >>> print(z + z) <Commuter: 374> 724 | Chapter 29: Operator Overloading # Not nested: doesn't recur to __radd__ In-Place Addition To also implement += in-place augmented addition, code either an __iadd__ or an __add__. The latter is used if the former is absent. In fact, the prior section’s Commuter class supports += already for this reason, but __iadd__ allows for more efficient in-place changes: >>> ... ... ... ... ... ... >>> >>> >>> >>> 7 >>> ... ... ... ... ... >>> >>> >>> >>> 7 class Number: def __init__(self, val): self.val = val def __iadd__(self, other): self.val += other return self # __iadd__ explicit: x += y # Usually returns self x = Number(5) x += 1 x += 1 x.val class Number: def __init__(self, val): self.val = val def __add__(self, other): return Number(self.val + other) # __add__ fallback: x = (x + y) # Propagates class type x = Number(5) x += 1 x += 1 x.val Every binary operator has similar right-side and in-place overloading methods that work the same (e.g., __mul__, __rmul__, and __imul__). Right-side methods are an advanced topic and tend to be fairly rarely used in practice; you only code them when you need operators to be commutative, and then only if you need to support such operators at all. For instance, a Vector class may use these tools, but an Employee or Button class probably would not. Call Expressions: __call__ The __call__ method is called when your instance is called. No, this isn’t a circular definition—if defined, Python runs a __call__ method for function call expressions applied to your instances, passing along whatever positional or keyword arguments were sent: >>> class Callee: ... def __call__(self, *pargs, **kargs): ... print('Called:', pargs, kargs) ... >>> C = Callee() >>> C(1, 2, 3) # Intercept instance calls # Accept arbitrary arguments # C is a callable object Call Expressions: __call__ | 725 Called: (1, 2, 3) {} >>> C(1, 2, 3, x=4, y=5) Called: (1, 2, 3) {'y': 5, 'x': 4} More formally, all the argument-passing modes we explored in Chapter 18 are supported by the __call__ method—whatever is passed to the instance is passed to this method, along with the usual implied instance argument. For example, the method definitions: class C: def __call__(self, a, b, c=5, d=6): ... # Normals and defaults class C: def __call__(self, *pargs, **kargs): ... # Collect arbitrary arguments class C: def __call__(self, *pargs, d=6, **kargs): ... # 3.0 keyword-only argument all match all the following instance calls: X = C() X(1, 2) X(1, 2, 3, 4) X(a=1, b=2, d=4) X(*[1, 2], **dict(c=3, d=4)) X(1, *(2,), c=3, **dict(d=4)) # Omit defaults # Positionals # Keywords # Unpack arbitrary arguments # Mixed modes The net effect is that classes and instances with a __call__ support the exact same argument syntax and semantics as normal functions and methods. Intercepting call expression like this allows class instances to emulate the look and feel of things like functions, but also retain state information for use during calls (we saw a similar example while exploring scopes in Chapter 17, but you should be more familiar with operator overloading here): >>> ... ... ... ... ... >>> >>> 6 >>> 8 class Prod: def __init__(self, value): self.value = value def __call__(self, other): return self.value * other x = Prod(2) x(3) # Accept just one argument # "Remembers" 2 in state # 3 (passed) * 2 (state) x(4) In this example, the __call__ may seem a bit gratuitous at first glance. A simple method can provide similar utility: >>> class Prod: ... def __init__(self, value): ... self.value = value ... def comp(self, other): ... return self.value * other ... 726 | Chapter 29: Operator Overloading >>> x = Prod(3) >>> x.comp(3) 9 >>> x.comp(4) 12 However, __call__ can become more useful when interfacing with APIs that expect functions—it allows us to code objects that conform to an expected function call interface, but also retain state information. In fact, it’s probably the third most commonly used operator overloading method, behind the __init__ constructor and the __str__ and __repr__ display-format alternatives. Function Interfaces and Callback-Based Code As an example, the tkinter GUI toolkit (named Tkinter in Python 2.6) allows you to register functions as event handlers (a.k.a. callbacks); when events occur, tkinter calls the registered objects. If you want an event handler to retain state between events, you can register either a class’s bound method or an instance that conforms to the expected interface with __call__. In this section’s code, both x.comp from the second example and x from the first can pass as function-like objects this way. I’ll have more to say about bound methods in the next chapter, but for now, here’s a hypothetical example of __call__ applied to the GUI domain. The following class defines an object that supports a function-call interface, but also has state information that remembers the color a button should change to when it is later pressed: class Callback: def __init__(self, color): self.color = color def __call__(self): print('turn', self.color) # Function + state information # Support calls with no arguments Now, in the context of a GUI, we can register instances of this class as event handlers for buttons, even though the GUI expects to be able to invoke event handlers as simple functions with no arguments: cb1 = Callback('blue') cb2 = Callback('green') # Remember blue B1 = Button(command=cb1) B2 = Button(command=cb2) # Register handlers # Register handlers When the button is later pressed, the instance object is called as a simple function, exactly like in the following calls. Because it retains state as instance attributes, though, it remembers what to do: cb1() cb2() # On events: prints 'blue' # Prints 'green' In fact, this is probably the best way to retain state information in the Python language—better than the techniques discussed earlier for functions (global variables, Call Expressions: __call__ | 727 enclosing function scope references, and default mutable arguments). With OOP, the state remembered is made explicit with attribute assignments. Before we move on, there are two other ways that Python programmers sometimes tie information to a callback function like this. One option is to use default arguments in lambda functions: cb3 = (lambda color='red': 'turn ' + color) # Or: defaults print(cb3()) The other is to use bound methods of a class. A bound method object is a kind of object that remembers the self instance and the referenced function. A bound method may therefore be called as a simple function without an instance later: class Callback: def __init__(self, color): self.color = color def changeColor(self): print('turn', self.color) # Class with state information # A normal named method cb1 = Callback('blue') cb2 = Callback('yellow') B1 = Button(command=cb1.changeColor) B2 = Button(command=cb2.changeColor) # Reference, but don't call # Remembers function+self In this case, when this button is later pressed it’s as if the GUI does this, which invokes the changeColor method to process the object’s state information: object = Callback('blue') cb = object.changeColor cb() # Registered event handler # On event prints 'blue' This technique is simpler, but less general than overloading calls with __call__; again, watch for more about bound methods in the next chapter. You’ll also see another __call__ example in Chapter 31, where we will use it to implement something known as a function decorator—a callable object often used to add a layer of logic on top of an embedded function. Because __call__ allows us to attach state information to a callable object, it’s a natural implementation technique for a function that must remember and call another function. Comparisons: __lt__, __gt__, and Others As suggested in Table 29-1, classes can define methods to catch all six comparison operators: <, >, <=, >=, ==, and !=. These methods are generally straightforward to use, but keep the following qualifications in mind: 728 | Chapter 29: Operator Overloading • Unlike the __add__/__radd__ pairings discussed earlier, there are no right-side variants of comparison methods. Instead, reflective methods are used when only one operand supports comparison (e.g., __lt__ and __gt__ are each other’s reflection). • There are no implicit relationships among the comparison operators. The truth of == does not imply that != is false, for example, so both __eq__ and __ne__ should be defined to ensure that both operators behave correctly. • In Python 2.6, a __cmp__ method is used by all comparisons if no more specific comparison methods are defined; it returns a number that is less than, equal to, or greater than zero, to signal less than, equal, and greater than results for the comparison of its two arguments (self and another operand). This method often uses the cmp(x, y) built-in to compute its result. Both the __cmp__ method and the cmp built-in function are removed in Python 3.0: use the more specific methods instead. We don’t have space for an in-depth exploration of comparison methods, but as a quick introduction, consider the following class and test code: class C: data = 'spam' def __gt__(self, other): return self.data > other def __lt__(self, other): return self.data < other X = C() print(X > 'ham') print(X < 'ham') # 3.0 and 2.6 version # True (runs __gt__) # False (runs __lt__) When run under Python 3.0 or 2.6, the prints at the end display the expected results noted in their comments, because the class’s methods intercept and implement comparison expressions. The 2.6 __cmp__ Method (Removed in 3.0) In Python 2.6, the __cmp__ method is used as a fallback if more specific methods are not defined: its integer result is used to evaluate the operator being run. The following produces the same result under 2.6, for example, but fails in 3.0 because __cmp__ is no longer used: class C: data = 'spam' def __cmp__(self, other): return cmp(self.data, other) # 2.6 only # __cmp__ not used in 3.0 # cmp not defined in 3.0 X = C() print(X > 'ham') print(X < 'ham') # True (runs __cmp__) # False (runs __cmp__) Comparisons: __lt__, __gt__, and Others | 729 Notice that this fails in 3.0 because __cmp__ is no longer special, not because the cmp built-in function is no longer present. If we change the prior class to the following to try to simulate the cmp call, the code still works in 2.6 but fails in 3.0: class C: data = 'spam' def __cmp__(self, other): return (self.data > other) - (self.data < other) So why, you might be asking, did I just show you a comparison method that is no longer supported in 3.0? While it would be easier to erase history entirely, this book is designed to support both 2.6 and 3.0 readers. Because __cmp__ may appear in code 2.6 readers must reuse or maintain, it’s fair game in this book. Moreover, __cmp__ was removed more abruptly than the __getslice__ method described earlier, and so may endure longer. If you use 3.0, though, or care about running your code under 3.0 in the future, don’t use __cmp__ anymore: use the more specific comparison methods instead. Boolean Tests: __bool__ and __len__ As mentioned earlier, classes may also define methods that give the Boolean nature of their instances—in Boolean contexts, Python first tries __bool__ to obtain a direct Boolean value and then, if that’s missing, tries __len__ to determine a truth value from the object length. The first of these generally uses object state or other information to produce a Boolean result: >>> class Truth: ... def __bool__(self): return True ... >>> X = Truth() >>> if X: print('yes!') ... yes! >>> class Truth: ... def __bool__(self): return False ... >>> X = Truth() >>> bool(X) False If this method is missing, Python falls back on length because a nonempty object is considered true (i.e., a nonzero length is taken to mean the object is true, and a zero length means it is false): >>> class Truth: ... def __len__(self): return 0 ... >>> X = Truth() >>> if not X: print('no!') 730 | Chapter 29: Operator Overloading ... no! If both methods are present Python prefers __bool__ over __len__, because it is more specific: >>> class Truth: ... def __bool__(self): return True ... def __len__(self): return 0 ... >>> X = Truth() >>> if X: print('yes!') ... yes! # 3.0 tries __bool__ first # 2.6 tries __len__ first If neither truth method is defined, the object is vacuously considered true (which has potential implications for metaphysically inclined readers!): >>> class Truth: ... pass ... >>> X = Truth() >>> bool(X) True And now that we’ve managed to cross over into the realm of philosophy, let’s move on to look at one last overloading context: object demise. Booleans in Python 2.6 Python 2.6 users should use __nonzero__ instead of __bool__ in all of the code in the section “Boolean Tests: __bool__ and __len__” on page 730. Python 3.0 renamed the 2.6 __nonzero__ method to __bool__, but Boolean tests work the same otherwise (both 3.0 and 2.6 use __len__ as a fallback). If you don’t use the 2.6 name, the very first test in this section will work the same for you anyhow, but only because __bool__ is not recognized as a special method name in 2.6, and objects are considered true by default! To witness this version difference live, you need to return False: C:\misc> c:\python30\python >>> class C: ... def __bool__(self): ... print('in bool') ... return False ... >>> X = C() >>> bool(X) in bool False >>> if X: print(99) ... in bool Boolean Tests: __bool__ and __len__ | 731 This works as advertised in 3.0. In 2.6, though, __bool__ is ignored and the object is always considered true: C:\misc> c:\python26\python >>> class C: ... def __bool__(self): ... print('in bool') ... return False ... >>> X = C() >>> bool(X) True >>> if X: print(99) ... 99 In 2.6, use __nonzero__ for Boolean values (or return 0 from the __len__ fallback method to designate false): C:\misc> c:\python26\python >>> class C: ... def __nonzero__(self): ... print('in nonzero') ... return False ... >>> X = C() >>> bool(X) in nonzero False >>> if X: print(99) ... in nonzero But keep in mind that __nonzero__ works in 2.6 only; if used in 3.0 it will be silently ignored and the object will be classified as true by default—just like using __bool__ in 2.6! Object Destruction: __del__ We’ve seen how the __init__ constructor is called whenever an instance is generated. Its counterpart, the destructor method __del__, is run automatically when an instance’s space is being reclaimed (i.e., at “garbage collection” time): >>> class Life: ... def __init__(self, name='unknown'): ... print('Hello', name) ... self.name = name ... def __del__(self): ... print('Goodbye', self.name) ... >>> brian = Life('Brian') Hello Brian >>> brian = 'loretta' Goodbye Brian 732 | Chapter 29: Operator Overloading Here, when brian is assigned a string, we lose the last reference to the Life instance and so trigger its destructor method. This works, and it may be useful for implementing some cleanup activities (such as terminating server connections). However, destructors are not as commonly used in Python as in some OOP languages, for a number of reasons. For one thing, because Python automatically reclaims all space held by an instance when the instance is reclaimed, destructors are not necessary for space management.* For another, because you cannot always easily predict when an instance will be reclaimed, it’s often better to code termination activities in an explicitly called method (or try/finally statement, described in the next part of the book); in some cases, there may be lingering references to your objects in system tables that prevent destructors from running. In fact, __del__ can be tricky to use for even more subtle reasons. Exceptions raised within it, for example, simply print a warning message to sys.stderr (the standard error stream) rather than triggering an exception event, because of the unpredictable context under which it is run by the garbage collector. In addition, cyclic (a.k.a. circular) references among objects may prevent garbage collection from happening when you expect it to; an optional cycle detector, enabled by default, can automatically collect such objects eventually, but only if they do not have __del__ methods. Since this is relatively obscure, we’ll ignore further details here; see Python’s standard manuals’ coverage of both __del__ and the gc garbage collector module for more information. Chapter Summary That’s as many overloading examples as we have space for here. Most of the other operator overloading methods work similarly to the ones we’ve explored, and all are just hooks for intercepting built-in type operations; some overloading methods, for example, have unique argument lists or return values. We’ll see a few others in action later in the book: • Chapter 33 uses the __enter__ and __exit__ with statement context manager methods. • Chapter 37 uses the __get__ and __set__ class descriptor fetch/set methods. • Chapter 39 uses the __new__ object creation method in the context of metaclasses. * In the current C implementation of Python, you also don’t need to close file objects held by the instance in destructors because they are automatically closed when reclaimed. However, as mentioned in Chapter 9, it’s better to explicitly call file close methods because auto-close-on-reclaim is a feature of the implementation, not of the language itself (this behavior can vary under Jython, for instance). Chapter Summary | 733 In addition, some of the methods we’ve studied here, such as __call__ and __str__, will be employed by later examples in this book. For complete coverage, though, I’ll defer to other documentation sources—see Python’s standard language manual or reference books for details on additional overloading methods. In the next chapter, we leave the realm of class mechanics behind to explore common design patterns—the ways that classes are commonly used and combined to optimize code reuse. Before you read on, though, take a moment to work though the chapter quiz below to review the concepts we’ve covered. Test Your Knowledge: Quiz 1. What two operator overloading methods can you use to support iteration in your classes? 2. What two operator overloading methods handle printing, and in what contexts? 3. How can you intercept slice operations in a class? 4. How can you catch in-place addition in a class? 5. When should you provide operator overloading? Test Your Knowledge: Answers 1. Classes can support iteration by defining (or inheriting) __getitem__ or __iter__. In all iteration contexts, Python tries to use __iter__ (which returns an object that supports the iteration protocol with a __next__ method) first: if no __iter__ is found by inheritance search, Python falls back on the __getitem__ indexing method (which is called repeatedly, with successively higher indexes). 2. The __str__ and __repr__ methods implement object print displays. The former is called by the print and str built-in functions; the latter is called by print and str if there is no __str__, and always by the repr built-in, interactive echoes, and nested appearances. That is, __repr__ is used everywhere, except by print and str when a __str__ is defined. A __str__ is usually used for user-friendly displays; __repr__ gives extra details or the object’s as-code form. 3. Slicing is caught by the __getitem__ indexing method: it is called with a slice object, instead of a simple index. In Python 2.6, __getslice__ (defunct in 3.0) may be used as well. 4. In-place addition tries __iadd__ first, and __add__ with an assignment second. The same pattern holds true for all binary operators. The __radd__ method is also available for right-side addition. 734 | Chapter 29: Operator Overloading 5. When a class naturally matches, or needs to emulate, a built-in type’s interfaces. For example, collections might imitate sequence or mapping interfaces. You generally shouldn’t implement expression operators if they don’t naturally map to your objects, though—use normally named methods instead. Test Your Knowledge: Answers | 735 CHAPTER 30 Designing with Classes So far in this part of the book, we’ve concentrated on using Python’s OOP tool, the class. But OOP is also about design issues—i.e., how to use classes to model useful objects. This chapter will touch on a few core OOP ideas and present some additional examples that are more realistic than those shown so far. Along the way, we’ll code some common OOP design patterns in Python, such as inheritance, composition, delegation, and factories. We’ll also investigate some designfocused class concepts, such as pseudoprivate attributes, multiple inheritance, and bound methods. Many of the design terms mentioned here require more explanation than I can provide in this book; if this material sparks your curiosity, I suggest exploring a text on OOP design or design patterns as a next step. Python and OOP Let’s begin with a review—Python’s implementation of OOP can be summarized by three ideas: Inheritance Inheritance is based on attribute lookup in Python (in X.name expressions). Polymorphism In X.method, the meaning of method depends on the type (class) of X. Encapsulation Methods and operators implement behavior; data hiding is a convention by default. By now, you should have a good feel for what inheritance is all about in Python. We’ve also talked about Python’s polymorphism a few times already; it flows from Python’s lack of type declarations. Because attributes are always resolved at runtime, objects that implement the same interfaces are interchangeable; clients don’t need to know what sorts of objects are implementing the methods they call. 737 Encapsulation means packaging in Python—that is, hiding implementation details behind an object’s interface. It does not mean enforced privacy, though that can be implemented with code, as we’ll see in Chapter 38. Encapsulation allows the implementation of an object’s interface to be changed without impacting the users of that object. Overloading by Call Signatures (or Not) Some OOP languages also define polymorphism to mean overloading functions based on the type signatures of their arguments. But because there are no type declarations in Python, this concept doesn’t really apply; polymorphism in Python is based on object interfaces, not types. You can try to overload methods by their argument lists, like this: class C: def meth(self, x): ... def meth(self, x, y, z): ... This code will run, but because the def simply assigns an object to a name in the class’s scope, the last definition of the method function is the only one that will be retained (it’s just as if you say X = 1 and then X = 2; X will be 2). Type-based selections can always be coded using the type-testing ideas we met in Chapters 4 and 9, or the argument list tools introduced in Chapter 18: class C: def meth(self, *args): if len(args) == 1: ... elif type(arg[0]) == int: ... You normally shouldn’t do this, though—as described in Chapter 16, you should write your code to expect an object interface, not a specific data type. That way, it will be useful for a broader category of types and applications, both now and in the future: class C: def meth(self, x): x.operation() # Assume x does the right thing It’s also generally considered better to use distinct method names for distinct operations, rather than relying on call signatures (no matter what language you code in). Although Python’s object model is straightforward, much of the art in OOP is in the way we combine classes to achieve a program’s goals. The next section begins a tour of some of the ways larger programs use classes to their advantage. 738 | Chapter 30: Designing with Classes OOP and Inheritance: “Is-a” Relationships We’ve explored the mechanics of inheritance in depth already, but I’d like to show you an example of how it can be used to model real-world relationships. From a programmer’s point of view, inheritance is kicked off by attribute qualifications, which trigger searches for names in instances, their classes, and then any superclasses. From a designer’s point of view, inheritance is a way to specify set membership: a class defines a set of properties that may be inherited and customized by more specific sets (i.e., subclasses). To illustrate, let’s put that pizza-making robot we talked about at the start of this part of the book to work. Suppose we’ve decided to explore alternative career paths and open a pizza restaurant. One of the first things we’ll need to do is hire employees to serve customers, prepare the food, and so on. Being engineers at heart, we’ve decided to build a robot to make the pizzas; but being politically and cybernetically correct, we’ve also decided to make our robot a full-fledged employee with a salary. Our pizza shop team can be defined by the four classes in the example file, employees.py. The most general class, Employee, provides common behavior such as bumping up salaries (giveRaise) and printing (__repr__). There are two kinds of employees, and so two subclasses of Employee: Chef and Server. Both override the inherited work method to print more specific messages. Finally, our pizza robot is modeled by an even more specific class: PizzaRobot is a kind of Chef, which is a kind of Employee. In OOP terms, we call these relationships “is-a” links: a robot is a chef, which is a(n) employee. Here’s the employees.py file: class Employee: def __init__(self, name, salary=0): self.name = name self.salary = salary def giveRaise(self, percent): self.salary = self.salary + (self.salary * percent) def work(self): print(self.name, "does stuff") def __repr__(self): return "<Employee: name=%s, salary=%s>" % (self.name, self.salary) class Chef(Employee): def __init__(self, name): Employee.__init__(self, name, 50000) def work(self): print(self.name, "makes food") class Server(Employee): def __init__(self, name): Employee.__init__(self, name, 40000) def work(self): print(self.name, "interfaces with customer") class PizzaRobot(Chef): OOP and Inheritance: “Is-a” Relationships | 739 def __init__(self, name): Chef.__init__(self, name) def work(self): print(self.name, "makes pizza") if __name__ == "__main__": bob = PizzaRobot('bob') print(bob) bob.work() bob.giveRaise(0.20) print(bob); print() # Make a robot named bob # Run inherited __repr__ # Run type-specific action # Give bob a 20% raise for klass in Employee, Chef, Server, PizzaRobot: obj = klass(klass.__name__) obj.work() When we run the self-test code included in this module, we create a pizza-making robot named bob, which inherits names from three classes: PizzaRobot, Chef, and Employee. For instance, printing bob runs the Employee.__repr__ method, and giving bob a raise invokes Employee.giveRaise because that’s where the inheritance search finds that method: C:\python\examples> python employees.py <Employee: name=bob, salary=50000> bob makes pizza <Employee: name=bob, salary=60000.0> Employee does stuff Chef makes food Server interfaces with customer PizzaRobot makes pizza In a class hierarchy like this, you can usually make instances of any of the classes, not just the ones at the bottom. For instance, the for loop in this module’s self-test code creates instances of all four classes; each responds differently when asked to work because the work method is different in each. Really, these classes just simulate real-world objects; work prints a message for the time being, but it could be expanded to do real work later. OOP and Composition: “Has-a” Relationships The notion of composition was introduced in Chapter 25. From a programmer’s point of view, composition involves embedding other objects in a container object, and activating them to implement container methods. To a designer, composition is another way to represent relationships in a problem domain. But, rather than set membership, composition has to do with components—parts of a whole. Composition also reflects the relationships between parts, called a “has-a” relationships. Some OOP design texts refer to composition as aggregation (or distinguish between the two terms by using aggregation to describe a weaker dependency between 740 | Chapter 30: Designing with Classes container and contained); in this text, a “composition” simply refers to a collection of embedded objects. The composite class generally provides an interface all its own and implements it by directing the embedded objects. Now that we’ve implemented our employees, let’s put them in the pizza shop and let them get busy. Our pizza shop is a composite object: it has an oven, and it has employees like servers and chefs. When a customer enters and places an order, the components of the shop spring into action—the server takes the order, the chef makes the pizza, and so on. The following example (the file pizzashop.py) simulates all the objects and relationships in this scenario: from employees import PizzaRobot, Server class Customer: def __init__(self, name): self.name = name def order(self, server): print(self.name, "orders from", server) def pay(self, server): print(self.name, "pays for item to", server) class Oven: def bake(self): print("oven bakes") class PizzaShop: def __init__(self): self.server = Server('Pat') self.chef = PizzaRobot('Bob') self.oven = Oven() def order(self, name): customer = Customer(name) customer.order(self.server) self.chef.work() self.oven.bake() customer.pay(self.server) if __name__ == "__main__": scene = PizzaShop() scene.order('Homer') print('...') scene.order('Shaggy') # Embed other objects # A robot named bob # Activate other objects # Customer orders from server # Make the composite # Simulate Homer's order # Simulate Shaggy's order The PizzaShop class is a container and controller; its constructor makes and embeds instances of the employee classes we wrote in the last section, as well as an Oven class defined here. When this module’s self-test code calls the PizzaShop order method, the embedded objects are asked to carry out their actions in turn. Notice that we make a new Customer object for each order, and we pass on the embedded Server object to Customer methods; customers come and go, but the server is part of the pizza shop composite. Also notice that employees are still involved in an inheritance relationship; composition and inheritance are complementary tools. OOP and Composition: “Has-a” Relationships | 741 When we run this module, our pizza shop handles two orders—one from Homer, and then one from Shaggy: C:\python\examples> python pizzashop.py Homer orders from <Employee: name=Pat, salary=40000> Bob makes pizza oven bakes Homer pays for item to <Employee: name=Pat, salary=40000> ... Shaggy orders from <Employee: name=Pat, salary=40000> Bob makes pizza oven bakes Shaggy pays for item to <Employee: name=Pat, salary=40000> Again, this is mostly just a toy simulation, but the objects and interactions are representative of composites at work. As a rule of thumb, classes can represent just about any objects and relationships you can express in a sentence; just replace nouns with classes, and verbs with methods, and you’ll have a first cut at a design. Stream Processors Revisited For a more realistic composition example, recall the generic data stream processor function we partially coded in the introduction to OOP in Chapter 25: def processor(reader, converter, writer): while 1: data = reader.read() if not data: break data = converter(data) writer.write(data) Rather than using a simple function here, we might code this as a class that uses composition to do its work to provide more structure and support inheritance. The following file, streams.py, demonstrates one way to code the class: class Processor: def __init__(self, reader, writer): self.reader = reader self.writer = writer def process(self): while 1: data = self.reader.readline() if not data: break data = self.converter(data) self.writer.write(data) def converter(self, data): assert False, 'converter must be defined' # Or raise exception This class defines a converter method that it expects subclasses to fill in; it’s an example of the abstract superclass model we outlined in Chapter 28 (more on assert in Part VII). Coded this way, reader and writer objects are embedded within the class instance (composition), and we supply the conversion logic in a subclass rather than passing in a converter function (inheritance). The file converters.py shows how: 742 | Chapter 30: Designing with Classes from streams import Processor class Uppercase(Processor): def converter(self, data): return data.upper() if __name__ == '__main__': import sys obj = Uppercase(open('spam.txt'), sys.stdout) obj.process() Here, the Uppercase class inherits the stream-processing loop logic (and anything else that may be coded in its superclasses). It needs to define only what is unique about it— the data conversion logic. When this file is run, it makes and runs an instance that reads from the file spam.txt and writes the uppercase equivalent of that file to the stdout stream: C:\lp4e> type spam.txt spam Spam SPAM! C:\lp4e> python converters.py SPAM SPAM SPAM! To process different sorts of streams, pass in different sorts of objects to the class construction call. Here, we use an output file instead of a stream: C:\lp4e> python >>> import converters >>> prog = converters.Uppercase(open('spam.txt'), open('spamup.txt', 'w')) >>> prog.process() C:\lp4e> type spamup.txt SPAM SPAM SPAM! But, as suggested earlier, we could also pass in arbitrary objects wrapped up in classes that define the required input and output method interfaces. Here’s a simple example that passes in a writer class that wraps up the text inside HTML tags: C:\lp4e> python >>> from converters import Uppercase >>> >>> class HTMLize: ... def write(self, line): ... print('<PRE>%s</PRE>' % line.rstrip()) ... >>> Uppercase(open('spam.txt'), HTMLize()).process() <PRE>SPAM</PRE> <PRE>SPAM</PRE> <PRE>SPAM!