Advanced Python topics

Lecture 21: More Advanced Python 

The set data type 

Command line arguments 

Redirection and pipe 

More on file operations 

More on string formatting 

More built-in functions: zip(), xrange() 

List comprehensions 

lambda functions 

Exceptions

COMP 116: Introduction to Scientific Programming 

The set Data Type 

‣ Set - unordered collections of unique elements. 

‣ Create a set 

• set(‘abcde’) 

• set([‘hello’, ‘world’, ‘my’, ‘friends’]) 

‣ Operations on a set 

• len(s): cardinality of set s 

• x in s: test x for membership in s 

• x not in s: test x for non-membership in s 

© 2013 Yinqian Zhang 2


Set Operations 

‣ Operations on a set (Con’t) 

• s.issubset(t): test whether every element in s is in t 

• s.issuperset(t): test whether every element in t is in s 

• s.union(t): new set with elements from both s and t 

• s.intersection(t): new set with elements common to s 

and t 

• s.difference(t): new set with elements in s but not in t 

• s.symmetric_difference(t): new set with elements in 

either s or t but not both 



Manipulate Elements in A Set 

‣ Operations on a set 

• s.update(t): return set s with elements added from t 

• s.add(x): add element x to set s 

• s.remove(x): remove x from set (error if not present) 

• s.discard(x): removes x from set s if present 

• s.pop(): remove and return an arbitrary element from s; 

(error if not present) 

• s.clear(): remove all elements from set s 



Compare Two Sets 

‣ s == t 

• if and only if every element of each set is contained in 

the other (each is a subset of the other). 

‣ s > t 

• if and only if t is a proper subset of s (is a subset, but is 

not equal). 

‣ Two arbitrary sets may not have >, < or == 

relationship. 



Command Line Arguments 

‣ Run a Python program with arguments 

• python example.py arg1 arg2 arg3 

‣ How to access to the command line arguments 

from within the Python program? 

• import sys 

for item in sys.argv: 

print item 



Exercises: Command Line Arguments 

‣ Write a program which accept two integers from 

the command line and print on the terminal the sum 

of the two integers. 



I/O Redirection 

‣ I/O redirection: redirecting the standard output (or 

standard input) to (or from) a file. 

‣ Example: 

• In the Python file test.py: 

 

print 'this is a test' 

• The following command redirects the output to a file 

 

python test.py > a.txt 

• The following command redirects and appends the 

output to a file 

 

python test.py >> a.txt 



I/O Redirection (Con’t) 

‣ Redirection standard input from a file 

• In the Python file test.py: 

 

a = raw_input() 

b = raw_input() 

print int(a) + int(b) 

• The content of file a.txt: 

1 

2 

• The following command redirects the input from a file 

 

python test.py < a.txt 



Pipe 

‣ A pipe joins the standard output stream from one 

process to the standard input stream of another 

process in the following manner: 

‣ Example: 

• command1 | command2 



Pipe (Con’t) 

‣ Exercises: using pipe to connect two python 

programs. 

• python test1.py | python test2.py 

# test1.py 

import random 

print random.randint(1, 10) 

print random.randint(1, 10) 

# test2.py 

a = raw_input() 

b = raw_input() 

print '%d + %d = %d' % (int(a), int(b), int(a) + int(b)) 



More on File Operations: tell() 

‣ The tell() method returns the current position 

within the file. 

• the next read or write will occur at that many bytes from 

the beginning of the file 

with open('a.txt', 'r') as f: 

while True: 

print f.tell() 

line = f.readline() 

if not line: 

break 



More on File Operations: seek() 

‣ The seek(offset[, from]) method changes the 

current file position. 

• offset: the number of bytes to be moved. 

• from: the reference position from where the bytes are to 

be moved. 

 

 

 

from = 0, the beginning of the file as the reference position 

from = 1, the current position as the reference position 

from = 2, the end of the file as the reference position. 



More on File Operations: seek() 

‣ Example of seek(): 

• Omit the first 10 characters of a file 


f.seek(10, 0) 

print f.read() 

• Read the only the last character of a file 


f.seek(-1, 2) 




More on String Formatting 

‣ x = 3.0; print '%f' % (1/x) 

• 0.333333 

‣ %[flags][width][.precision]code 

• code: d, f, … 

• flags: 

 

 

 

-: left justification 

+: numeric sign 

0: zero fills 

• width: total width of the formatted string 

• precision: number of digits after the decimal point 

‣ x = 3.0; print '%-15.4f' % (1/x) 



More Built-in Functions: zip() 

‣ zip(): access multiple sequences in parallel 

L1 = [1, 2, 3, 4] 

L2 = [5, 6, 7, 8] 

L3 = [9, 10, 11, 12] 

for i in zip(L1, L2, L3): 

print i 

#output 

#(1, 5, 9) 

#(2, 6, 10) 

#(3, 7, 11) 

#(4, 8, 12) 



More Built-in Functions: zip() 

‣ Construct a dictionary with zip() 

Keys = ['a', 'b', 'c'] 

Values = [1, 2, 3] 

D = dict(zip(Keys, Values)) 

print D 

# output 

# {'a': 1, 'c': 3, 'b': 2} 



More Built-in Functions: xrange() 

‣ xrange() 

• xrange() is similar to range(), but returns an xrange 

object instead of a list. 

• xrange() yields the same values as the corresponding 

list, without actually storing them all simultaneously. 

for i in xrange(100000): 

print i 



List Comprehensions 

‣ The basic format 

• L = [x + 3 for x in range(10)] 

• equals: 

L = [] 

for i in range(10): 

L.append(i+3) 

‣ Complicated formats 

• L = [x + y for x in range(10) for y in range(100, 200)] 

• L = [x for x in range(10) if x % 3 == 1] 



List Comprehensions (Con’t) 

‣ Using list comprehensions on files 

• Strip the new line characters of each line 

lines = [line.rstrip('\n') for line in open('a.txt', 'r')] 

print lines 

• Read lines start with certain requirements 

lines = [line.rstrip('\n') for line in open('a.txt', 'r') if 

line[0].isdigit()] 

print lines 



The lambda Functions 

‣ lambda functions: anonymous functions 

• lambda: arg1, arg2, arg3,..argN: expressions 

‣ A lambda function is an expression, not a 

statement. 

• appear in a list, or assigned to a variable 

‣ A lambda function’s body is a single expression, 

not a block of statements. 

• as simple as the return statement of def 

• limited logic without if, while statements 



The lambda Functions 

‣ Example: a simple function 

f = lambda x: x**2 

print f(2) 

‣ Example: max_of_two() 

max_of_two = lambda x, y: x if x > y else y 

print max_of_two(2, 3) 

‣ Example: Sort a dictionary by value: 

D = {'a':2, 'b':1, 'c':3} 

print sorted(D.items(), key=lambda x: x[1]) 



Exceptions 

‣ Python raises exceptions whenever it detects errors 

in programs at runtime. 

• The exception will be handled by the default exception 

handler, which stops the program and prints an error 

message. 

‣ In order to deal with the errors in your Python 

program, rather than having the program 

terminated, you need to deal with the exceptions. 



The try Statements 

‣ The try statement is a compound statement. 

‣ Basic form: 

try: 

 

except : 

 

except (, ): 

 

except: 

# all other types of exceptions 

 

else: 

# optional 

# if no exception occurs 



The try Statements 

import os 

try: 

os.mkdir('folder') 

except WindowsError: 

print 'folder already exists' 

with open('folder/a.txt', 'w+') as f:

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