Python Program

#!/usr/bin/python
# hello.py

# import modules used here -- sys is a very standard one
import sys

# Gather our code in a main() function
def main():
    print 'Hello there', sys.argv[1]
    # Command line args are in sys.argv[1], sys.argv[2] ...
    # sys.argv[0] is the script name itself and can be ignored

# Standard boilerplate to call the main() function to begin
# the program.
if __name__ == '__main__':
    main()
	

• hash sign (#) is used to start comment
• colon (:) is used to start a block
• indentation used to group block

Function / Indentation

def check(speed,mood):
    if speed >= 80:
        print 'License and registration please'
    if mood == 'terrible' or speed >= 100:
        print 'You have the right to remain silent.'
    elif mood == 'bad' or speed >= 90:
        print "I'm going to have to write you a ticket."
        write_ticket()
    else:
        print "Let's try to keep it under 80 ok?"

• indentation is not optional
• indentation is used to group block
• A logical block of statements should all have the same indentation

Standard Data Types

• Numbers
• String
• List
• Tuple
• Dictionary

Numbers

String

str = 'Hello World!'

print str          # Prints complete string
print str[0]       # Prints first character of the string
print str[2:5]     # Prints characters starting from 3rd to 5th
print str[2:]      # Prints string starting from 3rd character
print str * 2      # Prints string two times
print str + "TEST" # Prints concatenated string
		

This will produce the following result:

Hello World!
H
llo
llo World!
Hello World!Hello World!
Hello World!TEST
    	

List

list = [ 'abcd', 786 , 2.23, 'john', 70.2 ]
tinylist = [123, 'john']

print list          # Prints complete list
print list[0]       # Prints first element of the list
print list[1:3]     # Prints elements starting from 2nd till 3rd 
print list[2:]      # Prints elements starting from 3rd element
print tinylist * 2  # Prints list two times
print list + tinylist # Prints concatenated lists
		

This will produce the following result:

['abcd', 786, 2.23, 'john', 70.2]
abcd
[786, 2.23]
[2.23, 'john', 70.200000000000003]
[123, 'john', 123, 'john']
['abcd', 786, 2.23, 'john', 70.2, 123, 'john']
    	

List Comprehensions

nums = [1, 2, 3, 4]
squares = [n * n for n in nums] 

#The syntax is [ expr for var in list ], result is [1, 4, 9, 16]
		

## Select fruits containing 'a', change to upper case

fruits = ['apple', 'cherry', 'bannana', 'lemon']
afruits = [ s.upper() for s in fruits if 'a' in s ]

## ['APPLE', 'BANNANA']
    	

Tuples

tuple = ( 'abcd', 786 , 2.23, 'john', 70.2  )
tinytuple = (123, 'john')

print tuple           # Prints complete list
print tuple[0]        # Prints first element of the list
print tuple[1:3]      # Prints elements starting from 2nd till 3rd 
print tuple[2:]       # Prints elements starting from 3rd element
print tinytuple * 2   # Prints list two times
print tuple + tinytuple # Prints concatenated lists
		

This will produce the following result:

('abcd', 786, 2.23, 'john', 70.200000000000003)
abcd
(786, 2.23)
(2.23, 'john', 70.200000000000003)
(123, 'john', 123, 'john')
('abcd', 786, 2.23, 'john', 70.200000000000003, 123, 'john')
    	

Tuples

Following is invalid with tuple, because we attempted to update a tuple, which is not allowed. Similar case is possible with lists:

tuple = ( 'abcd', 786 , 2.23, 'john', 70.2  )
list = [ 'abcd', 786 , 2.23, 'john', 70.2  ]
tuple[2] = 1000    # Invalid syntax with tuple
list[2] = 1000     # Valid syntax with list
    	

Dictionary

dict = {}
dict['one'] = "This is one"
dict[2]     = "This is two"

tinydict = {'name': 'john','code':6734, 'dept': 'sales'}

print dict['one']       # Prints value for 'one' key
print dict[2]           # Prints value for 2 key
print tinydict          # Prints complete dictionary
print tinydict.keys()   # Prints all the keys
print tinydict.values() # Prints all the values
		

This will produce the following result:

This is one
This is two
{'dept': 'sales', 'code': 6734, 'name': 'john'}
['dept', 'code', 'name']
['sales', 6734, 'john']
    	

Python Operators

• Arithmetic Operators
• Comparison Operators
• Assignment Operators
• Logical Operators
• Bitwise Operators
• Membership Operators
• Identity Operators

Arithmetic Operators:

Comparison Operators

Assignment Operators

Bitwise Operators

Logical Operators

Membership Operators

Identity Operators

Control Flow

• If..elif..else
• While loop
• For loop

If..elif..else

While loop

For loop

For loop

Range

# print the numbers from 0 through 9
for i in range(10): ## equavilent to "for i in [0,1,2,3,4,5,6,7,8,9]"
  print i
            	

Function / Pass by reference vs value

# Function definition is here
def changeme( mylist ):
   # This changes a passed list into this function
   mylist.append([1,2,3,4]);
   print "Values inside the function: ", mylist
   return

# Now you can call changeme function
mylist = [10,20,30];
changeme( mylist );
print "Values outside the function: ", mylist
	    	

This would produce the following result

Values inside the function:  [10, 20, 30, [1, 2, 3, 4]]
Values outside the function:  [10, 20, 30, [1, 2, 3, 4]]
	    	

Default arguments

# Function definition is here
def printinfo( name, age = 35 ):
   # This prints a passed info into this function
   print 'Name: ', name
   print 'Age ', age
   return

# Now you can call printinfo function
printinfo( age=50, name='miki' )
printinfo( name='miki' )
	    	

This would produce the following result

Name:  miki
Age  50
Name:  miki
Age  35
	    	

The return Statement

# Function definition is here
def sum( arg1, arg2 ):
   # Add both the parameters and return them
   total = arg1 + arg2
   print "Inside the function : ", total
   return total

# Now you can call sum function
total = sum( 10, 20 )
print "Outside the function : ", total
	    	

This would produce the following result

Inside the function :  30
Outside the function :  30
	    	

File I/O

cat(filename)

# Echo the contents of a file
def cat(filename):
   f = open(filename, 'r')
   for line in f: # iterates over the lines of the file
      print line, # trailing , so print does not add an end-of-line char
	              # since 'line' already includes the end-of line.
   f.close()
	    	

File IO

readlines(): read file into a list, each element of list is a line in the file.

def cat(filename):
  f = open(filename,'r')
  list_of_lines = f.readlines()
  print list_of_lines
  f.close()
	    	

File IO

write(str): writes a string str to the file.

def write_file(filename,str):
  f = open(filename,'w')
  f.write(str)
  f.close()
	    	

Regular Expressions

import re

str = 'an example word:cat!!'
match = re.search(r'word:\w\w\w', str)
# If-statement after search() tests if it succeeded
if match:                      
  print 'found', match.group() ## 'found word:cat'
else:
  print 'did not find'
				

findall

import re

# Suppose we have a text with many email addresses
str = 'purple alice@google.com, blah monkey bob@abc.com blah dishwasher'

# Here re.findall() returns a list of all the found email strings
# ['alice@google.com', 'bob@abc.com']
emails = re.findall(r'[\w\.-]+@[\w\.-]+', str)
for email in emails:
  # do something with each found email string
  print email