</PRE> OOP and Composition: “Has-a” Relationships | 743 If you trace through this example’s control flow, you’ll see that we get both uppercase conversion (by inheritance) and HTML formatting (by composition), even though the core processing logic in the original Processor superclass knows nothing about either step. The processing code only cares that writers have a write method and that a method named convert is defined; it doesn’t care what those methods do when they are called. Such polymorphism and encapsulation of logic is behind much of the power of classes. As is, the Processor superclass only provides a file-scanning loop. In more realistic work, we might extend it to support additional programming tools for its subclasses, and, in the process, turn it into a full-blown framework. Coding such a tool once in a superclass enables you to reuse it in all of your programs. Even in this simple example, because so much is packaged and inherited with classes, all we had to code was the HTML formatting step; the rest was free. For another example of composition at work, see exercise 9 at the end of Chapter 31 and its solution in Appendix B; it’s similar to the pizza shop example. We’ve focused on inheritance in this book because that is the main tool that the Python language itself provides for OOP. But, in practice, composition is used as much as inheritance as a way to structure classes, especially in larger systems. As we’ve seen, inheritance and composition are often complementary (and sometimes alternative) techniques. Because composition is a design issue outside the scope of the Python language and this book, though, I’ll defer to other resources for more on this topic. Why You Will Care: Classes and Persistence I’ve mentioned Python’s pickle and shelve object persistence support a few times in this part of the book because it works especially well with class instances. In fact, these tools are often compelling enough to motivate the use of classes in general—by picking or shelving a class instance, we get data storage that contains both data and logic combined. For example, besides allowing us to simulate real-world interactions, the pizza shop classes developed in this chapter could also be used as the basis of a persistent restaurant database. Instances of classes can be stored away on disk in a single step using Python’s pickle or shelve modules. We used shelves to store instances of classes in the OOP tutorial in Chapter 27, but the object pickling interface is remarkably easy to use as well: import pickle object = someClass() file = open(filename, 'wb') pickle.dump(object, file) # Create external file # Save object in file import pickle file = open(filename, 'rb') object = pickle.load(file) # Fetch it back later 744 | Chapter 30: Designing with Classes Pickling converts in-memory objects to serialized byte streams (really, strings), which may be stored in files, sent across a network, and so on; unpickling converts back from byte streams to identical in-memory objects. Shelves are similar, but they automatically pickle objects to an access-by-key database, which exports a dictionary-like interface: import shelve object = someClass() dbase = shelve.open('filename') dbase['key'] = object # Save under key import shelve dbase = shelve.open('filename') object = dbase['key'] # Fetch it back later In our pizza shop example, using classes to model employees means we can get a simple database of employees and shops with little extra work—pickling such instance objects to a file makes them persistent across Python program executions: >>> from pizzashop import PizzaShop >>> shop = PizzaShop() >>> shop.server, shop.chef (<Employee: name=Pat, salary=40000>, <Employee: name=Bob, salary=50000>) >>> import pickle >>> pickle.dump(shop, open('shopfile.dat', 'wb')) This stores an entire composite shop object in a file all at once. To bring it back later in another session or program, a single step suffices as well. In fact, objects restored this way retain both state and behavior: >>> import pickle >>> obj = pickle.load(open('shopfile.dat', 'rb')) >>> obj.server, obj.chef (<Employee: name=Pat, salary=40000>, <Employee: name=Bob, salary=50000>) >>> obj.order('Sue') Sue orders from <Employee: name=Pat, salary=40000> Bob makes pizza oven bakes Sue pays for item to <Employee: name=Pat, salary=40000> See the standard library manual and later examples for more on pickles and shelves. OOP and Delegation: “Wrapper” Objects Beside inheritance and composition, object-oriented programmers often also talk about something called delegation, which usually implies controller objects that embed other objects to which they pass off operation requests. The controllers can take care of administrative activities, such as keeping track of accesses and so on. In Python, delegation is often implemented with the __getattr__ method hook; because it intercepts accesses to nonexistent attributes, a wrapper class (sometimes called a proxy class) can use __getattr__ to route arbitrary accesses to a wrapped object. The wrapper class retains the interface of the wrapped object and may add additional operations of its own. OOP and Delegation: “Wrapper” Objects | 745 Consider the file trace.py, for instance: class wrapper: def __init__(self, object): self.wrapped = object def __getattr__(self, attrname): print('Trace:', attrname) return getattr(self.wrapped, attrname) # Save object # Trace fetch # Delegate fetch Recall from Chapter 29 that __getattr__ gets the attribute name as a string. This code makes use of the getattr built-in function to fetch an attribute from the wrapped object by name string—getattr(X,N) is like X.N, except that N is an expression that evaluates to a string at runtime, not a variable. In fact, getattr(X,N) is similar to X.__dict__[N], but the former also performs an inheritance search, like X.N, while the latter does not (see “Namespace Dictionaries” on page 696 for more on the __dict__ attribute). You can use the approach of this module’s wrapper class to manage access to any object with attributes—lists, dictionaries, and even classes and instances. Here, the wrapper class simply prints a trace message on each attribute access and delegates the attribute request to the embedded wrapped object: >>> from trace import wrapper >>> x = wrapper([1,2,3]) >>> x.append(4) Trace: append >>> x.wrapped [1, 2, 3, 4] >>> x = wrapper({"a": 1, "b": 2}) >>> x.keys() Trace: keys ['a', 'b'] # Wrap a list # Delegate to list method # Print my member # Wrap a dictionary # Delegate to dictionary method The net effect is to augment the entire interface of the wrapped object, with additional code in the wrapper class. We can use this to log our method calls, route method calls to extra or custom logic, and so on. We’ll revive the notions of wrapped objects and delegated operations as one way to extend built-in types in Chapter 31. If you are interested in the delegation design pattern, also watch for the discussions in Chapters 31 and 38 of function decorators, a strongly related concept designed to augment a specific function or method call rather than the entire interface of an object, and class decorators, which serve as a way to automatically add such delegation-based wrappers to all instances of a class. 746 | Chapter 30: Designing with Classes Version skew note: In Python 2.6, operator overloading methods run by built-in operations are routed through generic attribute interception methods like __getattr__. Printing a wrapped object directly, for example, calls this method for __repr__ or __str__, which then passes the call on to the wrapped object. In Python 3.0, this no longer happens: printing does not trigger __getattr__, and a default display is used instead. In 3.0, new-style classes look up operator overloading methods in classes and skip the normal instance lookup entirely. We’ll return to this issue in Chapter 37, in the context of managed attributes; for now, keep in mind that you may need to redefine operator overloading methods in wrapper classes (either by hand, by tools, or by superclasses) if you want them to be intercepted in 3.0. Pseudoprivate Class Attributes Besides larger structuring goals, class designs often must address name usage too. In Part V, we learned that every name assigned at the top level of a module file is exported. By default, the same holds for classes—data hiding is a convention, and clients may fetch or change any class or instance attribute they like. In fact, attributes are all “public” and “virtual,” in C++ terms; they’re all accessible everywhere and are looked up dynamically at runtime.* That said, Python today does support the notion of name “mangling” (i.e., expansion) to localize some names in classes. Mangled names are sometimes misleadingly called “private attributes,” but really this is just a way to localize a name to the class that created it—name mangling does not prevent access by code outside the class. This feature is mostly intended to avoid namespace collisions in instances, not to restrict access to names in general; mangled names are therefore better called “pseudoprivate” than “private.” Pseudoprivate names are an advanced and entirely optional feature, and you probably won’t find them very useful until you start writing general tools or larger class hierarchies for use in multiprogrammer projects. In fact, they are not always used even when they probably should be—more commonly, Python programmers code internal names with a single underscore (e.g., _X), which is just an informal convention to let you know that a name shouldn’t be changed (it means nothing to Python itself). Because you may see this feature in other people’s code, though, you need to be somewhat aware of it, even if you don’t use it yourself. * This tends to scare people with a C++ background unnecessarily. In Python, it’s even possible to change or completely delete a class method at runtime. On the other hand, almost nobody ever does this in practical programs. As a scripting language, Python is more about enabling than restricting. Also, recall from our discussion of operator overloading in Chapter 29 that __getattr__ and __setattr__ can be used to emulate privacy, but are generally not used for this purpose in practice. More on this when we code a more realistic privacy decorator Chapter 38. Pseudoprivate Class Attributes | 747 Name Mangling Overview Here’s how name mangling works: names inside a class statement that start with two underscores but don’t end with two underscores are automatically expanded to include the name of the enclosing class. For instance, a name like __X within a class named Spam is changed to _Spam__X automatically: the original name is prefixed with a single underscore and the enclosing class’s name. Because the modified name contains the name of the enclosing class, it’s somewhat unique; it won’t clash with similar names created by other classes in a hierarchy. Name mangling happens only in class statements, and only for names that begin with two leading underscores. However, it happens for every name preceded with double underscores—both class attributes (like method names) and instance attribute names assigned to self attributes. For example, in a class named Spam, a method named __meth is mangled to _Spam__meth, and an instance attribute reference self.__X is transformed to self._Spam__X. Because more than one class may add attributes to an instance, this mangling helps avoid clashes—but we need to move on to an example to see how. Why Use Pseudoprivate Attributes? One of the main problems that the pseudoprivate attribute feature is meant to alleviate has to do with the way instance attributes are stored. In Python, all instance attributes wind up in the single instance object at the bottom of the class tree. This is different from the C++ model, where each class gets its own space for data members it defines. Within a class method in Python, whenever a method assigns to a self attribute (e.g., self.attr = value), it changes or creates an attribute in the instance (inheritance searches happen only on reference, not on assignment). Because this is true even if multiple classes in a hierarchy assign to the same attribute, collisions are possible. For example, suppose that when a programmer codes a class, she assumes that she owns the attribute name X in the instance. In this class’s methods, the name is set, and later fetched: class C1: def meth1(self): self.X = 88 def meth2(self): print(self.X) # I assume X is mine Suppose further that another programmer, working in isolation, makes the same assumption in a class that he codes: class C2: def metha(self): self.X = 99 def methb(self): print(self.X) # Me too Both of these classes work by themselves. The problem arises if the two classes are ever mixed together in the same class tree: 748 | Chapter 30: Designing with Classes class C3(C1, C2): ... I = C3() # Only 1 X in I! Now, the value that each class gets back when it says self.X will depend on which class assigned it last. Because all assignments to self.X refer to the same single instance, there is only one X attribute—I.X—no matter how many classes use that attribute name. To guarantee that an attribute belongs to the class that uses it, prefix the name with double underscores everywhere it is used in the class, as in this file, private.py: class C1: def meth1(self): def meth2(self): class C2: def metha(self): def methb(self): self.__X = 88 print(self.__X) # Now X is mine # Becomes _C1__X in I self.__X = 99 print(self.__X) # Me too # Becomes _C2__X in I class C3(C1, C2): pass I = C3() # Two X names in I I.meth1(); I.metha() print(I.__dict__) I.meth2(); I.methb() When thus prefixed, the X attributes will be expanded to include the names of their classes before being added to the instance. If you run a dir call on I or inspect its namespace dictionary after the attributes have been assigned, you’ll see the expanded names, _C1__X and _C2__X, but not X. Because the expansion makes the names unique within the instance, the class coders can safely assume that they truly own any names that they prefix with two underscores: % python private.py {'_C2__X': 99, '_C1__X': 88} 88 99 This trick can avoid potential name collisions in the instance, but note that it does not amount to true privacy. If you know the name of the enclosing class, you can still access either of these attributes anywhere you have a reference to the instance by using the fully expanded name (e.g., I._C1__X = 77). On the other hand, this feature makes it less likely that you will accidentally step on a class’s names. Pseudoprivate attributes are also useful in larger frameworks or tools, both to avoid introducing new method names that might accidentally hide definitions elsewhere in the class tree and to reduce the chance of internal methods being replaced by names defined lower in the tree. If a method is intended for use only within a class that may be mixed into other classes, the double underscore prefix ensures that the method won’t interfere with other names in the tree, especially in multiple-inheritance scenarios: class Super: def method(self): ... # A real application method class Tool: Pseudoprivate Class Attributes | 749 def __method(self): ... def other(self): self.__method() # Becomes _Tool__method # Use my internal method class Sub1(Tool, Super): ... def actions(self): self.method() # Runs Super.method as expected class Sub2(Tool): def __init__(self): self.method = 99 # Doesn't break Tool.__method We met multiple inheritance briefly in Chapter 25 and will explore it in more detail later in this chapter. Recall that superclasses are searched according to their left-to-right order in class header lines. Here, this means Sub1 prefers Tool attributes to those in Super. Although in this example we could force Python to pick the application class’s methods first by switching the order of the superclasses listed in the Sub1 class header, pseudoprivate attributes resolve the issue altogether. Pseudoprivate names also prevent subclasses from accidentally redefining the internal method’s names, as in Sub2. Again, I should note that this feature tends to be of use primarily for larger, multiprogrammer projects, and then only for selected names. Don’t be tempted to clutter your code unnecessarily; only use this feature for names that truly need to be controlled by a single class. For simpler programs, it’s probably overkill. For more examples that make use of the __X naming feature, see the lister.py mix-in classes introduced later in this chapter, in the section on multiple inheritance, as well as the discussion of Private class decorators in Chapter 38. If you care about privacy in general, you might want to review the emulation of private instance attributes sketched in the section “Attribute Reference: __getattr__ and __setattr__” on page 718 in Chapter 29, and watch for the Private class decorator in Chapter 38 that we will base upon this special method. Although it’s possible to emulate true access controls in Python classes, this is rarely done in practice, even for large systems. Methods Are Objects: Bound or Unbound Methods in general, and bound methods in particular, simplify the implementation of many design goals in Python. We met bound methods briefly while studying __call__ in Chapter 29. The full story, which we’ll flesh out here, turns out to be more general and flexible than you might expect. In Chapter 19, we learned how functions can be processed as normal objects. Methods are a kind of object too, and can be used generically in much the same way as other objects—they can be assigned, passed to functions, stored in data structures, and so on. Because class methods can be accessed from an instance or a class, though, they actually come in two flavors in Python: 750 | Chapter 30: Designing with Classes Unbound class method objects: no self Accessing a function attribute of a class by qualifying the class returns an unbound method object. To call the method, you must provide an instance object explicitly as the first argument. In Python 3.0, an unbound method is the same as a simple function and can be called though the class’s name; in 2.6 it’s a distinct type and cannot be called without providing an instance. Bound instance method objects: self + function pairs Accessing a function attribute of a class by qualifying an instance returns a bound method object. Python automatically packages the instance with the function in the bound method object, so you don’t need to pass an instance to call the method. Both kinds of methods are full-fledged objects; they can be transferred around a program at will, just like strings and numbers. Both also require an instance in their first argument when run (i.e., a value for self). This is why we had to pass in an instance explicitly when calling superclass methods from subclass methods in the previous chapter; technically, such calls produce unbound method objects. When calling a bound method object, Python provides an instance for you automatically—the instance used to create the bound method object. This means that bound method objects are usually interchangeable with simple function objects, and makes them especially useful for interfaces originally written for functions (see the sidebar “Why You Will Care: Bound Methods and Callbacks” on page 756 for a realistic example). To illustrate, suppose we define the following class: class Spam: def doit(self, message): print(message) Now, in normal operation, we make an instance and call its method in a single step to print the passed-in argument: object1 = Spam() object1.doit('hello world') Really, though, a bound method object is generated along the way, just before the method call’s parentheses. In fact, we can fetch a bound method without actually calling it. An object.name qualification is an object expression. In the following, it returns a bound method object that packages the instance (object1) with the method function (Spam.doit). We can assign this bound method pair to another name and then call it as though it were a simple function: object1 = Spam() x = object1.doit x('hello world') # Bound method object: instance+function # Same effect as object1.doit('...') Methods Are Objects: Bound or Unbound | 751 On the other hand, if we qualify the class to get to doit, we get back an unbound method object, which is simply a reference to the function object. To call this type of method, we must pass in an instance as the leftmost argument: object1 = Spam() t = Spam.doit t(object1, 'howdy') # Unbound method object (a function in 3.0: see ahead) # Pass in instance (if the method expects one in 3.0) By extension, the same rules apply within a class’s method if we reference self attributes that refer to functions in the class. A self.method expression is a bound method object because self is an instance object: class Eggs: def m1(self, n): print(n) def m2(self): x = self.m1 x(42) # Another bound method object # Looks like a simple function Eggs().m2() # Prints 42 Most of the time, you call methods immediately after fetching them with attribute qualification, so you don’t always notice the method objects generated along the way. But if you start writing code that calls objects generically, you need to be careful to treat unbound methods specially—they normally require an explicit instance object to be passed in.† Unbound Methods are Functions in 3.0 In Python 3.0, the language has dropped the notion of unbound methods. What we describe as an unbound method here is treated as a simple function in 3.0. For most purposes, this makes no difference to your code; either way, an instance will be passed to a method’s first argument when it’s called through an instance. Programs that do explicit type testing might be impacted, though—if you print the type of an instance-less class method, it displays “unbound method” in 2.6, and “function” in 3.0. Moreover, in 3.0 it is OK to call a method without an instance, as long as the method does not expect one and you call it only through the class and never through an instance. That is, Python 3.0 will pass along an instance to methods only for through-instance calls. When calling through a class, you must pass an instance manually only if the method expects one: C:\misc> c:\python30\python >>> class Selfless: † See the discussion of static and class methods in Chapter 31 for an optional exception to this rule. Like bound methods, static methods can masquerade as basic functions because they do not expect instances when called. Python supports three kinds of class methods—instance, static, and class—and 3.0 allows simple functions in classes, too. 752 | Chapter 30: Designing with Classes ... ... ... ... ... ... ... >>> >>> 9 >>> 9 >>> 7 def __init__(self, data): self.data = data def selfless(arg1, arg2): return arg1 + arg2 def normal(self, arg1, arg2): return self.data + arg1 + arg2 # A simple function in 3.0 # Instance expected when called X = Selfless(2) X.normal(3, 4) # Instance passed to self automatically Selfless.normal(X, 3, 4) # self expected by method: pass manually Selfless.selfless(3, 4) # No instance: works in 3.0, fails in 2.6! The last test in this fails in 2.6, because unbound methods require an instance to be passed by default; it works in 3.0 because such methods are treated as simple functions not requiring an instance. Although this removes some potential error trapping in 3.0 (what if a programmer accidentally forgets to pass an instance?), it allows class methods to be used as simple functions as long as they are not passed and do not expect a “self” instance argument. The following two calls still fail in both 3.0 and 2.6, though—the first (calling through an instance) automatically passes an instance to a method that does not expect one, while the second (calling through a class) does not pass an instance to a method that does expect one: >>> X.selfless(3, 4) TypeError: selfless() takes exactly 2 positional arguments (3 given) >>> Selfless.normal(3, 4) TypeError: normal() takes exactly 3 positional arguments (2 given) Because of this change, the staticmethod decorator described in the next chapter is not needed in 3.0 for methods without a self argument that are called only through the class name, and never through an instance—such methods are run as simple functions, without receiving an instance argument. In 2.6, such calls are errors unless an instance is passed manually (more on static methods in the next chapter). It’s important to be aware of the differences in behavior in 3.0, but bound methods are generally more important from a practical perspective anyway. Because they pair together the instance and function in a single object, they can be treated as callables generically. The next section demonstrates what this means in code. For a more visual illustration of unbound method treatment in Python 3.0 and 2.6, see also the lister.py example in the multiple inheritance section later in this chapter. Its classes print the value of methods fetched from both instances and classes, in both versions of Python. Methods Are Objects: Bound or Unbound | 753 Bound Methods and Other Callable Objects As mentioned earlier, bound methods can be processed as generic objects, just like simple functions—they can be passed around a program arbitrarily. Moreover, because bound methods combine both a function and an instance in a single package, they can be treated like any other callable object and require no special syntax when invoked. The following, for example, stores four bound method objects in a list and calls them later with normal call expressions: >>> ... ... ... ... ... ... ... >>> >>> >>> >>> 4 class Number: def __init__(self, base): self.base = base def double(self): return self.base * 2 def triple(self): return self.base * 3 # Class instance objects # State + methods x = Number(2) y = Number(3) z = Number(4) x.double() # Normal immediate calls >>> acts = [x.double, y.double, y.triple, z.double] >>> for act in acts: ... print(act()) ... 4 6 9 8 # List of bound methods # Calls are deferred # Call as though functions Like simple functions, bound method objects have introspection information of their own, including attributes that give access to the instance object and method function they pair. Calling the bound method simply dispatches the pair: >>> bound = x.double >>> bound.__self__, bound.__func__ (<__main__.Number object at 0x0278F610>, <function double at 0x027A4ED0>) >>> bound.__self__.base 2 >>> bound() # Calls bound.__func__(bound.__self__, ...) 4 In fact, bound methods are just one of a handful of callable object types in Python. As the following demonstrates, simple functions coded with a def or lambda, instances that inherit a __call__, and bound instance methods can all be treated and called the same way: >>> def square(arg): ... return arg ** 2 ... >>> class Sum: ... def __init__(self, val): 754 | Chapter 30: Designing with Classes # Simple functions (def or lambda) # Callable instances ... ... ... ... >>> ... ... ... ... ... >>> >>> >>> self.val = val def __call__(self, arg): return self.val + arg class Product: def __init__(self, val): self.val = val def method(self, arg): return self.val * arg # Bound methods sobject = Sum(2) pobject = Product(3) actions = [square, sobject, pobject.method] # Function, instance, method >>> for act in actions: ... print(act(5)) ... 25 7 15 >>> actions[-1](5) 15 >>> [act(5) for act in actions] [25, 7, 15] >>> list(map(lambda act: act(5), actions)) [25, 7, 15] # All 3 called same way # Call any 1-arg callable # Index, comprehensions, maps Technically speaking, classes belong in the callable objects category too, but we normally call them to generate instances rather than to do actual work, as shown here: >>> class Negate: ... def __init__(self, val): ... self.val = -val ... def __repr__(self): ... return str(self.val) ... >>> actions = [square, sobject, pobject.method, >>> for act in actions: ... print(act(5)) ... 25 7 15 -5 >>> [act(5) for act in actions] [25, 7, 15, −5] # Classes are callables too # But called for object, not work # Instance print format Negate] # Call a class too # Runs __repr__ not __str__! >>> table = {act(5): act for act in actions} # 2.6/3.0 dict comprehension >>> for (key, value) in table.items(): ... print('{0:2} => {1}'.format(key, value)) # 2.6/3.0 str.format ... -5 => <class '__main__.Negate'> 25 => <function square at 0x025D4978> 15 => <bound method Product.method of <__main__.Product object at 0x025D0F90>> 7 => <__main__.Sum object at 0x025D0F70> Methods Are Objects: Bound or Unbound | 755 As you can see, bound methods, and Python’s callable objects model in general, are some of the many ways that Python’s design makes for an incredibly flexible language. You should now understand the method object model. For other examples of bound methods at work, see the upcoming sidebar “Why You Will Care: Bound Methods and Callbacks” as well as the prior chapter’s discussion of callback handlers in the section on the method __call__. Why You Will Care: Bound Methods and Callbacks Because bound methods automatically pair an instance with a class method function, you can use them anywhere a simple function is expected. One of the most common places you’ll see this idea put to work is in code that registers methods as event callback handlers in the tkinter GUI interface (named Tkinter in Python 2.6). Here’s the simple case: def handler(): ...use globals for state... ... widget = Button(text='spam', command=handler) To register a handler for button click events, we usually pass a callable object that takes no arguments to the command keyword argument. Function names (and lambdas) work here, and so do class methods, as long as they are bound methods: class MyWidget: def handler(self): ...use self.attr for state... def makewidgets(self): b = Button(text='spam', command=self.handler) Here, the event handler is self.handler—a bound method object that remembers both self and MyGui.handler. Because self will refer to the original instance when handler is later invoked on events, the method will have access to instance attributes that can retain state between events. With simple functions, state normally must be retained in global variables or enclosing function scopes instead. See also the discussion of __call__ operator overloading in Chapter 29 for another way to make classes compatible with function-based APIs. Multiple Inheritance: “Mix-in” Classes Many class-based designs call for combining disparate sets of methods. In a class statement, more than one superclass can be listed in parentheses in the header line. When you do this, you use something called multiple inheritance—the class and its instances inherit names from all the listed superclasses. 756 | Chapter 30: Designing with Classes When searching for an attribute, Python’s inheritance search traverses all superclasses in the class header from left to right until a match is found. Technically, because any of the superclasses may have superclasses of its own, this search can be a bit more complex for larger class tress: • In classic classes (the default until Python 3.0), the attribute search proceeds depthfirst all the way to the top of the inheritance tree, and then from left to right. • In new-style classes (and all classes in 3.0), the attribute search proceeds across by tree levels, in a more breadth-first fashion (see the new-style class discussion in the next chapter). In either model, though, when a class has multiple superclasses, they are searched from left to right according to the order listed in the class statement header lines. In general, multiple inheritance is good for modeling objects that belong to more than one set. For instance, a person may be an engineer, a writer, a musician, and so on, and inherit properties from all such sets. With multiple inheritance, objects obtain the union of the behavior in all their superclasses. Perhaps the most common way multiple inheritance is used is to “mix in” generalpurpose methods from superclasses. Such superclasses are usually called mix-in classes—they provide methods you add to application classes by inheritance. In a sense, mix-in classes are similar to modules: they provide packages of methods for use in their client subclasses. Unlike simple functions in modules, though, methods in mix-ins also have access to the self instance, for using state information and other methods. The next section demonstrates one common use case for such tools. Coding Mix-in Display Classes As we’ve seen, Python’s default way to print a class instance object isn’t incredibly useful: >>> class Spam: ... def __init__(self): ... self.data1 = "food" ... >>> X = Spam() >>> print(X) <__main__.Spam object at 0x00864818> # No __repr__ or __str__ # Default: class, address # Displays "instance" in Python 2.6 As you saw in Chapter 29 when studying operator overloading, you can provide a __str__ or __repr__ method to implement a custom string representation of your own. But, rather than coding one of these in each and every class you wish to print, why not code it once in a general-purpose tool class and inherit it in all your classes? That’s what mix-ins are for. Defining a display method in a mix-in superclass once enables us to reuse it anywhere we want to see a custom display format. We’ve already seen tools that do related work: Multiple Inheritance: “Mix-in” Classes | 757 • Chapter 27’s AttrDisplay class formatted instance attributes in a generic __str__ method, but it did not climb class trees and was used in single-inheritance mode only. • Chapter 28’s classtree.py module defined functions for climbing and sketching class trees, but it did not display object attributes along the way and was not architected as an inheritable class. Here, we’re going to revisit these examples’ techniques and expand upon them to code a set of three mix-in classes that serve as generic display tools for listing instance attributes, inherited attributes, and attributes on all objects in a class tree. We’ll also use our tools in multiple-inheritance mode and deploy coding techniques that make classes better suited to use as generic tools. Listing instance attributes with __dict__ Let’s get started with the simple case—listing attributes attached to an instance. The following class, coded in the file lister.py, defines a mix-in called ListInstance that overloads the __str__ method for all classes that include it in their header lines. Because this is coded as a class, ListInstance is a generic tool whose formatting logic can be used for instances of any subclass: # File lister.py class ListInstance: """ Mix-in class that provides a formatted print() or str() of instances via inheritance of __str__, coded here; displays instance attrs only; self is the instance of lowest class; uses __X names to avoid clashing with client's attrs """ def __str__(self): return '<Instance of %s, address %s:\n%s>' % ( self.__class__.__name__, # My class's name id(self), # My address self.__attrnames()) # name=value list def __attrnames(self): result = '' for attr in sorted(self.__dict__): # Instance attr dict result += '\tname %s=%s\n' % (attr, self.__dict__ [attr]) retubrn result ListInstance uses some previously explored tricks to extract the instance’s class name and attributes: • Each instance has a built-in __class__ attribute that references the class from which it was created, and each class has a __name__ attribute that references the name in the header, so the expression self.__class__.__name__ fetches the name of an instance’s class. 758 | Chapter 30: Designing with Classes • This class does most of its work by simply scanning the instance’s attribute dictionary (remember, it’s exported in __dict__) to build up a string showing the names and values of all instance attributes. The dictionary’s keys are sorted to finesse any ordering differences across Python releases. In these respects, ListInstance is similar to Chapter 27’s attribute display; in fact, it’s largely just a variation on a theme. Our class here uses two additional techniques, though: • It displays the instance’s memory address by calling the id built-function, which returns any object’s address (by definition, a unique object identifier, which will be useful in later mutations of this code). • It uses the pseudoprivate naming pattern for its worker method: __attrnames. As we learned earlier in his chapter, Python automatically localizes any such name to its enclosing class by expanding the attribute name to include the class name (in this case, it becomes _ListInstance__attrnames). This holds true for both class attributes (like methods) and instance attributes attached to self. This behavior is useful in a general tool like this, as it ensures that its names don’t clash with any names used in its client subclasses. Because ListInstance defines a __str__ operator overloading method, instances derived from this class display their attributes automatically when printed, giving a bit more information than a simple address. Here is the class in action, in single-inheritance mode (this code works the same in both Python 3.0 and 2.6): >>> from lister import ListInstance >>> class Spam(ListInstance): ... def __init__(self): ... self.data1 = 'food' ... >>> x = Spam() >>> print(x) <Instance of Spam, address 40240880: name data1=food > # Inherit a __str__ method # print() and str() run __str__ You can also fetch the listing output as a string without printing it with str, and interactive echoes still use the default format: >>> str(x) '<Instance of Spam, address 40240880:\n\tname data1=food\n>' >>> x # The __repr__ still is a default <__main__.Spam object at 0x026606F0> The ListInstance class is useful for any classes you write—even classes that already have one or more superclasses. This is where multiple inheritance comes in handy: by adding ListInstance to the list of superclasses in a class header (i.e., mixing it in), you get its __str__ “for free” while still inheriting from the existing superclass(es). The file testmixin.py demonstrates: Multiple Inheritance: “Mix-in” Classes | 759 # File testmixin.py from lister import * class Super: def __init__(self): self.data1 = 'spam' def ham(self): pass class Sub(Super, ListInstance): def __init__(self): Super.__init__(self) self.data2 = 'eggs' self.data3 = 42 def spam(self): pass if __name__ == '__main__': X = Sub() print(X) # Get lister tool classes # Superclass __init__ # Create instance attrs # Mix in ham and a __str__ # listers have access to self # More instance attrs # Define another method here # Run mixed-in __str__ Here, Sub inherits names from both Super and ListInstance; it’s a composite of its own names and names in both its superclasses. When you make a Sub instance and print it, you automatically get the custom representation mixed in from ListInstance (in this case, this script’s output is the same under both Python 3.0 and 2.6, except for object addresses): C:\misc> C:\python30\python testmixin.py <Instance of Sub, address 40962576: name data1=spam name data2=eggs name data3=42 > ListInstance works in any class it’s mixed into because self refers to an instance of the subclass that pulls this class in, whatever that may be. In a sense, mix-in classes are the class equivalent of modules—packages of methods useful in a variety of clients. For example, here is Lister working again in single-inheritance mode on a different class’s instances, with import and attributes set outside the class: >>> import lister >>> class C(lister.ListInstance): pass ... >>> x = C() >>> x.a = 1; x.b = 2; x.c = 3 >>> print(x) <Instance of C, address 40961776: name a=1 name b=2 name c=3 > 760 | Chapter 30: Designing with Classes Besides the utility they provide, mix-ins optimize code maintenance, like all classes do. For example, if you later decide to extend ListInstance’s __str__ to also print all the class attributes that an instance inherits, you’re safe; because it’s an inherited method, changing __str__ automatically updates the display of each subclass that imports the class and mixes it in. Since it’s now officially “later,” let’s move on to the next section to see what such an extension might look like. Listing inherited attributes with dir As it is, our Lister mix-in displays instance attributes only (i.e., names attached to the instance object itself). It’s trivial to extend the class to display all the attributes accessible from an instance, though—both its own and those it inherits from its classes. The trick is to use the dir built-in function instead of scanning the instance’s __dict__ dictionary; the latter holds instance attributes only, but the former also collects all inherited attributes in Python 2.2 and later. The following mutation codes this scheme; I’ve renamed it to facilitate simple testing, but if this were to replace the original version, all existing clients would pick up the new display automatically: # File lister.py, continued class ListInherited: """ Use dir() to collect both instance attrs and names inherited from its classes; Python 3.0 shows more names than 2.6 because of the implied object superclass in the new-style class model; getattr() fetches inherited names not in self.__dict__; use __str__, not __repr__, or else this loops when printing bound methods! """ def __str__(self): return '<Instance of %s, address %s:\n%s>' % ( self.__class__.__name__, # My class's name id(self), # My address self.__attrnames()) # name=value list def __attrnames(self): result = '' for attr in dir(self): # Instance dir() if attr[:2] == '__' and attr[-2:] == '__': # Skip internals result += '\tname %s=<>\n' % attr else: result += '\tname %s=%s\n' % (attr, getattr(self, attr)) return result Notice that this code skips __X__ names’ values; most of these are internal names that we don’t generally care about in a generic listing like this. This version also must use the getattr built-in function to fetch attributes by name string instead of using instance attribute dictionary indexing—getattr employs the inheritance search protocol, and some of the names we’re listing here are not stored on the instance itself. Multiple Inheritance: “Mix-in” Classes | 761 To test the new version, change the testmixin.py file to use this new class instead: class Sub(Super, ListInherited): # Mix in a __str__ This file’s output varies per release. In Python 2.6, we get the following; notice the name mangling at work in the lister’s method name (I shortened its full value display to fit on this page): C:\misc> c:\python26\python testmixin.py <Instance of Sub, address 40073136: name _ListInherited__attrnames=<bound method Sub.__attrnames of <...more...>> name __doc__=<> name __init__=<> name __module__=<> name __str__=<> name data1=spam name data2=eggs name data3=42 name ham=<bound method Sub.ham of <__main__.Sub instance at 0x026377B0>> name spam=<bound method Sub.spam of <__main__.Sub instance at 0x026377B0>> > In Python 3.0, more attributes are displayed because all classes are “new-style” and inherit names from the implied object superclass (more on this in Chapter 31). Because so many names are inherited from the default superclass, I’ve omitted many here; run this on your own for the full listing: C:\misc> c:\python30\python testmixin.py <Instance of Sub, address 40831792: name _ListInherited__attrnames=<bound method Sub.__attrnames of <...more...>> name __class__=<> name __delattr__=<> name __dict__=<> name __doc__=<> name __eq__=<> ...more names omitted... name __repr__=<> name __setattr__=<> name __sizeof__=<> name __str__=<> name __subclasshook__=<> name __weakref__=<> name data1=spam name data2=eggs name data3=42 name ham=<bound method Sub.ham of <__main__.Sub object at 0x026F0B30>> name spam=<bound method Sub.spam of <__main__.Sub object at 0x026F0B30>> > One caution here—now that we’re displaying inherited methods too, we have to use __str__ instead of __repr__ to overload printing. With __repr__, this code will loop— displaying the value of a method triggers the __repr__ of the method’s class, in order to display the class. That is, if the lister’s __repr__ tries to display a method, displaying the method’s class will trigger the lister’s __repr__ again. Subtle, but true! Change 762 | Chapter 30: Designing with Classes __str__ to __repr__ here to see this for yourself. If you must use __repr__ in such a context, you can avoid the loops by using isinstance to compare the type of attribute values against types.MethodType in the standard library, to know which items to skip. Listing attributes per object in class trees Let’s code one last extension. As it is, our lister doesn’t tell us which class an inherited name comes from. As we saw in the classtree.py example near the end of Chapter 28, though, it’s straightforward to climb class inheritance trees in code. The following mixin class makes use of this same technique to display attributes grouped by the classes they live in—it sketches the full class tree, displaying attributes attached to each object along the way. It does so by traversing the inheritance tree from an instance’s __class__ to its class, and then from the class’s __bases__ to all superclasses recursively, scanning object __dicts__s along the way: # File lister.py, continued class ListTree: """ Mix-in that returns an __str__ trace of the entire class tree and all its objects' attrs at and above self; run by print(), str() returns constructed string; uses __X attr names to avoid impacting clients; uses generator expr to recurse to superclasses; uses str.format() to make substitutions clearer """ def __str__(self): self.__visited = {} return '<Instance of {0}, address {1}:\n{2}{3}>'.format( self.__class__.__name__, id(self), self.__attrnames(self, 0), self.__listclass(self.__class__, 4)) def __listclass(self, aClass, indent): dots = '.' * indent if aClass in self.__visited: return '\n{0}<Class {1}:, address {2}: (see above)>\n'.format( dots, aClass.__name__, id(aClass)) else: self.__visited[aClass] = True genabove = (self.__listclass(c, indent+4) for c in aClass.__bases__) return '\n{0}<Class {1}, address {2}:\n{3}{4}{5}>\n'.format( dots, aClass.__name__, id(aClass), self.__attrnames(aClass, indent), ''.join(genabove), dots) def __attrnames(self, obj, indent): Multiple Inheritance: “Mix-in” Classes | 763 spaces = ' ' * (indent + 4) result = '' for attr in sorted(obj.__dict__): if attr.startswith('__') and attr.endswith('__'): result += spaces + '{0}=<>\n'.format(attr) else: result += spaces + '{0}={1}\n'.format(attr, getattr(obj, attr)) return result Note the use of a generator expression to direct the recursive calls for superclasses; it’s activated by the nested string join method. Also see how this version uses the Python 3.0 and 2.6 string format method instead of % formatting expressions, to make substitutions clearer; when many substitutions are applied like this, explicit argument numbers may make the code easier to decipher. In short, in this version we exchange the first of the following lines for the second: return '<Instance of %s, address %s:\n%s%s>' % (...) # Expression return '<Instance of {0}, address {1}:\n{2}{3}>'.format(...) # Method Now, change testmixin.py to inherit from this new class again to test: class Sub(Super, ListTree): # Mix in a __str__ The file’s tree-sketcher output in Python 2.6 is then as follows: C:\misc> c:\python26\python testmixin.py <Instance of Sub, address 40728496: _ListTree__visited={} data1=spam data2=eggs data3=42 ....<Class Sub, address 40701168: __doc__=<> __init__=<> __module__=<> spam=<unbound method Sub.spam> ........<Class Super, address 40701120: __doc__=<> __init__=<> __module__=<> ham=<unbound method Super.ham> ........> ........<Class ListTree, address 40700688: _ListTree__attrnames=<unbound method ListTree.__attrnames> _ListTree__listclass=<unbound method ListTree.__listclass> __doc__=<> __module__=<> __str__=<> ........> ....> > 764 | Chapter 30: Designing with Classes Notice in this output how methods are unbound now under 2.6, because we fetch them from classes directly, instead of from instances. Also observe how the lister’s __visited table has its name mangled in the instance’s attribute dictionary; unless we’re very unlucky, this won’t clash with other data there. Under Python 3.0, we get extra attributes and superclasses again. Notice that unbound methods are simple functions in 3.0, as described in an earlier note in this chapter (and that again, I’ve deleted most built-in attributes in object to save space here; run this on your own for the complete listing): C:\misc> c:\python30\python testmixin.py <Instance of Sub, address 40635216: _ListTree__visited={} data1=spam data2=eggs data3=42 ....<Class Sub, address 40914752: __doc__=<> __init__=<> __module__=<> spam=<function spam at 0x026D53D8> ........<Class Super, address 40829952: __dict__=<> __doc__=<> __init__=<> __module__=<> __weakref__=<> ham=<function ham at 0x026D5228> ............<Class object, address 505114624: __class__=<> __delattr__=<> __doc__=<> __eq__=<> ...more omitted... __repr__=<> __setattr__=<> __sizeof__=<> __str__=<> __subclasshook__=<> ............> ........> ........<Class ListTree, address 40829496: _ListTree__attrnames=<function __attrnames at 0x026D5660> _ListTree__listclass=<function __listclass at 0x026D56A8> __dict__=<> __doc__=<> __module__=<> __str__=<> __weakref__=<> Multiple Inheritance: “Mix-in” Classes | 765 ............<Class object:, address 505114624: (see above)> ........> ....> > This version avoids listing the same class object twice by keeping a table of classes visited so far (this is why an object’s id is included—to serve as a key for a previously displayed item). Like the transitive module reloader of Chapter 24, a dictionary works to avoid repeats and cycles here because class objects may be dictionary keys; a set would provide similar functionality. This version also takes care to avoid large internal objects by skipping __X__ names again. If you comment out the test for these names, their values will display normally. Here’s an excerpt from the output in 2.6 with this temporary change made (it’s much larger in its entirety, and it gets even worse in 3.0, which is why these names are probably better skipped!): C:\misc> c:\python26\python testmixin.py ...more omitted... ........<Class ListTree, address 40700688: _ListTree__attrnames=<unbound method ListTree.__attrnames> _ListTree__listclass=<unbound method ListTree.__listclass> __doc__= Mix-in that returns the __str__ trace of the entire class tree and all its objects' attrs at and above self; run by print, str returns constructed string; uses __X attr names to avoid impacting clients; uses generator expr to recurse to superclasses; uses str.format() to make substitutions clearer ........> __module__=lister __str__=<unbound method ListTree.__str__> For more fun, try mixing this class into something more substantial, like the Button class of Python’s tkinter GUI toolkit module. In general, you’ll want to name List Tree first (leftmost) in a class header, so its __str__ is picked up; Button has one, too, and the leftmost superclass is searched first in multiple inheritance. The output of the following is fairly massive (18K characters), so run this code on your own to see the full listing (and if you’re using Python 2.6, recall that you should use Tkinter for the module name instead of tkinter): >>> from lister import ListTree >>> from tkinter import Button >>> class MyButton(ListTree, Button): pass ... >>> B = MyButton(text='spam') >>> open('savetree.txt', 'w').write(str(B)) 18247 >>> print(B) <Instance of MyButton, address 44355632: _ListTree__visited={} 766 | Chapter 30: Designing with Classes # Both classes have a __str__ # ListTree first: use its __str__ # Save to a file for later viewing # Print the display here > _name=44355632 _tclCommands=[] ...much more omitted... Of course, there’s much more we could do here (sketching the tree in a GUI might be a natural next step), but we’ll leave further work as a suggested exercise. We’ll also extend this code in the exercises at the end of this part of the book, to list superclass names in parentheses at the start of instance and class displays. The main point here is that OOP is all about code reuse, and mix-in classes are a powerful example. Like almost everything else in programming, multiple inheritance can be a useful device when applied well. In practice, though, it is an advanced feature and can become complicated if used carelessly or excessively. We’ll revisit this topic as a gotcha at the end of the next chapter. In that chapter, we’ll also meet the new-style class model, which modifies the search order for one special multiple inheritance case. Supporting slots: Because they scan instance dictionaries, the ListInstance and ListTree classes presented here don’t directly support attributes stored in slots—a newer and relatively rarely used option we’ll meet in the next chapter, where instance attributes are declared in a __slots__ class attribute. For example, if in textmixin.py we assign __slots__=['data1'] in Super and __slots__=['data3'] in Sub, only the data2 attribute is displayed in the instance by these two lister classes; ListTree also displays data1 and data3, but as attributes of the Super and Sub class objects and with a special format for their values (technically, they are class-level descriptors). To better support slot attributes in these classes, change the __dict__ scanning loops to also iterate through __slots__ lists using code the next chapter will present, and use the getattr built-in function to fetch values instead of __dict__ indexing (ListTree already does). Since instances inherit only the lowest class’s __slots__, you may also need to come up with a policy when __slots__ lists appear in multiple superclasses (ListTree already displays them as class attributes). ListInherited is immune to all this, because dir results combine both __dict__ names and all classes’ __slots__ names. Alternatively, as a policy we could simply let our code handle slot-based attributes as it currently does, rather than complicating it for a rare, advanced feature. Slots and normal instance attributes are different kinds of names. We’ll investigate slots further in the next chapter; I omitted addressing them in these examples to avoid a forward dependency (not counting this note, of course!)—not exactly a valid design goal, but reasonable for a book. Multiple Inheritance: “Mix-in” Classes | 767 Classes Are Objects: Generic Object Factories Sometimes, class-based designs require objects to be created in response to conditions that can’t be predicted when a program is written. The factory design pattern allows such a deferred approach. Due in large part to Python’s flexibility, factories can take multiple forms, some of which don’t seem special at all. Because classes are objects, it’s easy to pass them around a program, store them in data structures, and so on. You can also pass classes to functions that generate arbitrary kinds of objects; such functions are sometimes called factories in OOP design circles. Factories are a major undertaking in a strongly typed language such as C++ but are almost trivial to implement in Python. The call syntax we met in Chapter 18 can call any class with any number of constructor arguments in one step to generate any sort of instance:‡ def factory(aClass, *args): return aClass(*args) # Varargs tuple # Call aClass (or apply in 2.6 only) class Spam: def doit(self, message): print(message) class Person: def __init__(self, name, job): self.name = name self.job = job object1 = factory(Spam) object2 = factory(Person, "Guido", "guru") # Make a Spam object # Make a Person object In this code, we define an object generator function called factory. It expects to be passed a class object (any class will do) along with one or more arguments for the class’s constructor. The function uses special “varargs” call syntax to call the function and return an instance. The rest of the example simply defines two classes and generates instances of both by passing them to the factory function. And that’s the only factory function you’ll ever need to write in Python; it works for any class and any constructor arguments. One possible improvement worth noting is that to support keyword arguments in constructor calls, the factory can collect them with a **args argument and pass them along in the class call, too: def factory(aClass, *args, **kwargs): return aClass(*args, **kwargs) # +kwargs dict # Call aClass ‡ Actually, this syntax can invoke any callable object, including functions, classes, and methods. Hence, the factory function here can also run any callable object, not just a class (despite the argument name). Also, as we learned in Chapter 18, Python 2.6 has an alternative to aClass(*args): the apply(aClass, args) built-in call, which has been removed in Python 3.0 because of its redundancy and limitations. 768 | Chapter 30: Designing with Classes By now, you should know that everything is an “object” in Python, including things like classes, which are just compiler input in languages like C++. However, as mentioned at the start of this part of the book, only objects derived from classes are OOP objects in Python. Why Factories? So what good is the factory function (besides providing an excuse to illustrate class objects in this book)? Unfortunately, it’s difficult to show applications of this design pattern without listing much more code than we have space for here. In general, though, such a factory might allow code to be insulated from the details of dynamically configured object construction. For instance, recall the processor example presented in the abstract in Chapter 25, and then again as a composition example in this chapter. It accepts reader and writer objects for processing arbitrary data streams. The original version of this example manually passed in instances of specialized classes like FileWriter and SocketReader to customize the data streams being processed; later, we passed in hardcoded file, stream, and formatter objects. In a more dynamic scenario, external devices such as configuration files or GUIs might be used to configure the streams. In such a dynamic world, we might not be able to hardcode the creation of stream interface objects in our scripts, but might instead create them at runtime according to the contents of a configuration file. For example, the file might simply give the string name of a stream class to be imported from a module, plus an optional constructor call argument. Factory-style functions or code might come in handy here because they would allow us to fetch and pass in classes that are not hardcoded in our program ahead of time. Indeed, those classes might not even have existed at all when we wrote our code: classname = ...parse from config file... classarg = ...parse from config file... import streamtypes aclass = getattr(streamtypes, classname) reader = factory(aclass, classarg) processor(reader, ...) # Customizable code # Fetch from module # Or aclass(classarg) Here, the getattr built-in is again used to fetch a module attribute given a string name (it’s like saying obj.attr, but attr is a string). Because this code snippet assumes a single constructor argument, it doesn’t strictly need factory or apply—we could make an instance with just aclass(classarg). They may prove more useful in the presence of unknown argument lists, however, and the general factory coding pattern can improve the code’s flexibility. Classes Are Objects: Generic Object Factories | 769 Other Design-Related Topics In this chapter, we’ve seen inheritance, composition, delegation, multiple inheritance, bound methods, and factories—all common patterns used to combine classes in Python programs. We’ve really only scratched the surface here in the design patterns domain, though. Elsewhere in this book you’ll find coverage of other design-related topics, such as: • • • • • • Abstract superclasses (Chapter 28) Decorators (Chapters 31 and 38) Type subclasses (Chapter 31) Static and class methods (Chapter 31) Managed attributes (Chapter 37) Metaclasses (Chapters 31 and 39) For more details on design patterns, though, we’ll delegate to other resources on OOP at large. Although patterns are important in OOP work, and are often more natural in Python than other languages, they are not specific to Python itself. Chapter Summary In this chapter, we sampled common ways to use and combine classes to optimize their reusability and factoring benefits—what are usually considered design issues that are often independent of any particular programming language (though Python can make them easier to implement). We studied delegation (wrapping objects in proxy classes), composition (controlling embedded objects), and inheritance (acquiring behavior from other classes), as well as some more esoteric concepts such as pseudoprivate attributes, multiple inheritance, bound methods, and factories. The next chapter ends our look at classes and OOP by surveying more advanced classrelated topics; some of its material may be of more interest to tool writers than application programmers, but it still merits a review by most people who will do OOP in Python. First, though, another quick chapter quiz. Test Your Knowledge: Quiz 1. What is multiple inheritance? 2. What is delegation? 3. What is composition? 770 | Chapter 30: Designing with Classes 4. What are bound methods? 5. What are pseudoprivate attributes used for? Test Your Knowledge: Answers 1. Multiple inheritance occurs when a class inherits from more than one superclass; it’s useful for mixing together multiple packages of class-based code. The left-toright order in class statement headers determines the order of attribute searches. 2. Delegation involves wrapping an object in a proxy class, which adds extra behavior and passes other operations to the wrapped object. The proxy retains the interface of the wrapped object. 3. Composition is a technique whereby a controller class embeds and directs a number of objects, and provides an interface all its own; it’s a way to build up larger structures with classes. 4. Bound methods combine an instance and a method function; you can call them without passing in an instance object explicitly because the original instance is still available. 5. Pseudoprivate attributes (whose names begin with two leading underscores: __X) are used to localize names to the enclosing class. This includes both class attributes like methods defined inside the class, and self instance attributes assigned inside the class. Such names are expanded to include the class name, which makes them unique. Test Your Knowledge: Answers | 771 CHAPTER 31 Advanced Class Topics This chapter concludes our look at OOP in Python by presenting a few more advanced class-related topics: we will survey subclassing built-in types, “new-style” class changes and extensions, static and class methods, function decorators, and more. As we’ve seen, Python’s OOP model is, at its core, very simple, and some of the topics presented in this chapter are so advanced and optional that you may not encounter them very often in your Python applications-programming career. In the interest of completeness, though, we’ll round out our discussion of classes with a brief look at these advanced tools for OOP work. As usual, because this is the last chapter in this part of the book, it ends with a section on class-related “gotchas,” and the set of lab exercises for this part. I encourage you to work through the exercises to help cement the ideas we’ve studied here. I also suggest working on or studying larger OOP Python projects as a supplement to this book. As with much in computing, the benefits of OOP tend to become more apparent with practice. Content note: This chapter collects advanced class topics, but some are even too advanced for this chapter to cover well. Topics such as properties, descriptors, decorators, and metaclasses are only briefly mentioned here, and are covered more fully in the final part of this book. Be sure to look ahead for more complete examples and extended coverage of some of the subjects that fall into this chapter’s category. Extending Built-in Types Besides implementing new kinds of objects, classes are sometimes used to extend the functionality of Python’s built-in types to support more exotic data structures. For instance, to add queue insert and delete methods to lists, you can code classes that wrap (embed) a list object and export insert and delete methods that process the list specially, like the delegation technique we studied in Chapter 30. As of Python 2.2, you can also 773 use inheritance to specialize built-in types. The next two sections show both techniques in action. Extending Types by Embedding Remember those set functions we wrote in Chapters 16 and 18? Here’s what they look like brought back to life as a Python class. The following example (the file setwrapper.py) implements a new set object type by moving some of the set functions to methods and adding some basic operator overloading. For the most part, this class just wraps a Python list with extra set operations. But because it’s a class, it also supports multiple instances and customization by inheritance in subclasses. Unlike our earlier functions, using classes here allows us to make multiple self-contained set objects with preset data and behavior, rather than passing lists into functions manually: class Set: def __init__(self, value = []): self.data = [] self.concat(value) # Constructor # Manages a list def intersect(self, other): res = [] for x in self.data: if x in other: res.append(x) return Set(res) # other is any sequence # self is the subject def union(self, other): res = self.data[:] for x in other: if not x in res: res.append(x) return Set(res) # other is any sequence # Copy of my list # Add items in other def concat(self, value): for x in value: if not x in self.data: self.data.append(x) # value: list, Set... # Removes duplicates def def def def def len(self.data) self.data[key] self.intersect(other) self.union(other) 'Set:' + repr(self.data) __len__(self): __getitem__(self, key): __and__(self, other): __or__(self, other): __repr__(self): # Pick common items # Return a new Set return return return return return # len(self) # self[i] # self & other # self | other # print() To use this class, we make instances, call methods, and run defined operators as usual: x = Set([1, 3, 5, 7]) print(x.union(Set([1, 4, 7]))) print(x | Set([1, 4, 6])) 774 | Chapter 31: Advanced Class Topics # prints Set:[1, 3, 5, 7, 4] # prints Set:[1, 3, 5, 7, 4, 6] Overloading operations such as indexing enables instances of our Set class to masquerade as real lists. Because you will interact with and extend this class in an exercise at the end of this chapter, I won’t say much more about this code until Appendix B. Extending Types by Subclassing Beginning with Python 2.2, all the built-in types in the language can now be subclassed directly. Type-conversion functions such as list, str, dict, and tuple have become built-in type names—although transparent to your script, a type-conversion call (e.g., list('spam')) is now really an invocation of a type’s object constructor. This change allows you to customize or extend the behavior of built-in types with userdefined class statements: simply subclass the new type names to customize them. Instances of your type subclasses can be used anywhere that the original built-in type can appear. For example, suppose you have trouble getting used to the fact that Python list offsets begin at 0 instead of 1. Not to worry—you can always code your own subclass that customizes this core behavior of lists. The file typesubclass.py shows how: # Subclass built-in list type/class # Map 1..N to 0..N-1; call back to built-in version. class MyList(list): def __getitem__(self, offset): print('(indexing %s at %s)' % (self, offset)) return list.__getitem__(self, offset - 1) if __name__ == '__main__': print(list('abc')) x = MyList('abc') print(x) # __init__ inherited from list # __repr__ inherited from list print(x[1]) print(x[3]) # MyList.__getitem__ # Customizes list superclass method x.append('spam'); print(x) x.reverse(); print(x) # Attributes from list superclass In this file, the MyList subclass extends the built-in list’s __getitem__ indexing method only to map indexes 1 to N back to the required 0 to N−1. All it really does is decrement the submitted index and call back to the superclass’s version of indexing, but it’s enough to do the trick: % python typesubclass.py ['a', 'b', 'c'] ['a', 'b', 'c'] (indexing ['a', 'b', 'c'] at 1) a (indexing ['a', 'b', 'c'] at 3) c ['a', 'b', 'c', 'spam'] ['spam', 'c', 'b', 'a'] Extending Built-in Types | 775 This output also includes tracing text the class prints on indexing. Of course, whether changing indexing this way is a good idea in general is another issue—users of your MyList class may very well be confused by such a core departure from Python sequence behavior. The ability to customize built-in types this way can be a powerful asset, though. For instance, this coding pattern gives rise to an alternative way to code a set—as a subclass of the built-in list type, rather than a standalone class that manages an embedded list object, as shown earlier in this section. As we learned in Chapter 5, Python today comes with a powerful built-in set object, along with literal and comprehension syntax for making new sets. Coding one yourself, though, is still a great way to learn about type subclassing in general. The following class, coded in the file setsubclass.py, customizes lists to add just methods and operators related to set processing. Because all other behavior is inherited from the built-in list superclass, this makes for a shorter and simpler alternative: class Set(list): def __init__(self, value = []): list.__init__([]) self.concat(value) # Constructor # Customizes list # Copies mutable defaults def intersect(self, other): res = [] for x in self: if x in other: res.append(x) return Set(res) # other is any sequence # self is the subject def union(self, other): res = Set(self) res.concat(other) return res # other is any sequence # Copy me and my list def concat(self, value): for x in value: if not x in self: self.append(x) # value: list, Set . . . # Removes duplicates # Pick common items # Return a new Set def __and__(self, other): return self.intersect(other) def __or__(self, other): return self.union(other) def __repr__(self): return 'Set:' + list.__repr__(self) if __name__ == '__main__': x = Set([1,3,5,7]) y = Set([2,1,4,5,6]) print(x, y, len(x)) print(x.intersect(y), y.union(x)) print(x & y, x | y) x.reverse(); print(x) 776 | Chapter 31: Advanced Class Topics Here is the output of the self-test code at the end of this file. Because subclassing core types is an advanced feature, I’ll omit further details here, but I invite you to trace through these results in the code to study its behavior: % python setsubclass.py Set:[1, 3, 5, 7] Set:[2, 1, 4, 5, 6] 4 Set:[1, 5] Set:[2, 1, 4, 5, 6, 3, 7] Set:[1, 5] Set:[1, 3, 5, 7, 2, 4, 6] Set:[7, 5, 3, 1] There are more efficient ways to implement sets with dictionaries in Python, which replace the linear scans in the set implementations shown here with dictionary index operations (hashing) and so run much quicker. (For more details, see Programming Python.) If you’re interested in sets, also take another look at the set object type we explored in Chapter 5; this type provides extensive set operations as built-in tools. Set implementations are fun to experiment with, but they are no longer strictly required in Python today. For another type subclassing example, see the implementation of the bool type in Python 2.3 and later. As mentioned earlier in the book, bool is a subclass of int with two instances (True and False) that behave like the integers 1 and 0 but inherit custom stringrepresentation methods that display their names. The “New-Style” Class Model In Release 2.2, Python introduced a new flavor of classes, known as “new-style” classes; classes following the original model became known as “classic classes” when compared to the new kind. In 3.0 the class story has merged, but it remains split for Python 2.X users: • As of Python 3.0, all classes are automatically what we used to call “new-style,” whether they explicitly inherit from object or not. All classes inherit from object, whether implicitly or explicitly, and all objects are instances of object. • In Python 2.6 and earlier, classes must inherit from object (or another built-in type) to be considered “new-style” and obtain all new-style features. Because all classes are automatically new-style in 3.0, the features of new-style classes are simply normal class features. I’ve opted to keep their descriptions in this section separate, however, in deference to users of Python 2.X code—classes in such code acquire new-style features only when they are derived from object. In other words, when Python 3.0 users see descriptions of “new-style” features in this section, they should take them to be descriptions of existing features of their classes. For 2.6 readers, these are a set of optional extensions. In Python 2.6 and earlier, the only syntactic difference for new-style classes is that they are derived from either a built-in type, such as list, or a special built-in class known The “New-Style” Class Model | 777 as object. The built-in name object is provided to serve as a superclass for new-style classes if no other built-in type is appropriate to use: class newstyle(object): ...normal code... Any class derived from object, or any other built-in type, is automatically treated as a new-style class. As long as a built-in type is somewhere in the superclass tree, the new class is treated as a new-style class. Classes not derived from built-ins such as object are considered classic. New-style classes are only slightly different from classic classes, and the ways in which they differ are irrelevant to the vast majority of Python users. Moreover, the classic class model still available in 2.6 works exactly as it has for almost two decades. In fact, new-style classes are almost completely backward compatible with classic classes in syntax and behavior; they mostly just add a few advanced new features. However, because they modify a handful of class behaviors, they had to be introduced as a distinct tool so as to avoid impacting any existing code that depends on the prior behaviors. For example, some subtle differences, such as diamond pattern inheritance search and the behavior of built-in operations with managed attribute methods such as __getattr__, can cause some legacy code to fail if left unchanged. The next two sections provide overviews of the ways the new-style classes differ and the new tools they provide. Again, because all classes are new-style today, these topics represent changes to Python 2.X readers but simply additional advanced class topics to Python 3.0 readers. New-Style Class Changes New-style classes differ from classic classes in a number of ways, some of which are subtle but can impact existing 2.X code and coding styles. Here are some of the most prominent ways they differ: Classes and types merged Classes are now types, and types are now classes. In fact, the two are essentially synonyms. The type(I) built-in returns the class an instance is made from, instead of a generic instance type, and is normally the same as I.__class__. Moreover, classes are instances of the type class, type may be subclassed to customize class creation, and all classes (and hence types) inherit from object. Inheritance search order Diamond patterns of multiple inheritance have a slightly different search order— roughly, they are searched across before up, and more breadth-first than depthfirst. 778 | Chapter 31: Advanced Class Topics Attribute fetch for built-ins The __getattr__ and __getattribute__ methods are no longer run for attributes implicitly fetched by built-in operations. This means that they are not called for __X__ operator overloading method names—the search for such names begins at classes, not instances. New advanced tools New-style classes have a set of new class tools, including slots, properties, descriptors, and the __getattribute__ method. Most of these have very specific toolbuilding purposes. We discussed the third of these changes briefly in a sidebar in Chapter 27, and we’ll revisit it in depth in the contexts of attribute management in Chapter 37 and privacy decorators in Chapter 38. Because the first and second of the changes just listed can break existing 2.X code, though, let’s explore these in more detail before moving on to new-style additions. Type Model Changes In new-style classes, the distinction between type and class has vanished entirely. Classes themselves are types: the type object generates classes as its instances, and classes generate instances of their type. If fact, there is no real difference between builtin types like lists and strings and user-defined types coded as classes. This is why we can subclass built-in types, as shown earlier in this chapter—because subclassing a built-in type such as list qualifies a class as new-style, it becomes a user-defined type. Besides allowing us to subclass built-in types, one of the contexts where this becomes most obvious is when we do explicit type testing. With Python 2.6’s classic classes, the type of a class instance is a generic “instance,” but the types of built-in objects are more specific: C:\misc> c:\python26\python >>> class C: pass ... >>> I = C() >>> type(I) <type 'instance'> >>> I.__class__ <class __main__.C at 0x025085A0> # Classic classes in 2.6 # Instances are made from classes >>> type(C) # But classes are not the same as types <type 'classobj'> >>> C.__class__ AttributeError: class C has no attribute '__class__' >>> type([1, 2, 3]) <type 'list'> >>> type(list) <type 'type'> New-Style Class Changes | 779 >>> list.__class__ <type 'type'> But with new-style classes in 2.6, the type of a class instance is the class it’s created from, since classes are simply user-defined types—the type of an instance is its class, and the type of a user-defined class is the same as the type of a built-in object type. Classes have a __class__ attribute now, too, because they are instances of type: C:\misc> c:\python26\python >>> class C(object): pass ... >>> I = C() >>> type(I) <class '__main__.C'> >>> I.__class__ <class '__main__.C'> # New-style classes in 2.6 # Type of instance is class it's made from >>> type(C) <type 'type'> >>> C.__class__ <type 'type'> # Classes are user-defined types >>> type([1, 2, 3]) <type 'list'> >>> type(list) <type 'type'> >>> list.__class__ <type 'type'> # Built-in types work the same way The same is true for all classes in Python 3.0, since all classes are automatically newstyle, even if they have no explicit superclasses. In fact, the distinction between builtin types and user-defined class types melts away altogether in 3.0: C:\misc> c:\python30\python >>> class C: pass ... >>> I = C() >>> type(I) <class '__main__.C'> >>> I.__class__ <class '__main__.C'> # All classes are new-style in 3.0 # Type of instance is class it's made from >>> type(C) <class 'type'> >>> C.__class__ <class 'type'> # Class is a type, and type is a class >>> type([1, 2, 3]) <class 'list'> >>> type(list) <class 'type'> >>> list.__class__ <class 'type'> # Classes and built-in types work the same 780 | Chapter 31: Advanced Class Topics As you can see, in 3.0 classes are types, but types are also classes. Technically, each class is generated by a metaclass—a class that is normally either type itself, or a subclass of it customized to augment or manage generated classes. Besides impacting code that does type testing, this turns out to be an important hook for tool developers. We’ll talk more about metaclasses later in this chapter, and again in more detail in Chapter 39. Implications for type testing Besides providing for built-in type customization and metaclass hooks, the merging of classes and types in the new-style class model can impact code that does type testing. In Python 3.0, for example, the types of class instances compare directly and meaningfully, and in the same way as built-in type objects. This follows from the fact that classes are now types, and an instance’s type is the instance’s class: C:\misc> c:\python30\python >>> class C: pass ... >>> class D: pass ... >>> c = C() >>> d = D() >>> type(c) == type(d) False # 3.0: compares the instances' classes >>> type(c), type(d) (<class '__main__.C'>, <class '__main__.D'>) >>> c.__class__, d.__class__ (<class '__main__.C'>, <class '__main__.D'>) >>> c1, c2 = C(), C() >>> type(c1) == type(c2) True With classic classes in 2.6 and earlier, though, comparing instance types is almost useless, because all instances have the same “instance” type. To truly compare types, the instance __class__ attributes must be compared (if you care about portability, this works in 3.0, too, but it’s not required there): C:\misc> c:\python26\python >>> class C: pass ... >>> class D: pass ... >>> c = C() >>> d = D() >>> type(c) == type(d) True >>> c.__class__ == d.__class__ False # 2.6: all instances are same type # Must compare classes explicitly >>> type(c), type(d) (<type 'instance'>, <type 'instance'>) New-Style Class Changes | 781 >>> c.__class__, d.__class__ (<class __main__.C at 0x024585A0>, <class __main__.D at 0x024588D0>) And as you should expect by now, new-style classes in 2.6 work the same as all classes in 3.0 in this regard—comparing instance types compares the instances’ classes automatically: C:\misc> c:\python26\python >>> class C(object): pass ... >>> class D(object): pass ... >>> c = C() >>> d = D() >>> type(c) == type(d) False # 2.6 new-style: same as all in 3.0 >>> type(c), type(d) (<class '__main__.C'>, <class '__main__.D'>) >>> c.__class__, d.__class__ (<class '__main__.C'>, <class '__main__.D'>) Of course, as I’ve pointed out numerous times in this book, type checking is usually the wrong thing to do in Python programs (we code to object interfaces, not object types), and the more general isinstance built-in is more likely what you’ll want to use in the rare cases where instance class types must be queried. However, knowledge of Python’s type model can help demystify the class model in general. All objects derive from “object” One other ramification of the type change in the new-style class model is that because all classes derive (inherit) from the class object either implicitly or explicitly, and because all types are now classes, every object derives from the object built-in class, whether directly or through a superclass. Consider the following interaction in Python 3.0 (code an explicit object superclass in 2.6 to make this work equivalently): >>> class C: pass ... >>> X = C() >>> type(X) <class '__main__.C'> >>> type(C) <class 'type'> # Type is now class instance was created from As before, the type of a class instance is the class it was made from, and the type of a class is the type class because classes and types have merged. It is also true, though, that the instance and class are both derived from the built-in object class, since this is an implicit or explicit superclass of every class: 782 | Chapter 31: Advanced Class Topics >>> isinstance(X, object) True >>> isinstance(C, object) True # Classes always inherit from object The same holds true for built-in types like lists and strings, because types are classes in the new-style model—built-in types are now classes, and their instances derive from object, too: >>> type('spam') <class 'str'> >>> type(str) <class 'type'> >>> isinstance('spam', object) True >>> isinstance(str, object) True # Same for built-in types (classes) In fact, type itself derives from object, and object derives from type, even though the two are different objects—a circular relationship that caps the object model and stems from the fact that types are classes that generate classes: >>> type(type) <class 'type'> >>> type(object) <class 'type'> # All classes are types, and vice versa >>> isinstance(type, object) True >>> isinstance(object, type) True >>> type is object False # All classes derive from object, even type # Types make classes, and type is a class In practical terms, this model makes for fewer special cases than the prior type/class distinction of classic classes, and it allows us to write code that assumes and uses an object superclass. We’ll see examples of the latter later in the book; for now, let’s move on to explore other new-style changes. Diamond Inheritance Change One of the most visible changes in new-style classes is their slightly different inheritance search procedures for the so-called diamond pattern of multiple inheritance trees, where more than one superclass leads to the same higher superclass further above. The diamond pattern is an advanced design concept, is coded only rarely in Python practice, and has not been discussed in this book, so we won’t dwell on this topic in depth. In short, though, with classic classes, the inheritance search procedure is strictly depth first, and then left to right—Python climbs all the way to the top, hugging the left side of the tree, before it backs up and begins to look further to the right. In new-style classes, the search is more breadth-first in such cases—Python first looks in any superclasses New-Style Class Changes | 783 to the right of the first one searched before ascending all the way to the common superclass at the top. In other words, the search proceeds across by levels before moving up. The search algorithm is a bit more complex than this, but this is as much as most programmers need to know. Because of this change, lower superclasses can overload attributes of higher superclasses, regardless of the sort of multiple inheritance trees they are mixed into. Moreover, the new-style search rule avoids visiting the same superclass more than once when it is accessible from multiple subclasses. Diamond inheritance example To illustrate, consider this simplistic incarnation of the diamond multiple inheritance pattern for classic classes. Here, D’s superclasses B and C both lead to the same common ancestor, A: >>> class A: attr = 1 >>> class B(A): pass # Classic (Python 2.6) # B and C both lead to A >>> class C(A): attr = 2 >>> class D(B, C): pass >>> x = D() >>> x.attr 1 # Tries A before C # Searches x, D, B, A The attribute here is found in superclass A, because with classic classes, the inheritance search climbs as high as it can before backing up and moving right—Python will search D, B, A, and then C, but will stop when attr is found in A, above B. However, with new-style classes derived from a built-in like object, and all classes in 3.0, the search order is different: Python looks in C (to the right of B) before A (above B). That is, it searches D, B, C, and then A, and in this case, stops in C: >>> class A(object): attr = 1 # New-style ("object" not required in 3.0) >>> class B(A): pass >>> class C(A): attr = 2 >>> class D(B, C): pass # Tries C before A >>> x = D() 784 | Chapter 31: Advanced Class Topics >>> x.attr 2 # Searches x, D, B, C This change in the inheritance search procedure is based upon the assumption that if you mix in C lower in the tree, you probably intend to grab its attributes in preference to A’s. It also assumes that C is always intended to override A’s attributes in all contexts, which is probably true when it’s used standalone but may not be when it’s mixed into a diamond with classic classes—you might not even know that C may be mixed in like this when you code it. Since it is most likely that the programmer meant that C should override A in this case, though, new-style classes visit C first. Otherwise, C could be essentially pointless in a diamond context: it could not customize A and would be used only for names unique to C. Explicit conflict resolution Of course, the problem with assumptions is that they assume things. If this search order deviation seems too subtle to remember, or if you want more control over the search process, you can always force the selection of an attribute from anywhere in the tree by assigning or otherwise naming the one you want at the place where the classes are mixed together: >>> class A: attr = 1 # Classic >>> class B(A): pass >>> class C(A): attr = 2 >>> class D(B, C): attr = C.attr >>> x = D() >>> x.attr 2 # Choose C, to the right # Works like new-style (all 3.0) Here, a tree of classic classes is emulating the search order of new-style classes: the assignment to the attribute in D picks the version in C, thereby subverting the normal inheritance search path (D.attr will be lowest in the tree). New-style classes can similarly emulate classic classes by choosing the attribute above at the place where the classes are mixed together: >>> class A(object): attr = 1 # New-style >>> class B(A): pass >>> class C(A): New-Style Class Changes | 785 attr = 2 >>> class D(B, C): attr = B.attr >>> x = D() >>> x.attr 1 # Choose A.attr, above # Works like classic (default 2.6) If you are willing to always resolve conflicts like this, you can largely ignore the search order difference and not rely on assumptions about what you meant when you coded your classes. Naturally, attributes picked this way can also be method functions—methods are normal, assignable objects: >>> class A: def meth(s): print('A.meth') >>> class C(A): def meth(s): print('C.meth') >>> class B(A): pass >>> class D(B, C): pass >>> x = D() >>> x.meth() A.meth # Use default search order # Will vary per class type # Defaults to classic order in 2.6 >>> class D(B, C): meth = C.meth >>> x = D() >>> x.meth() C.meth # Pick C's method: new-style (and 3.0) >>> class D(B, C): meth = B.meth >>> x = D() >>> x.meth() A.meth # Pick B's method: classic Here, we select methods by explicitly assigning to names lower in the tree. We might also simply call the desired class explicitly; in practice, this pattern might be more common, especially for things like constructors: class D(B, C): def meth(self): ... C.meth(self) # Redefine lower # Pick C's method by calling Such selections by assignment or call at mix-in points can effectively insulate your code from this difference in class flavors. Explicitly resolving the conflicts this way ensures that your code won’t vary per Python version in the future (apart from perhaps needing to derive classes from object or a built-in type for the new-style tools in 2.6). 786 | Chapter 31: Advanced Class Topics Even without the classic/new-style class divergence, the explicit method resolution technique shown here may come in handy in multiple inheritance scenarios in general. For instance, if you want part of a superclass on the left and part of a superclass on the right, you might need to tell Python which same-named attributes to choose by using explicit assignments in subclasses. We’ll revisit this notion in a “gotcha” at the end of this chapter. Also note that diamond inheritance patterns might be more problematic in some cases than I’ve implied here (e.g., what if B and C both have required constructors that call to the constructor in A?). Since such contexts are rare in real-world Python, we’ll leave this topic outside this book’s scope (but see the super built-in function for hints—besides providing generic access to superclasses in single inheritance trees, super supports a cooperative mode for resolving some conflicts in multiple inheritance trees). Scope of search order change In sum, by default, the diamond pattern is searched differently for classic and new-style classes, and this is a nonbackward-compatible change. Keep in mind, though, that this change primarily affects diamond pattern cases of multiple inheritance; new-style class inheritance works unchanged for most other inheritance tree structures. Further, it’s not impossible that this entire issue may be of more theoretical than practical importance—because the new-style search wasn’t significant enough to address until Python 2.2 and didn’t become standard until 3.0, it seems unlikely to impact much Python code. Having said that, I should also note that even though you might not code diamond patterns in classes you write yourself, because the implied object superclass is above every class in 3.0, every case of multiple inheritance exhibits the diamond pattern today. That is, in new-style classes, object automatically plays the role that the class A does in the example we just considered. Hence the new-style search rule not only modifies logical semantics, but also optimizes performance by avoiding visiting the same class more than once. Just as important, the implied object superclass in the new-style model provides default methods for a variety of built-in operations, including the __str__ and __repr__ display format methods. Run a dir(object) to see which methods are provided. Without the new-style search order, in multiple inheritance cases the defaults in object would always override redefinitions in user-coded classes, unless they were always made in the leftmost superclass. In other words, the new-style class model itself makes using the new-style search order more critical! For a more visual example of the implied object superclass in 3.0, and other examples of diamond patterns created by it, see the ListTree class’s output in the lister.py example in the preceding chapter, as well as the classtree.py tree walker example in Chapter 28. New-Style Class Changes | 787 New-Style Class Extensions Beyond the changes described in the prior section (which, frankly, may be too academic and obscure to matter to many readers of this book), new-style classes provide a handful of more advanced class tools that have more direct and practical application. The following sections provide an overview of each of these additional features, available for new-style class in Python 2.6 and all classes in Python 3.0. Instance Slots By assigning a sequence of string attribute names to a special __slots__ class attribute, it is possible for a new-style class to both limit the set of legal attributes that instances of the class will have and optimize memory and speed performance. This special attribute is typically set by assigning a sequence of string names to the variable __slots__ at the top level of a class statement: only those names in the __slots__ list can be assigned as instance attributes. However, like all names in Python, instance attribute names must still be assigned before they can be referenced, even if they’re listed in __slots__. For example: >>> class limiter(object): ... __slots__ = ['age', 'name', 'job'] ... >>> x = limiter() >>> x.age AttributeError: age # Must assign before use >>> x.age = 40 >>> x.age 40 >>> x.ape = 1000 # Illegal: not in __slots__ AttributeError: 'limiter' object has no attribute 'ape' Slots are something of a break with Python’s dynamic nature, which dictates that any name may be created by assignment. However, this feature is envisioned as both a way to catch “typo” errors like this (assignments to illegal attribute names not in __slots__ are detected), as well as an optimization mechanism. Allocating a namespace dictionary for every instance object can become expensive in terms of memory if many instances are created and only a few attributes are required. To save space and speed execution (to a degree that can vary per program), instead of allocating a dictionary for each instance, slot attributes are stored sequentially for quicker lookup. Slots and generic code In fact, some instances with slots may not have a __dict__ attribute dictionary at all, which can make some metaprograms more complex (including some coded in this book). Tools that generically list attributes or access attributes by string name, for example, must be careful to use more storage-neutral tools than __dict__, such as the 788 | Chapter 31: Advanced Class Topics getattr, setattr, and dir built-in functions, which apply to attributes based on either __dict__ or __slots__ storage. In some cases, both attribute sources may need to be queried for completeness. For example, when slots are used, instances do not normally have an attribute dictionary—Python uses the class descriptors feature covered in Chapter 37 to allocate space for slot attributes in the instance instead. Only names in the slots list can be assigned to instances, but slot-based attributes can still be fetched and set by name using generic tools. In Python 3.0 (and in 2.6 for classes derived from object): >>> class C: ... __slots__ = ['a', 'b'] # __slots__ means no __dict__ by default ... >>> X = C() >>> X.a = 1 >>> X.a 1 >>> X.__dict__ AttributeError: 'C' object has no attribute '__dict__' >>> getattr(X, 'a') 1 >>> setattr(X, 'b', 2) # But getattr() and setattr() still work >>> X.b 2 >>> 'a' in dir(X) # And dir() finds slot attributes too True >>> 'b' in dir(X) True Without an attribute namespaces dictionary, it’s not possible to assign new names to instances that are not names in the slots list: >>> class D: ... __slots__ = ['a', 'b'] ... def __init__(self): self.d = 4 # Cannot add new names if no __dict__ ... >>> X = D() AttributeError: 'D' object has no attribute 'd' However, extra attributes can still be accommodated by including __dict__ in __slots__, in order to allow for an attribute namespace dictionary. In this case, both storage mechanisms are used, but generic tools such as getattr allow us to treat them as a single set of attributes: >>> class D: ... __slots__ = ['a', 'b', '__dict__'] # List __dict__ to include one too ... c = 3 # Class attrs work normally ... def __init__(self): self.d = 4 # d put in __dict__, a in __slots__ ... >>> X = D() >>> X.d 4 >>> X.__dict__ # Some objects have both __dict__ and __slots__ {'d': 4} # getattr() can fetch either type of attr New-Style Class Extensions | 789 >>> X.__slots__ ['a', 'b', '__dict__'] >>> X.c 3 >>> X.a # All instance attrs undefined until assigned AttributeError: a >>> X.a = 1 >>> getattr(X, 'a',), getattr(X, 'c'), getattr(X, 'd') (1, 3, 4) Code that wishes to list all instance attributes generically, though, may still need to allow for both storage forms, since dir also returns inherited attributes (this relies on dictionary iterators to collect keys): >>> for attr in list(X.__dict__) + X.__slots__: ... print(attr, '=>', getattr(X, attr)) d => 4 a => 1 b => 2 __dict__ => {'d': 4} Since either can be omitted, this is more correctly coded as follows (getattr allows for defaults): >>> for attr in list(getattr(X, '__dict__', [])) + getattr(X, '__slots__', []): ... print(attr, '=>', getattr(X, attr)) d => 4 a => 1 b => 2 __dict__ => {'d': 4} Multiple __slot__ lists in superclasses Note, however, that this code addresses only slot names in the lowest __slots__ attribute inherited by an instance. If multiple classes in a class tree have their own __slots__ attributes, generic programs must develop other policies for listing attributes (e.g., classifying slot names as attributes of classes, not instances). Slot declarations can appear in multiple classes in a class tree, but they are subject to a number of constraints that are somewhat difficult to rationalize unless you understand the implementation of slots as class-level descriptors (a tool we’ll study in detail in the last part of this book): • If a subclass inherits from a superclass without a __slots__, the __dict__ attribute of the superclass will always be accessible, making a __slots__ in the subclass meaningless. • If a class defines the same slot name as a superclass, the version of the name defined by the superclass slot will be accessible only by fetching its descriptor directly from the superclass. 790 | Chapter 31: Advanced Class Topics • Because the meaning of a __slots__ declaration is limited to the class in which it appears, subclasses will have a __dict__ unless they also define a __slots__. In terms of listing instance attributes generically, slots in multiple classes might require manual class tree climbs, dir usage, or a policy that treats slot names as a different category of names altogether: >>> ... ... >>> ... ... >>> >>> >>> (1, class E: __slots__ = ['c', 'd'] # Superclass has slots class D(E): __slots__ = ['a', '__dict__'] # So does its subclass X = D() X.a = 1; X.b = 2; X.c = 3 X.a, X.c 3) >>> E.__slots__ ['c', 'd'] >>> D.__slots__ ['a', '__dict__'] >>> X.__slots__ ['a', '__dict__'] >>> X.__dict__ {'b': 2} # The instance is the union # But slots are not concatenated # Instance inherits *lowest* __slots__ # And has its own an attr dict >>> for attr in list(getattr(X, '__dict__', [])) + getattr(X, '__slots__', []): ... print(attr, '=>', getattr(X, attr)) ... b => 2 # Superclass slots missed! a => 1 __dict__ => {'b': 2} >>> dir(X) # dir() includes all slot names [...many names omitted... 'a', 'b', 'c', 'd'] When such generality is possible, slots are probably best treated as class attributes, rather than trying to mold them to appear the same as normal instance attributes. For more on slots in general, see the Python standard manual set. Also watch for an example that allows for attributes based on both __slots__ and __dict__ storage in the Private decorator discussion of Chapter 38. For a prime example of why generic programs may need to care about slots, see the lister.py display mix-in classes example in the multiple inheritance section of the prior chapter; a note there describes the example’s slot concerns. In such a tool that attempts to list attributes generically, slot usage requires either extra code or the implementation of policies regarding the handling of slot-based attributes in general. New-Style Class Extensions | 791 Class Properties A mechanism known as properties provides another way for new-style classes to define automatically called methods for access or assignment to instance attributes. At least for specific attributes, this feature is an alternative to many current uses of the __getattr__ and __setattr__ overloading methods we studied in Chapter 29. Properties have a similar effect to these two methods, but they incur an extra method call for any accesses to names that require dynamic computation. Properties (and slots) are based on a new notion of attribute descriptors, which is too advanced for us to cover here. In short, a property is a type of object assigned to a class attribute name. A property is generated by calling the property built-in with three methods (handlers for get, set, and delete operations), as well as a docstring; if any argument is passed as None or omitted, that operation is not supported. Properties are typically assigned at the top level of a class statement [e.g., name = property(...)]. When thus assigned, accesses to the class attribute itself (e.g., obj.name) are automatically routed to one of the accessor methods passed into the property. For example, the __getattr__ method allows classes to intercept undefined attribute references: >>> class classic: ... def __getattr__(self, name): ... if name == 'age': ... return 40 ... else: ... raise AttributeError ... >>> x = classic() >>> x.age 40 >>> x.name AttributeError # Runs __getattr__ # Runs __getattr__ Here is the same example, coded with properties instead (note that properties are available for all classes but require the new-style object derivation in 2.6 to work properly for intercepting attribute assignments): >>> class newprops(object): ... def getage(self): ... return 40 ... age = property(getage, None, None, None) # get, set, del, docs ... >>> x = newprops() >>> x.age # Runs getage 40 >>> x.name # Normal fetch AttributeError: newprops instance has no attribute 'name' 792 | Chapter 31: Advanced Class Topics For some coding tasks, properties can be less complex and quicker to run than the traditional techniques. For example, when we add attribute assignment support, properties become more attractive—there’s less code to type, and no extra method calls are incurred for assignments to attributes we don’t wish to compute dynamically: >>> class newprops(object): ... def getage(self): ... return 40 ... def setage(self, value): ... print('set age:', value) ... self._age = value ... age = property(getage, setage, None, None) ... >>> x = newprops() >>> x.age # Runs getage 40 >>> x.age = 42 # Runs setage set age: 42 >>> x._age # Normal fetch; no getage call 42 >>> x.job = 'trainer' # Normal assign; no setage call >>> x.job # Normal fetch; no getage call 'trainer' The equivalent classic class incurs extra method calls for assignments to attributes not being managed and needs to route attribute assignments through the attribute dictionary (or, for new-style classes, to the object superclass’s __setattr__) to avoid loops: >>> class classic: ... def __getattr__(self, name): ... if name == 'age': ... return 40 ... else: ... raise AttributeError ... def __setattr__(self, name, value): ... print('set:', name, value) ... if name == 'age': ... self.__dict__['_age'] = value ... else: ... self.__dict__[name] = value ... >>> x = classic() >>> x.age 40 >>> x.age = 41 set: age 41 >>> x._age 41 >>> x.job = 'trainer' >>> x.job # On undefined reference # On all assignments # Runs __getattr__ # Runs __setattr__ # Defined: no __getattr__ call # Runs __setattr__ again # Defined: no __getattr__ call New-Style Class Extensions | 793 Properties seem like a win for this simple example. However, some applications of __getattr__ and __setattr__ may still require more dynamic or generic interfaces than properties directly provide. For example, in many cases, the set of attributes to be supported cannot be determined when the class is coded, and may not even exist in any tangible form (e.g., when delegating arbitrary method references to a wrapped/ embedded object generically). In such cases, a generic __getattr__ or a __setattr__ attribute handler with a passed-in attribute name may be preferable. Because such generic handlers can also handle simpler cases, properties are often an optional extension. For more details on both options, stay tuned for Chapter 37 in the final part of this book. As we’ll see there, it’s also possible to code properties using function decorator syntax, a topic introduced later in this chapter. __getattribute__ and Descriptors The __getattribute__ method, available for new-style classes only, allows a class to intercept all attribute references, not just undefined references, like __getattr__. It is also somewhat trickier to use than __getattr__: it is prone to loops, much like __setattr__, but in different ways. In addition to properties and operator overloading methods, Python supports the notion of attribute descriptors—classes with __get__ and __set__ methods, assigned to class attributes and inherited by instances, that intercept read and write accesses to specific attributes. Descriptors are in a sense a more general form of properties; in fact, properties are a simplified way to define a specific type of descriptor, one that runs functions on access. Descriptors are also used to implement the slots feature we met earlier. Because properties, __getattribute__, and descriptors are somewhat advanced topics, we’ll defer the rest of their coverage, as well as more on properties, to Chapter 37 in the final part of this book. Metaclasses Most of the changes and feature additions of new-style classes integrate with the notion of subclassable types mentioned earlier in this chapter, because subclassable types and new-style classes were introduced in conjunction with a merging of the type/class dichotomy in Python 2.2 and beyond. As we’ve seen, in 3.0, this merging is complete: classes are now types, and types are classes. Along with these changes, Python also grew a more coherent protocol for coding metaclasses, which are classes that subclass the type object and intercept class creation calls. As such, they provide a well-defined hook for management and augmentation of class objects. They are also an advanced topic that is optional for most Python programmers, so we’ll postpone further details here. We’ll meet metaclasses briefly later 794 | Chapter 31: Advanced Class Topics in this chapter in conjunction with class decorators, and we’ll explore them in full detail in Chapter 39, in the final part of this book. Static and Class Methods As of Python 2.2, it is possible to define two kinds of methods within a class that can be called without an instance: static methods work roughly like simple instance-less functions inside a class, and class methods are passed a class instead of an instance. Although this feature was added in conjunction with the new-style classes discussed in the prior sections, static and class methods work for classic classes too. To enable these method modes, special built-in functions called staticmethod and classmethod must be called within the class, or invoked with the decoration syntax we’ll meet later in this chapter. In Python 3.0, instance-less methods called only through a class name do not require a staticmethod declaration, but such methods called through instances do. Why the Special Methods? As we’ve learned, a class method is normally passed an instance object in its first argument, to serve as the implied subject of the method call. Today, though, there are two ways to modify this model. Before I explain what they are, I should explain why this might matter to you. Sometimes, programs need to process data associated with classes instead of instances. Consider keeping track of the number of instances created from a class, or maintaining a list of all of a class’s instances that are currently in memory. This type of information and its processing are associated with the class rather than its instances. That is, the information is usually stored on the class itself and processed in the absence of any instance. For such tasks, simple functions coded outside a class can often suffice—because they can access class attributes through the class name, they have access to class data and never require access to an instance. However, to better associate such code with a class, and to allow such processing to be customized with inheritance as usual, it would be better to code these types of functions inside the class itself. To make this work, we need methods in a class that are not passed, and do not expect, a self instance argument. Python supports such goals with the notion of static methods—simple functions with no self argument that are nested in a class and are designed to work on class attributes instead of instance attributes. Static methods never receive an automatic self argument, whether called through a class or an instance. They usually keep track of information that spans all instances, rather than providing behavior for instances. Static and Class Methods | 795 Although less commonly used, Python also supports the notion of class methods— methods of a class that are passed a class object in their first argument instead of an instance, regardless of whether they are called through an instance or a class. Such methods can access class data through their self class argument even if called through an instance. Normal methods (now known in formal circles as instance methods) still receive a subject instance when called; static and class methods do not. Static Methods in 2.6 and 3.0 The concept of static methods is the same in both Python 2.6 and 3.0, but its implementation requirements have evolved somewhat in Python 3.0. Since this book covers both versions, I need to explain the differences in the two underlying models before we get to the code. Really, we already began this story in the preceding chapter, when we explored the notion of unbound methods. Recall that both Python 2.6 and 3.0 always pass an instance to a method that is called through an instance. However, Python 3.0 treats methods fetched directly from a class differently than 2.6: • In Python 2.6, fetching a method from a class produces an unbound method, which cannot be called without manually passing an instance. • In Python 3.0, fetching a method from a class produces a simple function, which can be called normally with no instance present. In other words, Python 2.6 class methods always require an instance to be passed in, whether they are called through an instance or a class. By contrast, in Python 3.0 we are required to pass an instance to a method only if the method expects one—methods without a self instance argument can be called through the class without passing an instance. That is, 3.0 allows simple functions in a class, as long as they do not expect and are not passed an instance argument. The net effect is that: • In Python 2.6, we must always declare a method as static in order to call it without an instance, whether it is called through a class or an instance. • In Python 3.0, we need not declare such methods as static if they will be called through a class only, but we must do so in order to call them through an instance. To illustrate, suppose we want to use class attributes to count how many instances are generated from a class. The following file, spam.py, makes a first attempt—its class has a counter stored as a class attribute, a constructor that bumps up the counter by one each time a new instance is created, and a method that displays the counter’s value. Remember, class attributes are shared by all instances. Therefore, storing the counter in the class object itself ensures that it effectively spans all instances: class Spam: numInstances = 0 def __init__(self): Spam.numInstances = Spam.numInstances + 1 796 | Chapter 31: Advanced Class Topics def printNumInstances(): print("Number of instances created: ", Spam.numInstances) The printNumInstances method is designed to process class data, not instance data— it’s about all the instances, not any one in particular. Because of that, we want to be able to call it without having to pass an instance. Indeed, we don’t want to make an instance to fetch the number of instances, because this would change the number of instances we’re trying to fetch! In other words, we want a self-less “static” method. Whether this code works or not, though, depends on which Python you use, and which way you call the method—through the class or through an instance. In 2.6 (and 2.X in general), calls to a self-less method function through both the class and instances fail (I’ve omitted some error text here for space): C:\misc> c:\python26\python >>> from spam import Spam >>> a = Spam() >>> b = Spam() >>> c = Spam() # Cannot call unbound class methods in 2.6 # Methods expect a self object by default >>> Spam.printNumInstances() TypeError: unbound method printNumInstances() must be called with Spam instance as first argument (got nothing instead) >>> a.printNumInstances() TypeError: printNumInstances() takes no arguments (1 given) The problem here is that unbound instance methods aren’t exactly the same as simple functions in 2.6. Even though there are no arguments in the def header, the method still expects an instance to be passed in when it’s called, because the function is associated with a class. In Python 3.0 (and later 3.X releases), calls to self-less methods made through classes work, but calls from instances fail: C:\misc> c:\python30\python >>> from spam import Spam >>> a = Spam() >>> b = Spam() >>> c = Spam() # Can call functions in class in 3.0 # Calls through instances still pass a self >>> Spam.printNumInstances() # Differs in 3.0 Number of instances created: 3 >>> a.printNumInstances() TypeError: printNumInstances() takes no arguments (1 given) That is, calls to instance-less methods like printNumInstances made through the class fail in Python 2.6 but work in Python 3.0. On the other hand, calls made through an instance fail in both Pythons, because an instance is automatically passed to a method that does not have an argument to receive it: Spam.printNumInstances() instance.printNumInstances() # Fails in 2.6, works in 3.0 # Fails in both 2.6 and 3.0 If you’re able to use 3.0 and stick with calling self-less methods through classes only, you already have a static method feature. However, to allow self-less methods to be Static and Class Methods | 797 called through classes in 2.6 and through instances in both 2.6 and 3.0, you need to either adopt other designs or be able to somehow mark such methods as special. Let’s look at both options in turn. Static Method Alternatives Short of marking a self-less method as special, there are a few different coding structures that can be tried. If you want to call functions that access class members without an instance, perhaps the simplest idea is to just make them simple functions outside the class, not class methods. This way, an instance isn’t expected in the call. For example, the following mutation of spam.py works the same in Python 3.0 and 2.6 (albeit displaying extra parentheses in 2.6 for its print statement): def printNumInstances(): print("Number of instances created: ", Spam.numInstances) class Spam: numInstances = 0 def __init__(self): Spam.numInstances = Spam.numInstances + 1 >>> import spam >>> a = spam.Spam() >>> b = spam.Spam() >>> c = spam.Spam() >>> spam.printNumInstances() Number of instances created: >>> spam.Spam.numInstances 3 3 # But function may be too far removed # And cannot be changed via inheritance Because the class name is accessible to the simple function as a global variable, this works fine. Also, note that the name of the function becomes global, but only to this single module; it will not clash with names in other files of the program. Prior to static methods in Python, this structure was the general prescription. Because Python already provides modules as a namespace-partitioning tool, one could argue that there’s not typically any need to package functions in classes unless they implement object behavior. Simple functions within modules like the one here do much of what instance-less class methods could, and are already associated with the class because they live in the same module. Unfortunately, this approach is still less than ideal. For one thing, it adds to this file’s scope an extra name that is used only for processing a single class. For another, the function is much less directly associated with the class; in fact, its definition could be hundreds of lines away. Perhaps worse, simple functions like this cannot be customized by inheritance, since they live outside a class’s namespace: subclasses cannot directly replace or extend such a function by redefining it. 798 | Chapter 31: Advanced Class Topics We might try to make this example work in a version-neutral way by using a normal method and always calling it through (or with) an instance, as usual: class Spam: numInstances = 0 def __init__(self): Spam.numInstances = Spam.numInstances + 1 def printNumInstances(self): print("Number of instances created: ", Spam.numInstances) >>> from spam import Spam >>> a, b, c = Spam(), Spam(), Spam() >>> a.printNumInstances() Number of instances created: 3 >>> Spam.printNumInstances(a) Number of instances created: 3 >>> Spam().printNumInstances() Number of instances created: 4 # But fetching counter changes counter! Unfortunately, as mentioned earlier, such an approach is completely unworkable if we don’t have an instance available, and making an instance changes the class data, as illustrated in the last line here. A better solution would be to somehow mark a method inside a class as never requiring an instance. The next section shows how. Using Static and Class Methods Today, there is another option for coding simple functions associated with a class that may be called through either the class or its instances. As of Python 2.2, we can code classes with static and class methods, neither of which requires an instance argument to be passed in when invoked. To designate such methods, classes call the built-in functions staticmethod and classmethod, as hinted in the earlier discussion of new-style classes. Both mark a function object as special—i.e., as requiring no instance if static and requiring a class argument if a class method. For example: class Methods: def imeth(self, x): print(self, x) # Normal instance method: passed a self def smeth(x): print(x) # Static: no instance passed def cmeth(cls, x): print(cls, x) # Class: gets class, not instance smeth = staticmethod(smeth) cmeth = classmethod(cmeth) # Make smeth a static method # Make cmeth a class method Notice how the last two assignments in this code simply reassign the method names smeth and cmeth. Attributes are created and changed by any assignment in a class statement, so these final assignments simply overwrite the assignments made earlier by the defs. Static and Class Methods | 799 Technically, Python now supports three kinds of class-related methods: instance, static, and class. Moreover, Python 3.0 extends this model by also allowing simple functions in a class to serve the role of static methods without extra protocol, when called through a class. Instance methods are the normal (and default) case that we’ve seen in this book. An instance method must always be called with an instance object. When you call it through an instance, Python passes the instance to the first (leftmost) argument automatically; when you call it through a class, you must pass along the instance manually (for simplicity, I’ve omitted some class imports in interactive sessions like this one): >>> obj = Methods() # Make an instance >>> obj.imeth(1) <__main__.Methods object...> 1 # Normal method, call through instance # Becomes imeth(obj, 1) >>> Methods.imeth(obj, 2) <__main__.Methods object...> 2 # Normal method, call through class # Instance passed explicitly By contrast, static methods are called without an instance argument. Unlike simple functions outside a class, their names are local to the scopes of the classes in which they are defined, and they may be looked up by inheritance. Instance-less functions can be called through a class normally in Python 3.0, but never by default in 2.6. Using the staticmethod built-in allows such methods to also be called through an instance in 3.0 and through both a class and an instance in Python 2.6 (the first of these works in 3.0 without staticmethod, but the second does not): >>> Methods.smeth(3) 3 # Static method, call through class # No instance passed or expected >>> obj.smeth(4) 4 # Static method, call through instance # Instance not passed Class methods are similar, but Python automatically passes the class (not an instance) in to a class method’s first (leftmost) argument, whether it is called through a class or an instance: >>> Methods.cmeth(5) <class '__main__.Methods'> 5 # Class method, call through class # Becomes cmeth(Methods, 5) >>> obj.cmeth(6) <class '__main__.Methods'> 6 # Class method, call through instance # Becomes cmeth(Methods, 6) Counting Instances with Static Methods Now, given these built-ins, here is the static method equivalent of this section’s instance-counting example—it marks the method as special, so it will never be passed an instance automatically: 800 | Chapter 31: Advanced Class Topics class Spam: numInstances = 0 # Use static method for class data def __init__(self): Spam.numInstances += 1 def printNumInstances(): print("Number of instances:", Spam.numInstances) printNumInstances = staticmethod(printNumInstances) Using the static method built-in, our code now allows the self-less method to be called through the class or any instance of it, in both Python 2.6 and 3.0: >>> a = Spam() >>> b = Spam() >>> c = Spam() >>> Spam.printNumInstances() Number of instances: 3 >>> a.printNumInstances() Number of instances: 3 # Call as simple function # Instance argument not passed Compared to simply moving printNumInstances outside the class, as prescribed earlier, this version requires an extra staticmethod call; however, it localizes the function name in the class scope (so it won’t clash with other names in the module), moves the function code closer to where it is used (inside the class statement), and allows subclasses to customize the static method with inheritance—a more convenient approach than importing functions from the files in which superclasses are coded. The following subclass and new testing session illustrate: class Sub(Spam): def printNumInstances(): # Override a static method print("Extra stuff...") # But call back to original Spam.printNumInstances() printNumInstances = staticmethod(printNumInstances) >>> a = Sub() >>> b = Sub() >>> a.printNumInstances() Extra stuff... Number of instances: 2 >>> Sub.printNumInstances() Extra stuff... Number of instances: 2 >>> Spam.printNumInstances() Number of instances: 2 # Call from subclass instance # Call from subclass itself Moreover, classes can inherit the static method without redefining it—it is run without an instance, regardless of where it is defined in a class tree: >>> class Other(Spam): pass # Inherit static method verbatim >>> c = Other() >>> c.printNumInstances() Number of instances: 3 Static and Class Methods | 801 Counting Instances with Class Methods Interestingly, a class method can do similar work here—the following has the same behavior as the static method version listed earlier, but it uses a class method that receives the instance’s class in its first argument. Rather than hardcoding the class name, the class method uses the automatically passed class object generically: class Spam: numInstances = 0 # Use class method instead of static def __init__(self): Spam.numInstances += 1 def printNumInstances(cls): print("Number of instances:", cls.numInstances) printNumInstances = classmethod(printNumInstances) This class is used in the same way as the prior versions, but its printNumInstances method receives the class, not the instance, when called from both the class and an instance: >>> a, b = Spam(), Spam() >>> a.printNumInstances() Number of instances: 2 >>> Spam.printNumInstances() Number of instances: 2 # Passes class to first argument # Also passes class to first argument When using class methods, though, keep in mind that they receive the most specific (i.e., lowest) class of the call’s subject. This has some subtle implications when trying to update class data through the passed-in class. For example, if in module test.py we subclass to customize as before, augment Spam.printNumInstances to also display its cls argument, and start a new testing session: class Spam: numInstances = 0 # Trace class passed in def __init__(self): Spam.numInstances += 1 def printNumInstances(cls): print("Number of instances:", cls.numInstances, cls) printNumInstances = classmethod(printNumInstances) class Sub(Spam): def printNumInstances(cls): # Override a class method print("Extra stuff...", cls) # But call back to original Spam.printNumInstances() printNumInstances = classmethod(printNumInstances) class Other(Spam): pass # Inherit class method verbatim the lowest class is passed in whenever a class method is run, even for subclasses that have no class methods of their own: >>> x, y = Sub(), Spam() >>> x.printNumInstances() Extra stuff... <class 'test.Sub'> Number of instances: 2 <class 'test.Spam'> 802 | Chapter 31: Advanced Class Topics # Call from subclass instance >>> Sub.printNumInstances() Extra stuff... <class 'test.Sub'> Number of instances: 2 <class 'test.Spam'> >>> y.printNumInstances() Number of instances: 2 <class 'test.Spam'> # Call from subclass itself In the first call here, a class method call is made through an instance of the Sub subclass, and Python passes the lowest class, Sub, to the class method. All is well in this case— since Sub’s redefinition of the method calls the Spam superclass’s version explicitly, the superclass method in Spam receives itself in its first argument. But watch what happens for an object that simply inherits the class method: >>> z = Other() >>> z.printNumInstances() Number of instances: 3 <class 'test.Other'> This last call here passes Other to Spam’s class method. This works in this example because fetching the counter finds it in Spam by inheritance. If this method tried to assign to the passed class’s data, though, it would update Object, not Spam! In this specific case, Spam is probably better off hardcoding its own class name to update its data, rather than relying on the passed-in class argument. Counting instances per class with class methods In fact, because class methods always receive the lowest class in an instance’s tree: • Static methods and explicit class names may be a better solution for processing data local to a class. • Class methods may be better suited to processing data that may differ for each class in a hierarchy. Code that needs to manage per-class instance counters, for example, might be best off leveraging class methods. In the following, the top-level superclass uses a class method to manage state information that varies for and is stored on each class in the tree— similar in spirit to the way instance methods manage state information in class instances: class Spam: numInstances = 0 def count(cls): cls.numInstances += 1 def __init__(self): self.count() count = classmethod(count) class Sub(Spam): numInstances = 0 def __init__(self): Spam.__init__(self) class Other(Spam): numInstances = 0 # Per-class instance counters # cls is lowest class above instance # Passes self.__class__ to count # Redefines __init__ # Inherits __init__ Static and Class Methods | 803 >>> >>> >>> >>> (1, >>> (1, x = Spam() y1, y2 = Sub(), Sub() z1, z2, z3 = Other(), Other(), Other() x.numInstances, y1.numInstances, z1.numInstances 2, 3) Spam.numInstances, Sub.numInstances, Other.numInstances 2, 3) Static and class methods have additional advanced roles, which we will finesse here; see other resources for more use cases. In recent Python versions, though, the static and class method designations have become even simpler with the advent of function decoration syntax—a way to apply one function to another that has roles well beyond the static method use case that was its motivation. This syntax also allows us to augment classes in Python 2.6 and 3.0—to initialize data like the numInstances counter in the last example, for instance. The next section explains how. Decorators and Metaclasses: Part 1 Because the staticmethod call technique described in the prior section initially seemed obscure to some users, a feature was eventually added to make the operation simpler. Function decorators provide a way to specify special operation modes for functions, by wrapping them in an extra layer of logic implemented as another function. Function decorators turn out to be general tools: they are useful for adding many types of logic to functions besides the static method use case. For instance, they may be used to augment functions with code that logs calls made to them, checks the types of passed arguments during debugging, and so on. In some ways, function decorators are similar to the delegation design pattern we explored in Chapter 30, but they are designed to augment a specific function or method call, not an entire object interface. Python provides some built-in function decorators for operations such as marking static methods, but programmers can also code arbitrary decorators of their own. Although they are not strictly tied to classes, user-defined function decorators often are coded as classes to save the original functions, along with other data, as state information. There’s also a more recent related extension available in Python 2.6 and 3.0: class decorators are directly tied to the class model, and their roles overlap with metaclasses. Function Decorator Basics Syntactically, a function decorator is a sort of runtime declaration about the function that follows. A function decorator is coded on a line by itself just before the def statement that defines a function or method. It consists of the @ symbol, followed by what we call a metafunction—a function (or other callable object) that manages another function. Static methods today, for example, may be coded with decorator syntax like this: 804 | Chapter 31: Advanced Class Topics class C: @staticmethod def meth(): ... # Decoration syntax Internally, this syntax has the same effect as the following (passing the function through the decorator and assigning the result back to the original name): class C: def meth(): ... meth = staticmethod(meth) # Rebind name Decoration rebinds the method name to the decorator’s result. The net effect is that calling the method function’s name later actually triggers the result of its staticmethod decorator first. Because a decorator can return any sort of object, this allows the decorator to insert a layer of logic to be run on every call. The decorator function is free to return either the original function itself, or a new object that saves the original function passed to the decorator to be invoked indirectly after the extra logic layer runs. With this addition, here’s a better way to code our static method example from the prior section in either Python 2.6 or 3.0 (the classmethod decorator is used the same way): class Spam: numInstances = 0 def __init__(self): Spam.numInstances = Spam.numInstances + 1 @staticmethod def printNumInstances(): print("Number of instances created: ", Spam.numInstances) a = Spam() b = Spam() c = Spam() Spam.printNumInstances() a.printNumInstances() # Calls from both classes and instances work now! # Both print "Number of instances created: 3" Keep in mind that staticmethod is still a built-in function; it may be used in decoration syntax, just because it takes a function as argument and returns a callable. In fact, any such function can be used in this way—even user-defined functions we code ourselves, as the next section explains. A First Function Decorator Example Although Python provides a handful of built-in functions that can be used as decorators, we can also write custom decorators of our own. Because of their wide utility, we’re going to devote an entire chapter to coding decorators in the next part of this book. As a quick example, though, let’s look at a simple user-defined decorator at work. Decorators and Metaclasses: Part 1 | 805 Recall from Chapter 29 that the __call__ operator overloading method implements a function-call interface for class instances. The following code uses this to define a class that saves the decorated function in the instance and catches calls to the original name. Because this is a class, it also has state information (a counter of calls made): class tracer: def __init__(self, func): self.calls = 0 self.func = func def __call__(self, *args): self.calls += 1 print('call %s to %s' % (self.calls, self.func.__name__)) self.func(*args) @tracer def spam(a, b, c): print(a, b, c) # Same as spam = tracer(spam) # Wrap spam in a decorator object spam(1, 2, 3) spam('a', 'b', 'c') spam(4, 5, 6) # Really calls the tracer wrapper object # Invokes __call__ in class # __call__ adds logic and runs original object Because the spam function is run through the tracer decorator, when the original spam name is called it actually triggers the __call__ method in the class. This method counts and logs the call, and then dispatches it to the original wrapped function. Note how the *name argument syntax is used to pack and unpack the passed-in arguments; because of this, this decorator can be used to wrap any function with any number of positional arguments. The net effect, again, is to add a layer of logic to the original spam function. Here is the script’s output—the first line comes from the tracer class, and the second comes from the spam function: call 1 to spam 1 2 3 call 2 to spam a b c call 3 to spam 4 5 6 Trace through this example’s code for more insight. As it is, this decorator works for any function that takes positional arguments, but it does not return the decorated function’s result, doesn’t handle keyword arguments, and cannot decorate class method functions (in short, for methods its __call__ would be passed a tracer instance only). As we’ll see in Part VIII, there are a variety of ways to code function decorators, including nested def statements; some of the alternatives are better suited to methods than the version shown here. 806 | Chapter 31: Advanced Class Topics Class Decorators and Metaclasses Function decorators turned out to be so useful that Python 2.6 and 3.0 expanded the model, allowing decorators to be applied to classes as well as functions. In short, class decorators are similar to function decorators, but they are run at the end of a class statement to rebind a class name to a callable. As such, they can be used to either manage classes just after they are created, or insert a layer of wrapper logic to manage instances when they are later created. Symbolically, the code structure: def decorator(aClass): ... @decorator class C: ... is mapped to the following equivalent: def decorator(aClass): ... class C: ... C = decorator(C) The class decorator is free to augment the class itself, or return an object that intercepts later instance construction calls. For instance, in the example in the section “Counting instances per class with class methods” on page 803, we could use this hook to automatically augment the classes with instance counters and any other data required: def count(aClass): aClass.numInstances = 0 return aClass # Return class itself, instead of a wrapper @count class Spam: ... # Same as Spam = count(Spam) @count class Sub(Spam): ... # numInstances = 0 not needed here @count class Other(Spam): ... Metaclasses are a similarly advanced class-based tool whose roles often intersect with those of class decorators. They provide an alternate model, which routes the creation of a class object to a subclass of the top-level type class, at the conclusion of a class statement: class Meta(type): def __new__(meta, classname, supers, classdict): ... class C(metaclass=Meta): ... Decorators and Metaclasses: Part 1 | 807 In Python 2.6, the effect is the same, but the coding differs—use a class attribute instead of a keyword argument in the class header: class C: __metaclass__ = Meta ... The metaclass generally redefines the __new__ or __init__ method of the type class, in order to assume control of the creation or initialization of a new class object. The net effect, as with class decorators, is to define code to be run automatically at class creation time. Both schemes are free to augment a class or return an arbitrary object to replace it—a protocol with almost limitless class-based possibilities. For More Details Naturally, there’s much more to the decorator and metaclass stories than I’ve shown here. Although they are a general mechanism, decorators and metaclasses are advanced features of interest primarily to tool writers, not application programmers, so we’ll defer additional coverage until the final part of this book: • Chapter 37 shows how to code properties using function decorator syntax. • Chapter 38 has much more on decorators, including more comprehensive examples. • Chapter 39 covers metaclasses, and more on the class and instance management story. Although these chapters cover advanced topics, they’ll also provide us with a chance to see Python at work in more substantial examples than much of the rest of the book was able to provide. Class Gotchas Most class issues can be boiled down to namespace issues (which makes sense, given that classes are just namespaces with a few extra tricks). Some of the topics we’ll cover in this section are more like case studies of advanced class usage than real problems, and one or two of these gotchas have been eased by recent Python releases. Changing Class Attributes Can Have Side Effects Theoretically speaking, classes (and class instances) are mutable objects. Like built-in lists and dictionaries, they can be changed in-place by assigning to their attributes— and as with lists and dictionaries, this means that changing a class or instance object may impact multiple references to it. 808 | Chapter 31: Advanced Class Topics That’s usually what we want (and is how objects change their state in general), but awareness of this issue becomes especially critical when changing class attributes. Because all instances generated from a class share the class’s namespace, any changes at the class level are reflected in all instances, unless they have their own versions of the changed class attributes. Because classes, modules, and instances are all just objects with attribute namespaces, you can normally change their attributes at runtime by assignments. Consider the following class. Inside the class body, the assignment to the name a generates an attribute X.a, which lives in the class object at runtime and will be inherited by all of X’s instances: >>> ... ... >>> >>> 1 >>> 1 class X: a = 1 # Class attribute I = X() I.a # Inherited by instance X.a So far, so good—this is the normal case. But notice what happens when we change the class attribute dynamically outside the class statement: it also changes the attribute in every object that inherits from the class. Moreover, new instances created from the class during this session or program run also get the dynamically set value, regardless of what the class’s source code says: >>> >>> 2 >>> >>> 2 X.a = 2 I.a # May change more than X # I changes too J = X() J.a # J inherits from X's runtime values # (but assigning to J.a changes a in J, not X or I) Is this a useful feature or a dangerous trap? You be the judge. As we learned in Chapter 26, you can actually get work done by changing class attributes without ever making a single instance; this technique can simulate the use of “records” or “structs” in other languages. As a refresher, consider the following unusual but legal Python program: class X: pass class Y: pass # Make a few attribute namespaces X.a X.b X.c Y.a # Use class attributes as variables # No instances anywhere to be found = = = = 1 2 3 X.a + X.b + X.c for X.i in range(Y.a): print(X.i) # Prints 0..5 Here, the classes X and Y work like “fileless” modules—namespaces for storing variables we don’t want to clash. This is a perfectly legal Python programming trick, but it’s less appropriate when applied to classes written by others; you can’t always be sure that class attributes you change aren’t critical to the class’s internal behavior. If you’re out Class Gotchas | 809 to simulate a C struct, you may be better off changing instances than classes, as that way only one object is affected: class Record: pass X = Record() X.name = 'bob' X.job = 'Pizza maker' Changing Mutable Class Attributes Can Have Side Effects, Too This gotcha is really an extension of the prior. Because class attributes are shared by all instances, if a class attribute references a mutable object, changing that object in-place from any instance impacts all instances at once: >>> class C: ... shared = [] ... def __init__(self): ... self.perobj = [] ... >>> x = C() >>> y = C() >>> y.shared, y.perobj ([], []) # Class attribute # Instance attribute # Two instances # Implicitly share class attrs >>> x.shared.append('spam') >>> x.perobj.append('spam') >>> x.shared, x.perobj (['spam'], ['spam']) # Impacts y's view too! # Impacts x's data only >>> y.shared, y.perobj (['spam'], []) >>> C.shared ['spam'] # y sees change made through x # Stored on class and shared This effect is no different than many we’ve seen in this book already: mutable objects are shared by simple variables, globals are shared by functions, module-level objects are shared by multiple importers, and mutable function arguments are shared by the caller and the callee. All of these are cases of general behavior—multiple references to a mutable object—and all are impacted if the shared object is changed in-place from any reference. Here, this occurs in class attributes shared by all instances via inheritance, but it’s the same phenomenon at work. It may be made more subtle by the different behavior of assignments to instance attributes themselves: x.shared.append('spam') x.shared = 'spam' # Changes shared object attached to class in-place # Changed or creates instance attribute attached to x but again, this is not a problem, it’s just something to be aware of; shared mutable class attributes can have many valid uses in Python programs. 810 | Chapter 31: Advanced Class Topics Multiple Inheritance: Order Matters This may be obvious by now, but it’s worth underscoring: if you use multiple inheritance, the order in which superclasses are listed in the class statement header can be critical. Python always searches superclasses from left to right, according to their order in the header line. For instance, in the multiple inheritance example we studied in Chapter 30, suppose that the Super class implemented a __str__ method, too: class ListTree: def __str__(self): ... class Super: def __str__(self): ... class Sub(ListTree, Super): # Get ListTree's __str__ by listing it first x = Sub() # Inheritance searches ListTree before Super Which class would we inherit it from—ListTree or Super? As inheritance searches proceed from left to right, we would get the method from whichever class is listed first (leftmost) in Sub’s class header. Presumably, we would list ListTree first because its whole purpose is its custom __str__ (indeed, we had to do this in Chapter 30 when mixing this class with a tkinter.Button that had a __str__ of its own). But now suppose Super and ListTree have their own versions of other same-named attributes, too. If we want one name from Super and another from ListTree, the order in which we list them in the class header won’t help—we will have to override inheritance by manually assigning to the attribute name in the Sub class: class ListTree: def __str__(self): ... def other(self): ... class Super: def __str__(self): ... def other(self): ... class Sub(ListTree, Super): other = Super.other def __init__(self): ... # Get ListTree's __str__ by listing it first # But explicitly pick Super's version of other x = Sub() # Inheritance searches Sub before ListTree/Super Here, the assignment to other within the Sub class creates Sub.other—a reference back to the Super.other object. Because it is lower in the tree, Sub.other effectively hides ListTree.other, the attribute that the inheritance search would normally find. Similarly, if we listed Super first in the class header to pick up its other, we would need to select ListTree’s method explicitly: Class Gotchas | 811 # Get Super's other by order # Explicitly pick Lister.__str__ class Sub(Super, ListTree): __str__ = Lister.__str__ Multiple inheritance is an advanced tool. Even if you understood the last paragraph, it’s still a good idea to use it sparingly and carefully. Otherwise, the meaning of a name may come to depend on the order in which classes are mixed in an arbitrarily far-removed subclass. (For another example of the technique shown here in action, see the discussion of explicit conflict resolution in “The “New-Style” Class Model” on page 777.) As a rule of thumb, multiple inheritance works best when your mix-in classes are as self-contained as possible—because they may be used in a variety of contexts, they should not make assumptions about names related to other classes in a tree. The pseudoprivate __X attributes feature we studied in Chapter 30 can help by localizing names that a class relies on owning and limiting the names that your mix-in classes add to the mix. In this example, for instance, if ListTree only means to export its custom __str__, it can name its other method __other to avoid clashing with like-named classes in the tree. Methods, Classes, and Nested Scopes This gotcha went away in Python 2.2 with the introduction of nested function scopes, but I’ve retained it here for historical perspective, for readers working with older Python releases, and because it demonstrates what happens to the new nested function scope rules when one layer of the nesting is a class. Classes introduce local scopes, just as functions do, so the same sorts of scope behavior can happen in a class statement body. Moreover, methods are further nested functions, so the same issues apply. Confusion seems to be especially common when classes are nested. In the following example (the file nester.py), the generate function returns an instance of the nested Spam class. Within its code, the class name Spam is assigned in the generate function’s local scope. However, in versions of Python prior to 2.2, within the class’s method function the class name Spam is not visible—method has access only to its own local scope, the module surrounding generate, and built-in names: def generate(): class Spam: count = 1 def method(self): print(Spam.count) return Spam() # Fails prior to Python 2.2, works later # Name Spam not visible: # not local (def), global (module), built-in generate().method() C:\python\examples> python nester.py ...error text omitted... 812 | Chapter 31: Advanced Class Topics Print(Spam.count) NameError: Spam # Not local (def), global (module), built-in This example works in Python 2.2 and later because the local scopes of all enclosing function defs are automatically visible to nested defs (including nested method defs, as in this example). However, it doesn’t work before 2.2 (we’ll look at some possible solutions momentarily). Note that even in 2.2 and later, method defs cannot see the local scope of the enclosing class; they can only see the local scopes of enclosing defs. That’s why methods must go through the self instance or the class name to reference methods and other attributes defined in the enclosing class statement. For example, code in the method must use self.count or Spam.count, not just count. If you’re using a release prior to 2.2, there are a variety of ways to get the preceding example to work. One of the simplest is to move the name Spam out to the enclosing module’s scope with a global declaration. Because method sees global names in the enclosing module, references to Spam will work: def generate(): global Spam class Spam: count = 1 def method(self): print(Spam.count) return Spam() generate().method() # Force Spam to module scope # Works: in global (enclosing module) # Prints 1 A better alternative would be to restructure the code such that the class Spam is defined at the top level of the module by virtue of its nesting level, rather than using global declarations. The nested method function and the top-level generate will then find Spam in their global scopes: def generate(): return Spam() class Spam: count = 1 def method(self): print(Spam.count) # Define at top level of module # Works: in global (enclosing module) generate().method() In fact, this approach is recommended for all Python releases—code tends to be simpler in general if you avoid nesting classes and functions. If you want to get complicated and tricky, you can also get rid of the Spam reference in method altogether by using the special __class__ attribute, which returns an instance’s class object: def generate(): class Spam: Class Gotchas | 813 count = 1 def method(self): print(self.__class__.count) return Spam() # Works: qualify to get class generate().method() Delegation-Based Classes in 3.0: __getattr__ and built-ins We met this issue briefly in our class tutorial in Chapter 27 and our delegation coverage in Chapter 30: classes that use the __getattr__ operator overloading method to delegate attribute fetches to wrapped objects will fail in Python 3.0 unless operator overloading methods are redefined in the wrapper class. In Python 3.0 (and 2.6, when new-style classes are used), the names of operator overloading methods implicitly fetched by built-in operations are not routed through generic attribute-interception methods. The __str__ method used by printing, for example, never invokes __getattr__. Instead, Python 3.0 looks up such names in classes and skips the normal runtime instance lookup mechanism entirely. To work around this, such methods must be redefined in wrapper classes, either by hand, with tools, or by definition in superclasses. We’ll revisit this gotcha in Chapters 37 and 38. “Overwrapping-itis” When used well, the code reuse features of OOP make it excel at cutting development time. Sometimes, though, OOP’s abstraction potential can be abused to the point of making code difficult to understand. If classes are layered too deeply, code can become obscure; you may have to search through many classes to discover what an operation does. For example, I once worked in a C++ shop with thousands of classes (some machinegenerated), and up to 15 levels of inheritance. Deciphering method calls in such a complex system was often a monumental task: multiple classes had to be consulted for even the most basic of operations. In fact, the logic of the system was so deeply wrapped that understanding a piece of code in some cases required days of wading through related files. The most general rule of thumb of Python programming applies here, too: don’t make things complicated unless they truly must be. Wrapping your code in multiple layers of classes to the point of incomprehensibility is always a bad idea. Abstraction is the basis of polymorphism and encapsulation, and it can be a very effective tool when used well. However, you’ll simplify debugging and aid maintainability if you make your class interfaces intuitive, avoid making your code overly abstract, and keep your class hierarchies short and flat unless there is a good reason to do otherwise. 814 | Chapter 31: Advanced Class Topics Chapter Summary This chapter presented a handful of advanced class-related topics, including subclassing built-in types, new-style classes, static methods, and decorators. Most of these are optional extensions to the OOP model in Python, but they may become more useful as you start writing larger object-oriented programs. As mentioned earlier, our discussion of some of the more advanced class tools continues in the final part of this book; be sure to look ahead if you need more details on properties, descriptors, decorators, and metaclasses. This is the end of the class part of this book, so you’ll find the usual lab exercises at the end of the chapter—be sure to work through them to get some practice coding real classes. In the next chapter, we’ll begin our look at our last core language topic, exceptions. Exceptions are Python’s mechanism for communicating errors and other conditions to your code. This is a relatively lightweight topic, but I’ve saved it for last because exceptions are supposed to be coded as classes today. Before we tackle that final core subject, though, take a look at this chapter’s quiz and the lab exercises. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. 6. Name two ways to extend a built-in object type. What are function decorators used for? How do you code a new-style class? How are new-style and classic classes different? How are normal and static methods different? How long should you wait before lobbing a “Holy Hand Grenade”? Test Your Knowledge: Answers 1. You can embed a built-in object in a wrapper class, or subclass the built-in type directly. The latter approach tends to be simpler, as most original behavior is automatically inherited. 2. Function decorators are generally used to add to an existing function a layer of logic that is run each time the function is called. They can be used to log or count calls to a function, check its argument types, and so on. They are also used to “declare” static methods—simple functions in a class that are not passed an instance when called. Test Your Knowledge: Answers | 815 3. New-style classes are coded by inheriting from the object built-in class (or any other built-in type). In Python 3.0, all classes are new-style automatically, so this derivation is not required; in 2.6, classes with this derivation are new-style and those without it are “classic.” 4. New-style classes search the diamond pattern of multiple inheritance trees differently—they essentially search breadth-first (across), instead of depth-first (up). New-style classes also change the result of the type built-in for instances and classes, do not run generic attribute fetch methods such as __getattr__ for builtin operation methods, and support a set of advanced extra tools including properties, descriptors, and __slots__ instance attribute lists. 5. Normal (instance) methods receive a self argument (the implied instance), but static methods do not. Static methods are simple functions nested in class objects. To make a method static, it must either be run through a special built-in function or be decorated with decorator syntax. Python 3.0 allows simple functions in a class to be called through the class without this step, but calls through instances still require static method declaration. 6. Three seconds. (Or, more accurately: “And the Lord spake, saying, ‘First shalt thou take out the Holy Pin. Then, shalt thou count to three, no more, no less. Three shalt be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, nor either count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in my sight, shall snuff it.’”)* Test Your Knowledge: Part VI Exercises These exercises ask you to write a few classes and experiment with some existing code. Of course, the problem with existing code is that it must be existing. To work with the set class in exercise 5, either pull the class source code off this book’s website (see the Preface for a pointer) or type it up by hand (it’s fairly brief). These programs are starting to get more sophisticated, so be sure to check the solutions at the end of the book for pointers. You’ll find them in Appendix B, under “Part VI, Classes and OOP” on page 1122. 1. Inheritance. Write a class called Adder that exports a method add(self, x, y) that prints a “Not Implemented” message. Then, define two subclasses of Adder that implement the add method: ListAdder With an add method that returns the concatenation of its two list arguments * This quote is from Monty Python and the Holy Grail. 816 | Chapter 31: Advanced Class Topics DictAdder With an add method that returns a new dictionary containing the items in both its two dictionary arguments (any definition of addition will do) Experiment by making instances of all three of your classes interactively and calling their add methods. Now, extend your Adder superclass to save an object in the instance with a constructor (e.g., assign self.data a list or a dictionary), and overload the + operator with an __add__ method to automatically dispatch to your add methods (e.g., X + Y triggers X.add(X.data,Y)). Where is the best place to put the constructors and operator overloading methods (i.e., in which classes)? What sorts of objects can you add to your class instances? In practice, you might find it easier to code your add methods to accept just one real argument (e.g., add(self,y)), and add that one argument to the instance’s current data (e.g., self.data + y). Does this make more sense than passing two arguments to add? Would you say this makes your classes more “object-oriented”? 2. Operator overloading. Write a class called Mylist that shadows (“wraps”) a Python list: it should overload most list operators and operations, including +, indexing, iteration, slicing, and list methods such as append and sort. See the Python reference manual for a list of all possible methods to support. Also, provide a constructor for your class that takes an existing list (or a Mylist instance) and copies its components into an instance member. Experiment with your class interactively. Things to explore: a. Why is copying the initial value important here? b. Can you use an empty slice (e.g., start[:]) to copy the initial value if it’s a Mylist instance? c. Is there a general way to route list method calls to the wrapped list? d. Can you add a Mylist and a regular list? How about a list and a Mylist instance? e. What type of object should operations like + and slicing return? What about indexing operations? f. If you are working with a more recent Python release (version 2.2 or later), you may implement this sort of wrapper class by embedding a real list in a standalone class, or by extending the built-in list type with a subclass. Which is easier, and why? 3. Subclassing. Make a subclass of Mylist from exercise 2 called MylistSub, which extends Mylist to print a message to stdout before each overloaded operation is called and counts the number of calls. MylistSub should inherit basic method behavior from Mylist. Adding a sequence to a MylistSub should print a message, increment the counter for + calls, and perform the superclass’s method. Also, introduce a new method that prints the operation counters to stdout, and experiment with your class interactively. Do your counters count calls per instance, or per class (for all instances of the class)? How would you program the other option)? Test Your Knowledge: Part VI Exercises | 817 (Hint: it depends on which object the count members are assigned to: class members are shared by instances, but self members are per-instance data.) 4. Metaclass methods. Write a class called Meta with methods that intercept every attribute qualification (both fetches and assignments), and print messages listing their arguments to stdout. Create a Meta instance, and experiment with qualifying it interactively. What happens when you try to use the instance in expressions? Try adding, indexing, and slicing the instance of your class. (Note: a fully generic approach based upon __getattr__ will work in 2.6 but not 3.0, for reasons noted in Chapter 30 and restated in the solution to this exercise.) 5. Set objects. Experiment with the set class described in “Extending Types by Embedding” on page 774. Run commands to do the following sorts of operations: a. Create two sets of integers, and compute their intersection and union by using & and | operator expressions. b. Create a set from a string, and experiment with indexing your set. Which methods in the class are called? c. Try iterating through the items in your string set using a for loop. Which methods run this time? d. Try computing the intersection and union of your string set and a simple Python string. Does it work? e. Now, extend your set by subclassing to handle arbitrarily many operands using the *args argument form. (Hint: see the function versions of these algorithms in Chapter 18.) Compute intersections and unions of multiple operands with your set subclass. How can you intersect three or more sets, given that & has only two sides? f. How would you go about emulating other list operations in the set class? (Hint: __add__ can catch concatenation, and __getattr__ can pass most list method calls to the wrapped list.) 6. Class tree links. In “Namespaces: The Whole Story” on page 693 in Chapter 28 and in “Multiple Inheritance: “Mix-in” Classes” on page 756 in Chapter 30, I mentioned that classes have a __bases__ attribute that returns a tuple of their superclass objects (the ones listed in parentheses in the class header). Use __bases__ to extend the lister.py mix-in classes we wrote in Chapter 30 so that they print the names of the immediate superclasses of the instance’s class. When you’re done, the first line of the string representation should look like this (your address may vary): <Instance of Sub(Super, Lister), address 7841200: 7. Composition. Simulate a fast-food ordering scenario by defining four classes: Lunch A container and controller class 818 | Chapter 31: Advanced Class Topics Customer The actor who buys food Employee The actor from whom a customer orders Food What the customer buys To get you started, here are the classes and methods you’ll be defining: class Lunch: def __init__(self) def order(self, foodName) def result(self) # Make/embed Customer and Employee # Start a Customer order simulation # Ask the Customer what Food it has class Customer: def __init__(self) # Initialize my food to None def placeOrder(self, foodName, employee) # Place order with an Employee def printFood(self) # Print the name of my food class Employee: def takeOrder(self, foodName) # Return a Food, with requested name class Food: def __init__(self, name) # Store food name The order simulation should work as follows: a. The Lunch class’s constructor should make and embed an instance of Customer and an instance of Employee, and it should export a method called order. When called, this order method should ask the Customer to place an order by calling its placeOrder method. The Customer’s placeOrder method should in turn ask the Employee object for a new Food object by calling Employee’s takeOrder method. b. Food objects should store a food name string (e.g., “burritos”), passed down from Lunch.order, to Customer.placeOrder, to Employee.takeOrder, and finally to Food’s constructor. The top-level Lunch class should also export a method called result, which asks the customer to print the name of the food it received from the Employee via the order (this can be used to test your simulation). Note that Lunch needs to pass either the Employee or itself to the Customer to allow the Customer to call Employee methods. Experiment with your classes interactively by importing the Lunch class, calling its order method to run an interaction, and then calling its result method to verify that the Customer got what he or she ordered. If you prefer, you can also simply code test cases as self-test code in the file where your classes are defined, using the module __name__ trick of Chapter 24. In this simulation, the Customer is the active agent; how would your classes change if Employee were the object that initiated customer/employee interaction instead? Test Your Knowledge: Part VI Exercises | 819 Figure 31-1. A zoo hierarchy composed of classes linked into a tree to be searched by attribute inheritance. Animal has a common “reply” method, but each class may have its own custom “speak” method called by “reply”. 3. Zoo animal hierarchy. Consider the class tree shown in Figure 31-1. Code a set of six class statements to model this taxonomy with Python inheritance. Then, add a speak method to each of your classes that prints a unique message, and a reply method in your top-level Animal superclass that simply calls self.speak to invoke the category-specific message printer in a subclass below (this will kick off an independent inheritance search from self). Finally, remove the speak method from your Hacker class so that it picks up the default above it. When you’re finished, your classes should work this way: % python >>> from zoo import Cat, Hacker >>> spot = Cat() >>> spot.reply() meow >>> data = Hacker() >>> data.reply() Hello world! # Animal.reply; calls Cat.speak # Animal.reply; calls Primate.speak 4. The Dead Parrot Sketch. Consider the object embedding structure captured in Figure 31-2. Code a set of Python classes to implement this structure with composition. Code your Scene object to define an action method, and embed instances of the Customer, Clerk, and Parrot classes (each of which should define a line method that prints a unique message). The embedded objects may either inherit from a common superclass that defines line and simply provide message text, or define line themselves. In the end, your classes should operate like this: % python >>> import parrot >>> parrot.Scene().action() customer: "that's one ex-bird!" 820 | Chapter 31: Advanced Class Topics # Activate nested objects clerk: "no it isn't..." parrot: None Figure 31-2. A scene composite with a controller class (Scene) that embeds and directs instances of three other classes (Customer, Clerk, Parrot). The embedded instance’s classes may also participate in an inheritance hierarchy; composition and inheritance are often equally useful ways to structure classes for code reuse. Why You Will Care: OOP by the Masters When I teach Python classes, I invariably find that about halfway through the class, people who have used OOP in the past are following along intensely, while people who have not are beginning to glaze over (or nod off completely). The point behind the technology just isn’t apparent. In a book like this, I have the luxury of including material like the new Big Picture overview in Chapter 25, and the gradual tutorial of Chapter 27—in fact, you should probably review that section if you’re starting to feel like OOP is just some computer science mumbo-jumbo. In real classes, however, to help get the newcomers on board (and keep them awake), I have been known to stop and ask the experts in the audience why they use OOP. The answers they’ve given might help shed some light on the purpose of OOP, if you’re new to the subject. Here, then, with only a few embellishments, are the most common reasons to use OOP, as cited by my students over the years: Code reuse This one’s easy (and is the main reason for using OOP). By supporting inheritance, classes allow you to program by customization instead of starting each project from scratch. Encapsulation Wrapping up implementation details behind object interfaces insulates users of a class from code changes. Structure Classes provide new local scopes, which minimizes name clashes. They also provide a natural place to write and look for implementation code, and to manage object state. Test Your Knowledge: Part VI Exercises | 821 Maintenance Classes naturally promote code factoring, which allows us to minimize redundancy. Thanks both to the structure and code reuse support of classes, usually only one copy of the code needs to be changed. Consistency Classes and inheritance allow you to implement common interfaces, and hence create a common look and feel in your code; this eases debugging, comprehension, and maintenance. Polymorphism This is more a property of OOP than a reason for using it, but by supporting code generality, polymorphism makes code more flexible and widely applicable, and hence more reusable. Other And, of course, the number one reason students gave for using OOP: it looks good on a résumé! (OK, I threw this one in as a joke, but it is important to be familiar with OOP if you plan to work in the software field today.) Finally, keep in mind what I said at the beginning of this part of the book: you won’t fully appreciate OOP until you’ve used it for awhile. Pick a project, study larger examples, work through the exercises—do whatever it takes to get your feet wet with OO code; it’s worth the effort. 822 | Chapter 31: Advanced Class Topics PART VII Exceptions and Tools CHAPTER 32 Exception Basics This part of the book deals with exceptions, which are events that can modify the flow of control through a program. In Python, exceptions are triggered automatically on errors, and they can be triggered and intercepted by your code. They are processed by four statements we’ll study in this part, the first of which has two variations (listed separately here) and the last of which was an optional extension until Python 2.6 and 3.0: try/except Catch and recover from exceptions raised by Python, or by you. try/finally Perform cleanup actions, whether exceptions occur or not. raise Trigger an exception manually in your code. assert Conditionally trigger an exception in your code. with/as Implement context managers in Python 2.6 and 3.0 (optional in 2.5). This topic was saved until nearly the end of the book because you need to know about classes to code exceptions of your own. With a few exceptions (pun intended), though, you’ll find that exception handling is simple in Python because it’s integrated into the language itself as another high-level tool. Why Use Exceptions? In a nutshell, exceptions let us jump out of arbitrarily large chunks of a program. Consider the hypothetical pizza-making robot we discussed earlier in the book. Suppose we took the idea seriously and actually built such a machine. To make a pizza, our culinary automaton would need to execute a plan, which we would implement as a 825 Python program: it would take an order, prepare the dough, add toppings, bake the pie, and so on. Now, suppose that something goes very wrong during the “bake the pie” step. Perhaps the oven is broken, or perhaps our robot miscalculates its reach and spontaneously combusts. Clearly, we want to be able to jump to code that handles such states quickly. As we have no hope of finishing the pizza task in such unusual cases, we might as well abandon the entire plan. That’s exactly what exceptions let you do: you can jump to an exception handler in a single step, abandoning all function calls begun since the exception handler was entered. Code in the exception handler can then respond to the raised exception as appropriate (by calling the fire department, for instance!). One way to think of an exception is as a sort of structured “super go to.” An exception handler ( try statement) leaves a marker and executes some code. Somewhere further ahead in the program, an exception is raised that makes Python jump back to that marker, abandoning any active functions that were called after the marker was left. This protocol provides a coherent way to respond to unusual events. Moreover, because Python jumps to the handler statement immediately, your code is simpler—there is usually no need to check status codes after every call to a function that could possibly fail. Exception Roles In Python programs, exceptions are typically used for a variety of purposes. Here are some of their most common roles: Error handling Python raises exceptions whenever it detects errors in programs at runtime. You can catch and respond to the errors in your code, or ignore the exceptions that are raised. If an error is ignored, Python’s default exception-handling behavior kicks in: it stops the program and prints an error message. If you don’t want this default behavior, code a try statement to catch and recover from the exception—Python will jump to your try handler when the error is detected, and your program will resume execution after the try. Event notification Exceptions can also be used to signal valid conditions without you having to pass result flags around a program or test them explicitly. For instance, a search routine might raise an exception on failure, rather than returning an integer result code (and hoping that the code will never be a valid result). Special-case handling Sometimes a condition may occur so rarely that it’s hard to justify convoluting your code to handle it. You can often eliminate special-case code by handling unusual cases in exception handlers in higher levels of your program. 826 | Chapter 32: Exception Basics Termination actions As you’ll see, the try/finally statement allows you to guarantee that required closing-time operations will be performed, regardless of the presence or absence of exceptions in your programs. Unusual control flows Finally, because exceptions are a sort of high-level “go to,” you can use them as the basis for implementing exotic control flows. For instance, although the language does not explicitly support backtracking, it can be implemented in Python by using exceptions and a bit of support logic to unwind assignments.* There is no “go to” statement in Python (thankfully!), but exceptions can sometimes serve similar roles. We’ll see such typical use cases in action later in this part of the book. For now, let’s get started with a look at Python’s exception-processing tools. Exceptions: The Short Story Compared to some other core language topics we’ve met in this book, exceptions are a fairly lightweight tool in Python. Because they are so simple, let’s jump right into some code. Default Exception Handler Suppose we write the following function: >>> def fetcher(obj, index): ... return obj[index] ... There’s not much to this function—it simply indexes an object on a passed-in index. In normal operation, it returns the result of a legal index: >>> x = 'spam' >>> fetcher(x, 3) 'm' # Like x[3] However, if we ask this function to index off the end of the string, an exception will be triggered when the function tries to run obj[index]. Python detects out-of-bounds indexing for sequences and reports it by raising (triggering) the built-in IndexError exception: * True backtracking is an advanced topic that is not part of the Python language, so I won’t say much more about it here (even the generator functions and expressions we met in Chapter 20 are not true backtracking— they simply respond to next(G) requests). Roughly, backtracking undoes all computations before it jumps; Python exceptions do not (i.e., variables assigned between the time a try statement is entered and the time an exception is raised are not reset to their prior values). See a book on artificial intelligence or the Prolog or Icon programming languages if you’re curious. Exceptions: The Short Story | 827 >>> fetcher(x, 4) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in fetcher IndexError: string index out of range # Default handler - shell interface Because our code does not explicitly catch this exception, it filters back up to the top level of the program and invokes the default exception handler, which simply prints the standard error message. By this point in the book, you’ve probably seen your share of standard error messages. They include the exception that was raised, along with a stack trace—a list of all the lines and functions that were active when the exception occurred. The error message text here was printed by Python 3.0; it can vary slightly per release, and even per interactive shell. When coding interactively in the basic shell interface, the filename is just “<stdin>,” meaning the standard input stream. When working in the IDLE GUI’s interactive shell, the filename is “<pyshell>”, and source lines are displayed, too. Either way, file line numbers are not very meaningful when there is no file (we’ll see more interesting error messages later in this part of the book): >>> fetcher(x, 4) # Default handler - IDLE GUI interface Traceback (most recent call last): File "<pyshell#6>", line 1, in <module> fetcher(x, 4) File "<pyshell#3>", line 2, in fetcher return obj[index] IndexError: string index out of range In a more realistic program launched outside the interactive prompt, after printing an error message the default handler at the top also terminates the program immediately. That course of action makes sense for simple scripts; errors often should be fatal, and the best you can do when they occur is inspect the standard error message. Catching Exceptions Sometimes, this isn’t what you want, though. Server programs, for instance, typically need to remain active even after internal errors. If you don’t want the default exception behavior, wrap the call in a try statement to catch exceptions yourself: >>> try: ... fetcher(x, 4) ... except IndexError: ... print('got exception') ... got exception >>> # Catch and recover Now, Python jumps to your handler (the block under the except clause that names the exception raised) automatically when an exception is triggered while the try block is running. When working interactively like this, after the except clause runs, we wind up back at the Python prompt. In a more realistic program, try statements not only catch exceptions, but also recover from them: 828 | Chapter 32: Exception Basics >>> def catcher(): ... try: ... fetcher(x, 4) ... except IndexError: ... print('got exception') ... print('continuing') ... >>> catcher() got exception continuing >>> This time, after the exception is caught and handled, the program resumes execution after the entire try statement that caught it—which is why we get the “continuing” message here. We don’t see the standard error message, and the program continues on its way normally. Raising Exceptions So far, we’ve been letting Python raise exceptions for us by making mistakes (on purpose this time!), but our scripts can raise exceptions too—that is, exceptions can be raised by Python or by your program, and can be caught or not. To trigger an exception manually, simply run a raise statement. User-triggered exceptions are caught the same way as those Python raises. The following may not be the most useful Python code ever penned, but it makes the point: >>> try: ... raise IndexError ... except IndexError: ... print('got exception') ... got exception # Trigger exception manually As usual, if they’re not caught, user-triggered exceptions are propagated up to the toplevel default exception handler and terminate the program with a standard error message: >>> raise IndexError Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError As we’ll see in the next chapter, the assert statement can be used to trigger exceptions, too—it’s a conditional raise, used mostly for debugging purposes during development: >>> assert False, 'Nobody expects the Spanish Inquisition!' Traceback (most recent call last): File "<stdin>", line 1, in <module> AssertionError: Nobody expects the Spanish Inquisition! Exceptions: The Short Story | 829 User-Defined Exceptions The raise statement introduced in the prior section raises a built-in exception defined in Python’s built-in scope. As you’ll learn later in this part of the book, you can also define new exceptions of your own that are specific to your programs. User-defined exceptions are coded with classes, which inherit from a built-in exception class: usually the class named Exception. Class-based exceptions allow scripts to build exception categories, inherit behavior, and have attached state information: >>> ... ... >>> ... ... >>> ... ... ... ... got >>> class Bad(Exception): pass def doomed(): raise Bad() # User-defined exception # Raise an instance try: doomed() except Bad: print('got Bad') # Catch class name Bad Termination Actions Finally, try statements can say “finally”—that is, they may include finally blocks. These look like except handlers for exceptions, but the try/finally combination specifies termination actions that always execute “on the way out,” regardless of whether an exception occurs in the try block: >>> try: ... fetcher(x, 3) ... finally: ... print('after fetch') ... 'm' after fetch >>> # Termination actions Here, if the try block finishes without an exception, the finally block will run, and the program will resume after the entire try. In this case, this statement seems a bit silly—we might as well have simply typed the print right after a call to the function, and skipped the try altogether: fetcher(x, 3) print('after fetch') There is a problem with coding this way, though: if the function call raises an exception, the print will never be reached. The try/finally combination avoids this pitfall—when an exception does occur in a try block, finally blocks are executed while the program is being unwound: 830 | Chapter 32: Exception Basics >>> def after(): ... try: ... fetcher(x, 4) ... finally: ... print('after fetch') ... print('after try?') ... >>> after() after fetch Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in after File "<stdin>", line 2, in fetcher IndexError: string index out of range >>> Here, we don’t get the “after try?” message because control does not resume after the try/finally block when an exception occurs. Instead, Python jumps back to run the finally action, and then propagates the exception up to a prior handler (in this case, to the default handler at the top). If we change the call inside this function so as not to trigger an exception, the finally code still runs, but the program continues after the try: >>> def after(): ... try: ... fetcher(x, 3) ... finally: ... print('after fetch') ... print('after try?') ... >>> after() after fetch after try? >>> In practice, try/except combinations are useful for catching and recovering from exceptions, and try/finally combinations come in handy to guarantee that termination actions will fire regardless of any exceptions that may occur in the try block’s code. For instance, you might use try/except to catch errors raised by code that you import from a third-party library, and try/finally to ensure that calls to close files or terminate server connections are always run. We’ll see some such practical examples later in this part of the book. Although they serve conceptually distinct purposes, as of Python 2.5, we can now mix except and finally clauses in the same try statement—the finally is run on the way out regardless of whether an exception was raised, and regardless of whether the exception was caught by an except clause. As we’ll learn in the next chapter, Python 2.6 and 3.0 provide an alternative to try/ finally when using some types of objects. The with/as statement runs an object’s context management logic to guarantee that termination actions occur: Exceptions: The Short Story | 831 >>> with open('lumberjack.txt', 'w') as file: ... file.write('The larch!\n') # Always close file on exit Although this option requires fewer lines of code, it’s only applicable when processing certain object types, so try/finally is a more general termination structure. On the other hand, with/as may also run startup actions and supports user-defined context management code. Why You Will Care: Error Checks One way to see how exceptions are useful is to compare coding styles in Python and languages without exceptions. For instance, if you want to write robust programs in the C language, you generally have to test return values or status codes after every operation that could possibly go astray, and propagate the results of the tests as your programs run: doStuff() { # C program if (doFirstThing() == ERROR) # Detect errors everywhere return ERROR; # even if not handled here if (doNextThing() == ERROR) return ERROR; ... return doLastThing(); } main() { if (doStuff() == ERROR) badEnding(); else goodEnding(); } In fact, realistic C programs often have as much code devoted to error detection as to doing actual work. But in Python, you don’t have to be so methodical (and neurotic!). You can instead wrap arbitrarily vast pieces of a program in exception handlers and simply write the parts that do the actual work, assuming all is well: def doStuff(): doFirstThing() doNextThing() ... doLastThing() # Python code # We don't care about exceptions here, # so we don't need to detect them if __name__ == '__main__': try: doStuff() # This is where we care about results, except: # so it's the only place we must check badEnding() else: goodEnding() 832 | Chapter 32: Exception Basics Because control jumps immediately to a handler when an exception occurs, there’s no need to instrument all your code to guard for errors. Moreover, because Python detects errors automatically, your code usually doesn’t need to check for errors in the first place. The upshot is that exceptions let you largely ignore the unusual cases and avoid error-checking code. Chapter Summary And that is the majority of the exception story; exceptions really are a simple tool. To summarize, Python exceptions are a high-level control flow device. They may be raised by Python, or by your own programs. In both cases, they may be ignored (to trigger the default error message), or caught by try statements (to be processed by your code). The try statement comes in two logical formats that, as of Python 2.5, can be combined—one that handles exceptions, and one that executes finalization code regardless of whether exceptions occur or not. Python’s raise and assert statements trigger exceptions on demand (both built-ins and new exceptions we define with classes); the with/as statement is an alternative way to ensure that termination actions are carried out for objects that support it. In the rest of this part of the book, we’ll fill in some of the details about the statements involved, examine the other sorts of clauses that can appear under a try, and discuss class-based exception objects. The next chapter begins our tour by taking a closer look at the statements we introduced here. Before you turn the page, though, here are a few quiz questions to review. Test Your Knowledge: Quiz 1. 2. 3. 4. 5. Name three things that exception processing is good for. What happens to an exception if you don’t do anything special to handle it? How can your script recover from an exception? Name two ways to trigger exceptions in your script. Name two ways to specify actions to be run at termination time, whether an exception occurs or not. Test Your Knowledge: Answers 1. Exception processing is useful for error handling, termination actions, and event notification. It can also simplify the handling of special cases and can be used to implement alternative control flows. In general, exception processing also cuts Test Your Knowledge: Answers | 833 2. 3. 4. 5. down on the amount of error-checking code your program may require—because all errors filter up to handlers, you may not need to test the outcome of every operation. Any uncaught exception eventually filters up to the default exception handler Python provides at the top of your program. This handler prints the familiar error message and shuts down your program. If you don’t want the default message and shutdown, you can code try/except statements to catch and recover from exceptions that are raised. Once an exception is caught, the exception is terminated and your program continues. The raise and assert statements can be used to trigger an exception, exactly as if it had been raised by Python itself. In principle, you can also raise an exception by making a programming mistake, but that’s not usually an explicit goal! The try/finally statement can be used to ensure actions are run after a block of code exits, regardless of whether it raises an exception or not. The with/as statement can also be used to ensure termination actions are run, but only when processing object types that support it. 834 | Chapter 32: Exception Basics CHAPTER 33 Exception Coding Details In the prior chapter we took a quick look at exception-related statements in action. Here, we’re going to dig a bit deeper—this chapter provides a more formal introduction to exception processing syntax in Python. Specifically, we’ll explore the details behind the try, raise, assert, and with statements. As we’ll see, although these statements are mostly straightforward, they offer powerful tools for dealing with exceptions in Python code. One procedural note up front: The exception story has changed in major ways in recent years. As of Python 2.5, the finally clause can appear in the same try statement as except and else clauses (previously, they could not be combined). Also, as of Python 3.0 and 2.6, the new with context manager statement has become official, and user-defined exceptions must now be coded as class instances, which should inherit from a built-in exception superclass. Moreover, 3.0 sports slightly modified syntax for the raise statement and except clauses. I will focus on the state of exceptions in Python 2.6 and 3.0 in this edition, but because you are still very likely to see the original techniques in code for some time to come, along the way I’ll point out how things have evolved in this domain. The try/except/else Statement Now that we’ve seen the basics, it’s time for the details. In the following discussion, I’ll first present try/except/else and try/finally as separate statements, because in versions of Python prior to 2.5 they serve distinct roles and cannot be combined. As mentioned in the preceding note, in Python 2.5 and later except and finally can be mixed in a single try statement; I’ll explain the implications of this change after we’ve explored the two original forms in isolation. The try is a compound statement; its most complete form is sketched below. It starts with a try header line, followed by a block of (usually) indented statements, then one 835 or more except clauses that identify exceptions to be caught, and an optional else clause at the end. The words try, except, and else are associated by indenting them to the same level (i.e., lining them up vertically). For reference, here’s the general format in Python 3.0: try: <statements> except <name1>: <statements> except (name2, name3): <statements> except <name4> as <data>: <statements> except: <statements> else: <statements> # Run this main action first # Run if name1 is raised during try block # Run if any of these exceptions occur # Run if name4 is raised, and get instance raised # Run for all (other) exceptions raised # Run if no exception was raised during try block In this statement, the block under the try header represents the main action of the statement—the code you’re trying to run. The except clauses define handlers for exceptions raised during the try block, and the else clause (if coded) provides a handler to be run if no exceptions occur. The <data> entry here has to do with a feature of raise statements and exception classes, which we will discuss later in this chapter. Here’s how try statements work. When a try statement is entered, Python marks the current program context so it can return to it if an exception occurs. The statements nested under the try header are run first. What happens next depends on whether exceptions are raised while the try block’s statements are running: • If an exception does occur while the try block’s statements are running, Python jumps back to the try and runs the statements under the first except clause that matches the raised exception. Control resumes below the entire try statement after the except block runs (unless the except block raises another exception). • If an exception happens in the try block and no except clause matches, the exception is propagated up to the last matching try statement that was entered in the program or, if it’s the first such statement, to the top level of the process (in which case Python kills the program and prints a default error message). • If no exception occurs while the statements under the try header run, Python runs the statements under the else line (if present), and control then resumes below the entire try statement. In other words, except clauses catch any exceptions that happen while the try block is running, and the else clause runs only if no exceptions happen while the try block runs. except clauses are focused exception handlers—they catch exceptions that occur only within the statements in the associated try block. However, as the try block’s state- ments can call functions coded elsewhere in a program, the source of an exception may be outside the try statement itself. I’ll have more to say about this when we explore try nesting in Chapter 35. 836 | Chapter 33: Exception Coding Details try Statement Clauses When you write a try statement, a variety of clauses can appear after the try header. Table 33-1 summarizes all the possible forms—you must use at least one. We’ve already met some of these: as you know, except clauses catch exceptions, finally clauses run on the way out, and else clauses run if no exceptions are encountered. Syntactically, there may be any number of except clauses, but you can code else only if there is at least one except, and there can be only one else and one finally. Through Python 2.4, the finally clause must appear alone (without else or except); the try/ finally is really a different statement. As of Python 2.5, however, a finally can appear in the same statement as except and else (more on the ordering rules later in this chapter when we meet the unified try statement). Table 33-1. try statement clause forms Clause form Interpretation except: Catch all (or all other) exception types. except name: Catch a specific exception only. except name as value: Catch the listed exception and its instance. except (name1, name2): Catch any of the listed exceptions. except (name1, name2) as value: Catch any listed exception and its instance. else: Run if no exceptions are raised. finally: Always perform this block. We’ll explore the entries with the extra as value part when we meet the raise statement. They provide access to the objects that are raised as exceptions. The first and fourth entries in Table 33-1 are new here: • except clauses that list no exception name (except:) catch all exceptions not previously listed in the try statement. • except clauses that list a set of exceptions in parentheses (except (e1, e2, e3):) catch any of the listed exceptions. Because Python looks for a match within a given try by inspecting the except clauses from top to bottom, the parenthesized version has the same effect as listing each exception in its own except clause, but you have to code the statement body only once. Here’s an example of multiple except clauses at work, which demonstrates just how specific your handlers can be: try: action() except NameError: ... except IndexError: ... The try/except/else Statement | 837 except KeyError: ... except (AttributeError, TypeError, SyntaxError): ... else: ... In this example, if an exception is raised while the call to the action function is running, Python returns to the try and searches for the first except that names the exception raised. It inspects the except clauses from top to bottom and left to right, and runs the statements under the first one that matches. If none match, the exception is propagated past this try. Note that the else runs only when no exception occurs in action—it does not run when an exception without a matching except is raised. If you really want a general “catch-all” clause, an empty except does the trick: try: action() except NameError: ... except IndexError: ... except: ... else: ... # Handle NameError # Handle IndexError # Handle all other exceptions # Handle the no-exception case The empty except clause is a sort of wildcard feature—because it catches everything, it allows your handlers to be as general or specific as you like. In some scenarios, this form may be more convenient than listing all possible exceptions in a try. For example, the following catches everything without listing anything: try: action() except: ... # Catch all possible exceptions Empty excepts also raise some design issues, though. Although convenient, they may catch unexpected system exceptions unrelated to your code, and they may inadvertently intercept exceptions meant for another handler. For example, even system exit calls in Python trigger exceptions, and you usually want these to pass. That said, this structure may also catch genuine programming mistakes for you which you probably want to see an error message. We’ll revisit this as a gotcha at the end of this part of the book. For now, I’ll just say “use with care.” Python 3.0 introduced an alternative that solves one of these problems—catching an exception named Exception has almost the same effect as an empty except, but ignores exceptions related to system exits: try: action() except Exception: ... # Catch all possible exceptions, except exits 838 | Chapter 33: Exception Coding Details This has most of the same convenience of the empty except, but also most of the same dangers. We’ll explore how this form works its voodoo in the next chapter, when we study exception classes. Version skew note: Python 3.0 requires the except E as V: handler clause form listed in Table 33-1 and used in this book, rather than the older except E, V: form. The latter form is still available (but not recommended) in Python 2.6: if used, it’s converted to the former. The change was made to eliminate errors that occur when confusing the older form with two alternate exceptions, properly coded in 2.6 as except (E1, E2):. Because 3.0 supports the as form only, commas in a handler clause are always taken to mean a tuple, regardless of whether parentheses are used or not, and the values are interpreted as alternative exceptions to be caught. This change also modifies the scoping rules: with the new as syntax, the variable V is deleted at the end of the except block. The try else Clause The purpose of the else clause is not always immediately obvious to Python newcomers. Without it, though, there is no way to tell (without setting and checking Boolean flags) whether the flow of control has proceeded past a try statement because no exception was raised, or because an exception occurred and was handled: try: ...run code... except IndexError: ...handle exception... # Did we get here because the try failed or not? Much like the way else clauses in loops make the exit cause more apparent, the else clause provides syntax in a try that makes what has happened obvious and unambiguous: try: ...run code... except IndexError: ...handle exception... else: ...no exception occurred... You can almost emulate an else clause by moving its code into the try block: try: ...run code... ...no exception occurred... except IndexError: ...handle exception... This can lead to incorrect exception classifications, though. If the “no exception occurred” action triggers an IndexError, it will register as a failure of the try block and The try/except/else Statement | 839 erroneously trigger the exception handler below the try (subtle, but true!). By using an explicit else clause instead, you make the logic more obvious and guarantee that except handlers will run only for real failures in the code you’re wrapping in a try, not for failures in the else case’s action. Example: Default Behavior Because the control flow through a program is easier to capture in Python than in English, let’s run some examples that further illustrate exception basics. I’ve mentioned that exceptions not caught by try statements percolate up to the top level of the Python process and run Python’s default exception-handling logic (i.e., Python terminates the running program and prints a standard error message). Let’s look at an example. Running the following module file, bad.py, generates a divide-by-zero exception: def gobad(x, y): return x / y def gosouth(x): print(gobad(x, 0)) gosouth(1) Because the program ignores the exception it triggers, Python kills the program and prints a message: % python bad.py Traceback (most recent call last): File "bad.py", line 7, in <module> gosouth(1) File "bad.py", line 5, in gosouth print(gobad(x, 0)) File "bad.py", line 2, in gobad return x / y ZeroDivisionError: int division or modulo by zero I ran this in a shell widow with Python 3.0. The message consists of a stack trace (“Traceback”) and the name of and details about the exception that was raised. The stack trace lists all lines active when the exception occurred, from oldest to newest. Note that because we’re not working at the interactive prompt, in this case the file and line number information is more useful. For example, here we can see that the bad divide happens at the last entry in the trace—line 2 of the file bad.py, a return statement.* Because Python detects and reports all errors at runtime by raising exceptions, exceptions are intimately bound up with the ideas of error handling and debugging in general. * As mentioned in the prior chapter, the text of error messages and stack traces tends to vary slightly over time and shells. Don’t be alarmed if your error messages don’t exactly match mine. When I ran this example in Python 3.0’s IDLE GUI, for instance, its error message text showed filenames with full absolute directory paths. 840 | Chapter 33: Exception Coding Details If you’ve worked through this book’s examples, you’ve undoubtedly seen an exception or two along the way—even typos usually generate a SyntaxError or other exception when a file is imported or executed (that’s when the compiler is run). By default, you get a useful error display like the one just shown, which helps you track down the problem. Often, this standard error message is all you need to resolve problems in your code. For more heavy-duty debugging jobs, you can catch exceptions with try statements, or use one of the debugging tools that I introduced in Chapter 3 and will summarize again in Chapter 35 (such as the pdb standard library module). Example: Catching Built-in Exceptions Python’s default exception handling is often exactly what you want—especially for code in a top-level script file, an error generally should terminate your program immediately. For many programs, there is no need to be more specific about errors in your code. Sometimes, though, you’ll want to catch errors and recover from them instead. If you don’t want your program terminated when Python raises an exception, simply catch it by wrapping the program logic in a try. This is an important capability for programs such as network servers, which must keep running persistently. For example, the following code catches and recovers from the TypeError Python raises immediately when you try to concatenate a list and a string (the + operator expects the same sequence type on both sides): def kaboom(x, y): print(x + y) # Trigger TypeError try: kaboom([0,1,2], "spam") except TypeError: print('Hello world!') print('resuming here') # Catch and recover here # Continue here if exception or not When the exception occurs in the function kaboom, control jumps to the try statement’s except clause, which prints a message. Since an exception is “dead” after it’s been caught like this, the program continues executing below the try rather than being terminated by Python. In effect, the code processes and clears the error, and your script recovers: % python kaboom.py Hello world! resuming here Notice that once you’ve caught an error, control resumes at the place where you caught it (i.e., after the try); there is no direct way to go back to the place where the exception occurred (here, in the function kaboom). In a sense, this makes exceptions more like The try/except/else Statement | 841 simple jumps than function calls—there is no way to return to the code that triggered the error. The try/finally Statement The other flavor of the try statement is a specialization that has to do with finalization actions. If a finally clause is included in a try, Python will always run its block of statements “on the way out” of the try statement, whether an exception occurred while the try block was running or not. Its general form is: try: <statements> finally: <statements> # Run this action first # Always run this code on the way out With this variant, Python begins by running the statement block associated with the try header line. What happens next depends on whether an exception occurs during the try block: • If no exception occurs while the try block is running, Python jumps back to run the finally block and then continues execution past below the try statement. • If an exception does occur during the try block’s run, Python still comes back and runs the finally block, but it then propagates the exception up to a higher try or the top-level default handler; the program does not resume execution below the try statement. That is, the finally block is run even if an exception is raised, but unlike an except, the finally does not terminate the exception—it continues being raised after the finally block runs. The try/finally form is useful when you want to be completely sure that an action will happen after some code runs, regardless of the exception behavior of the program. In practice, it allows you to specify cleanup actions that always must occur, such as file closes and server disconnects. Note that the finally clause cannot be used in the same try statement as except and else in Python 2.4 and earlier, so the try/finally is best thought of as a distinct statement form if you are using an older release. In Python 2.5, and later, however, finally can appear in the same statement as except and else, so today there is really a single try statement with many optional clauses (more about this shortly). Whichever version you use, though, the finally clause still serves the same purpose—to specify “cleanup” actions that must always be run, regardless of any exceptions. As we’ll also see later in this chapter, in Python 2.6 and 3.0, the new with statement and its context managers provide an object-based way to do similar work for exit actions. Unlike finally, this new statement also supports entry actions, but it is limited in scope to objects that implement the context manager protocol. 842 | Chapter 33: Exception Coding Details Example: Coding Termination Actions with try/finally We saw some simple try/finally examples in the prior chapter. Here’s a more realistic example that illustrates a typical role for this statement: class MyError(Exception): pass def stuff(file): raise MyError() file = open('data', 'w') try: stuff(file) finally: file.close() print('not reached') # Open an output file # Raises exception # Always close file to flush output buffers # Continue here only if no exception In this code, we’ve wrapped a call to a file-processing function in a try with a finally clause to make sure that the file is always closed, and thus finalized, whether the function triggers an exception or not. This way, later code can be sure that the file’s output buffer’s content has been flushed from memory to disk. A similar code structure can guarantee that server connections are closed, and so on. As we learned in Chapter 9, file objects are automatically closed on garbage collection; this is especially useful for temporary files that we don’t assign to variables. However, it’s not always easy to predict when garbage collection will occur, especially in larger programs. The try statement makes file closes more explicit and predictable and pertains to a specific block of code. It ensures that the file will be closed on block exit, regardless of whether an exception occurs or not. This particular example’s function isn’t all that useful (it just raises an exception), but wrapping calls in try/finally statements is a good way to ensure that your closing-time (i.e., termination) activities always run. Again, Python always runs the code in your finally blocks, regardless of whether an exception happens in the try block.† When the function here raises its exception, the control flow jumps back and runs the finally block to close the file. The exception is then propagated on to either another try or the default top-level handler, which prints the standard error message and shuts down the program; the statement after this try is never reached. If the function here did not raise an exception, the program would still execute the finally block to close the file, but it would then continue below the entire try statement. Notice that the user-defined exception here is again defined with a class—as we’ll see in the next chapter, exceptions today must all be class instances in both 2.6 and 3.0. † Unless Python crashes completely, of course. It does a good job of avoiding this, though, by checking all possible errors as a program runs. When a program does crash hard, it is usually due to a bug in linked-in C extension code, outside of Python’s scope. The try/finally Statement | 843 Unified try/except/finally In all versions of Python prior to Release 2.5 (for its first 15 years of life, more or less), the try statement came in two flavors and was really two separate statements—we could either use a finally to ensure that cleanup code was always run, or write except blocks to catch and recover from specific exceptions and optionally specify an else clause to be run if no exceptions occurred. That is, the finally clause could not be mixed with except and else. This was partly because of implementation issues, and partly because the meaning of mixing the two seemed obscure—catching and recovering from exceptions seemed a disjoint concept from performing cleanup actions. In Python 2.5 and later, though (including 2.6 and 3.0, the versions used in this book), the two statements have merged. Today, we can mix finally, except, and else clauses in the same statement. That is, we can now write a statement of this form: try: # Merged form main-action except Exception1: handler1 except Exception2: handler2 ... else: else-block finally: finally-block The code in this statement’s main-action block is executed first, as usual. If that code raises an exception, all the except blocks are tested, one after another, looking for a match to the exception raised. If the exception raised is Exception1, the handler1 block is executed; if it’s Exception2, handler2 is run, and so on. If no exception is raised, the else-block is executed. No matter what’s happened previously, the finally-block is executed once the main action block is complete and any raised exceptions have been handled. In fact, the code in the finally-block will be run even if there is an error in an exception handler or the else-block and a new exception is raised. As always, the finally clause does not end the exception—if an exception is active when the finally-block is executed, it continues to be propagated after the finallyblock runs, and control jumps somewhere else in the program (to another try, or to the default top-level handler). If no exception is active when the finally is run, control resumes after the entire try statement. The net effect is that the finally is always run, regardless of whether: • An exception occurred in the main action and was handled. • An exception occurred in the main action and was not handled. 844 | Chapter 33: Exception Coding Details • No exceptions occurred in the main action. • A new exception was triggered in one of the handlers. Again, the finally serves to specify cleanup actions that must always occur on the way out of the try, regardless of what exceptions have been raised or handled. Unified try Statement Syntax When combined like this, the try statement must have either an except or a finally, and the order of its parts must be like this: try -> except -> else -> finally where the else and finally are optional, and there may be zero or more except, but there must be at least one except if an else appears. Really, the try statement consists of two parts: excepts with an optional else, and/or the finally. In fact, it’s more accurate to describe the merged statement’s syntactic form this way (square brackets mean optional and star means zero-or-more here): try: statements except [type [as value]]: statements [except [type [as value]]: statements]* [else: statements] [finally: statements] try: # Format 1 # [type [, value]] in Python 2 # Format 2 statements finally: statements Because of these rules, the else can appear only if there is at least one except, and it’s always possible to mix except and finally, regardless of whether an else appears or not. It’s also possible to mix finally and else, but only if an except appears too (though the except can omit an exception name to catch everything and run a raise statement, described later, to reraise the current exception). If you violate any of these ordering rules, Python will raise a syntax error exception before your code runs. Combining finally and except by Nesting Prior to Python 2.5, it is actually possible to combine finally and except clauses in a try by syntactically nesting a try/except in the try block of a try/finally statement (we’ll explore this technique more fully in Chapter 35). In fact, the following has the same effect as the new merged form shown at the start of this section: Unified try/except/finally | 845 # Nested equivalent to merged form try: try: main-action except Exception1: handler1 except Exception2: handler2 ... else: no-error finally: cleanup Again, the finally block is always run on the way out, regardless of what happened in the main action and regardless of any exception handlers run in the nested try (trace through the four cases listed previously to see how this works the same). Since an else always requires an except, this nested form even sports the same mixing constraints of the unified statement form outlined in the preceding section. However, this nested equivalent is more obscure and requires more code than the new merged form (one four-character line, at least). Mixing finally into the same statement makes your code easier to write and read, so this is the generally preferred technique today. Unified try Example Here’s a demonstration of the merged try statement form at work. The following file, mergedexc.py, codes four common scenarios, with print statements that describe the meaning of each: sep = '-' * 32 + '\n' print(sep + 'EXCEPTION RAISED AND CAUGHT') try: x = 'spam'[99] except IndexError: print('except run') finally: print('finally run') print('after run') print(sep + 'NO EXCEPTION RAISED') try: x = 'spam'[3] except IndexError: print('except run') finally: print('finally run') print('after run') print(sep + 'NO EXCEPTION RAISED, WITH ELSE') try: 846 | Chapter 33: Exception Coding Details x = 'spam'[3] except IndexError: print('except run') else: print('else run') finally: print('finally run') print('after run') print(sep + 'EXCEPTION RAISED BUT NOT CAUGHT') try: x = 1 / 0 except IndexError: print('except run') finally: print('finally run') print('after run') When this code is run, the following output is produced in Python 3.0 (actually, its behavior and output are the same in 2.6, because the print calls each print a single item). Trace through the code to see how exception handling produces the output of each of the four tests here: c:\misc> C:\Python30\python mergedexc.py -------------------------------EXCEPTION RAISED AND CAUGHT except run finally run after run -------------------------------NO EXCEPTION RAISED finally run after run -------------------------------NO EXCEPTION RAISED, WITH ELSE else run finally run after run -------------------------------EXCEPTION RAISED BUT NOT CAUGHT finally run Traceback (most recent call last): File "mergedexc.py", line 36, in <module> x = 1 / 0 ZeroDivisionError: int division or modulo by zero This example uses built-in operations in the main action to trigger exceptions (or not), and it relies on the fact that Python always checks for errors as code is running. The next section shows how to raise exceptions manually instead. Unified try/except/finally | 847 The raise Statement To trigger exceptions explicitly, you can code raise statements. Their general form is simple—a raise statement consists of the word raise, optionally followed by the class to be raised or an instance of it: raise <instance> raise <class> raise # Raise instance of class # Make and raise instance of class # Reraise the most recent exception As mentioned earlier, exceptions are always instances of classes in Python 2.6 and 3.0. Hence, the first raise form here is the most common—we provide an instance directly, either created before the raise or within the raise statement itself. If we pass a class instead, Python calls the class with no constructor arguments, to create an instance to be raised; this form is equivalent to adding parentheses after the class reference. The last form reraises the most recently raised exception; it’s commonly used in exception handlers to propagate exceptions that have been caught. To make this clearer, let’s look at some examples. With built-in exceptions, the following two forms are equivalent—both raise an instance of the exception class named, but the first creates the instance implicitly: raise IndexError raise IndexError() # Class (instance created) # Instance (created in statement) We can also create the instance ahead of time—because the raise statement accepts any kind of object reference, the following two examples raise IndexError just like the prior two: exc = IndexError() raise exc # Create instance ahead of time excs = [IndexError, TypeError] raise excs[0] When an exception is raised, Python sends the raised instance along with the exception. If a try includes an except name as X: clause, the variable X will be assigned the instance provided in the raise: try: ... except IndexError as X: ... # X assigned the raised instance object The as is optional in a try handler (if it’s omitted, the instance is simply not assigned to a name), but including it allows the handler to access both data in the instance and methods in the exception class. This model works the same for user-defined exceptions we code with classes—the following, for example, passes to the exception class constructor arguments that become available in the handler through the assigned instance: 848 | Chapter 33: Exception Coding Details class MyExc(Exception): pass ... raise MyExc('spam') # Exception class with constructor args ... try: ... except MyExc as X: # Instance attributes available in handler print(X.args) Because this encroaches on the next chapter’s topic, though, I’ll defer further details until then. Regardless of how you name them, exceptions are always identified by instance objects, and at most one is active at any given time. Once caught by an except clause anywhere in the program, an exception dies (i.e., won’t propagate to another try), unless it’s reraised by another raise statement or error. Propagating Exceptions with raise A raise statement that does not include an exception name or extra data value simply reraises the current exception. This form is typically used if you need to catch and handle an exception but don’t want the exception to die in your code: >>> try: ... raise IndexError('spam') ... except IndexError: ... print('propagating') ... raise ... propagating Traceback (most recent call last): File "<stdin>", line 2, in <module> IndexError: spam # Exceptions remember arguments # Reraise most recent exception Running a raise this way reraises the exception and propagates it to a higher handler (or the default handler at the top, which stops the program with a standard error message). Notice how the argument we passed to the exception class shows up in the error messages; you’ll learn why this happens in the next chapter. Python 3.0 Exception Chaining: raise from Python 3.0 (but not 2.6) also allows raise statements to have an optional from clause: raise exception from otherexception When the from is used, the second expression specifies another exception class or instance to attach to the raised exception’s __cause__ attribute. If the raised exception is not caught, Python prints both exceptions as part of the standard error message: >>> try: ... 1 / 0 ... except Exception as E: The raise Statement | 849 ... raise TypeError('Bad!') from E ... Traceback (most recent call last): File "<stdin>", line 2, in <module> ZeroDivisionError: int division or modulo by zero The above exception was the direct cause of the following exception: Traceback (most recent call last): File "<stdin>", line 4, in <module> TypeError: Bad! When an exception is raised inside an exception handler, a similar procedure is followed implicitly: the previous exception is attached to the new exception’s __context__ attribute and is again displayed in the standard error message if the exception goes uncaught. This is an advanced and still somewhat obscure extension, so see Python’s manuals for more details. Version skew note: Python 3.0 no longer supports the raise Exc, Args form that is still available in Python 2.6. In 3.0, use the raise Exc(Args) instance-creation call form described in this book instead. The equivalent comma form in 2.6 is legacy syntax provided for compatibility with the now defunct string-based exceptions model, and it’s deprecated in 3.0. If used, it is converted to the 3.0 call form. As in earlier releases, a raise Exc form is also allowed—it is converted to raise Exc() in both versions, calling the class constructor with no arguments. The assert Statement As a somewhat special case for debugging purposes, Python includes the assert statement. It is mostly just syntactic shorthand for a common raise usage pattern, and an assert can be thought of as a conditional raise statement. A statement of the form: assert <test>, <data> # The <data> part is optional works like the following code: if __debug__: if not <test>: raise AssertionError(<data>) In other words, if the test evaluates to false, Python raises an exception: the data item (if it’s provided) is used as the exception’s constructor argument. Like all exceptions, the AssertionError exception will kill your program if it’s not caught with a try, in which case the data item shows up as part of the error message. As an added feature, assert statements may be removed from a compiled program’s byte code if the -O Python command-line flag is used, thereby optimizing the program. AssertionError is a built-in exception, and the __debug__ flag is a built-in name that is 850 | Chapter 33: Exception Coding Details automatically set to True unless the -O flag is used. Use a command line like python –O main.py to run in optimized mode and disable asserts. Example: Trapping Constraints (but Not Errors!) Assertions are typically used to verify program conditions during development. When displayed, their error message text automatically includes source code line information and the value listed in the assert statement. Consider the file asserter.py: def f(x): assert x < 0, 'x must be negative' return x ** 2 % python >>> import asserter >>> asserter.f(1) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "asserter.py", line 2, in f assert x < 0, 'x must be negative' AssertionError: x must be negative It’s important to keep in mind that assert is mostly intended for trapping user-defined constraints, not for catching genuine programming errors. Because Python traps programming errors itself, there is usually no need to code asserts to catch things like outof-bounds indexes, type mismatches, and zero divides: def reciprocal(x): assert x != 0 return 1 / x # A useless assert! # Python checks for zero automatically Such asserts are generally superfluous—because Python raises exceptions on errors automatically, you might as well let it do the job for you.‡ For another example of common assert usage, see the abstract superclass example in Chapter 28; there, we used assert to make calls to undefined methods fail with a message. with/as Context Managers Python 2.6 and 3.0 introduced a new exception-related statement—the with, and its optional as clause. This statement is designed to work with context manager objects, which support a new method-based protocol. This feature is also available as an option in 2.5, enabled with an import of this form: from __future__ import with_statement ‡ In most cases, at least. As suggested earlier in the book, if a function has to perform long-running or unrecoverable actions before it reaches the place where an exception will be triggered, you still might want to test for errors. Even in this case, though, be careful not to make your tests overly specific or restrictive, or you will limit your code’s utility. with/as Context Managers | 851 In short, the with/as statement is designed to be an alternative to a common try/ finally usage idiom; like that statement, it is intended for specifying termination-time or “cleanup” activities that must run regardless of whether an exception occurs in a processing step. Unlike try/finally, though, the with statement supports a richer object-based protocol for specifying both entry and exit actions around a block of code. Python enhances some built-in tools with context managers, such as files that automatically close themselves and thread locks that automatically lock and unlock, but programmers can code context managers of their own with classes, too. Basic Usage The basic format of the with statement looks like this: with expression [as variable]: with-block The expression here is assumed to return an object that supports the context management protocol (more on this protocol in a moment). This object may also return a value that will be assigned to the name variable if the optional as clause is present. Note that the variable is not necessarily assigned the result of the expression; the result of the expression is the object that supports the context protocol, and the variable may be assigned something else intended to be used inside the statement. The object returned by the expression may then run startup code before the with-block is started, as well as termination code after the block is done, regardless of whether the block raised an exception or not. Some built-in Python objects have been augmented to support the context management protocol, and so can be used with the with statement. For example, file objects (covered in Chapter 9) have a context manager that automatically closes the file after the with block regardless of whether an exception is raised: with open(r'C:\misc\data') as myfile: for line in myfile: print(line) ...more code here... Here, the call to open returns a simple file object that is assigned to the name myfile. We can use myfile with the usual file tools—in this case, the file iterator reads line by line in the for loop. However, this object also supports the context management protocol used by the with statement. After this with statement has run, the context management machinery guarantees that the file object referenced by myfile is automatically closed, even if the for loop raised an exception while processing the file. Although file objects are automatically closed on garbage collection, it’s not always straightforward to know when that will occur. The with statement in this role is an alternative that allows us to be sure that the close will occur after execution of a specific 852 | Chapter 33: Exception Coding Details block of code. As we saw earlier, we can achieve a similar effect with the more general and explicit try/finally statement, but it requires four lines of administrative code instead of one in this case: myfile = open(r'C:\misc\data') try: for line in myfile: print(line) ...more code here... finally: myfile.close() We won’t cover Python’s multithreading modules in this book (for more on that topic, see follow-up application-level texts such as Programming Python), but the lock and condition synchronization objects they define may also be used with the with statement, because they support the context management protocol: lock = threading.Lock() with lock: # critical section of code ...access shared resources... Here, the context management machinery guarantees that the lock is automatically acquired before the block is executed and released once the block is complete, regardless of exception outcomes. As introduced in Chapter 5, the decimal module also uses context managers to simplify saving and restoring the current decimal context, which specifies the precision and rounding characteristics for calculations: with decimal.localcontext() as ctx: ctx.prec = 2 x = decimal.Decimal('1.00') / decimal.Decimal('3.00') After this statement runs, the current thread’s context manager stat