A Byte of Python

A Byte of Python

$ python -V
Python 2.7.6
print "Hello World"
$ python
Python 2.7.6 (default, Feb 23 2014, 16:08:15)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print "hello world"
hello world
>>>
print "hello world"
print "hello world"
$ python hello.py
hello world
print 'hello world' # Note that print is a statement
# Note that print is a statement
print 'hello world'
'''This is a multi-line string. This is the first line.
This is the second line.
"What's your name?," I asked.
He said "Bond, James Bond."
'''
age = 20
name = 'Swaroop'

print '{0} was {1} years old when he wrote this book'.format(name, age)
print 'Why is {0} playing with that python?'.format(name)
$ python str_format.py
Swaroop was 20 years old when he wrote this book
Why is Swaroop playing with that python?
name + ' is ' + str(age) + ' years old'
age = 20
name = 'Swaroop'

print '{} was {} years old when he wrote this book'.format(name, age)
print 'Why is {} playing with that python?'.format(name)
# 소수점 이하 셋째 자리까지 부동 소숫점 숫자 표기 (0.333)
print '{0:.3f}'.format(1.0/3)
# 밑줄(_)로 11칸을 채우고 가운데 정렬(^)하기 (___hello___)
print '{0:_^11}'.format('hello')
# 사용자 지정 키워드를 이용해 (Swaroop wrote A Byte of Python) 표기
print '{name} wrote {book}'.format(name='Swaroop',
                                   book='A Byte of Python')
0.333
___hello___
Swaroop wrote A Byte of Python
print "a",
print "b",
a b
'This is the first line\nThis is the second line'
"This is the first sentence. \
This is the second sentence."
"This is the first sentence. This is the second sentence."
r"Newlines are indicated by \n"
# Filename : var.py
i = 5
print i
i = i + 1
print i

s = '''This is a multi-line string.
This is the second line.'''
print s
5
6
This is a multi-line string.
This is the second line.
i = 5
print i
i = 5;
print i;
i = 5; print i;
i = 5; print i
s = 'This is a string. \
This continues the string.'
print s
This is a string. This continues the string.
print \
i
print i
i = 5
# 다음 행에서 오류가 발생합니다! 행 앞에 잘못된 공백이 한 칸 있습니다.
 print 'Value is ', i
print 'I repeat, the value is ', i
  File "whitespace.py", line 5
    print 'Value is ', i
    ^
IndentationError: unexpected indent
>>> 2 + 3
5
>>> 3 * 5
15
>>>
a = 2
a = a * 3
a = 2
a *= 3
length = 5
breadth = 2

area = length * breadth
print 'Area is', area
print 'Perimeter is', 2 * (length + breadth)
$ python expression.py
Area is 10
Perimeter is 14
number = 23
guess = int(raw_input('Enter an integer : '))

if guess == number:
    # New block starts here
    print 'Congratulations, you guessed it.'
    print '(but you do not win any prizes!)'
    # New block ends here
elif guess < number:
    # Another block
    print 'No, it is a little higher than that'
    # You can do whatever you want in a block ...
else:
    print 'No, it is a little lower than that'
    # you must have guessed > number to reach here

print 'Done'
# This last statement is always executed,
# after the if statement is executed.
$ python if.py
Enter an integer : 50
No, it is a little lower than that
Done

$ python if.py
Enter an integer : 22
No, it is a little higher than that
Done

$ python if.py
Enter an integer : 23
Congratulations, you guessed it.
(but you do not win any prizes!)
Done
if True:
    print 'Yes, it is true'
number = 23
running = True

while running:
    guess = int(raw_input('Enter an integer : '))

    if guess == number:
        print 'Congratulations, you guessed it.'
        # this causes the while loop to stop
        running = False
    elif guess < number:
        print 'No, it is a little higher than that.'
    else:
        print 'No, it is a little lower than that.'
else:
    print 'The while loop is over.'
    # Do anything else you want to do here

print 'Done'
$ python while.py
Enter an integer : 50
No, it is a little lower than that.
Enter an integer : 22
No, it is a little higher than that.
Enter an integer : 23
Congratulations, you guessed it.
The while loop is over.
Done
for i in range(1, 5):
    print i
else:
    print 'The for loop is over'
$ python for.py
1
2
3
4
The for loop is over
while True:
    s = raw_input('Enter something : ')
    if s == 'quit':
        break
    print 'Length of the string is', len(s)
print 'Done'
$ python break.py
Enter something : Programming is fun
Length of the string is 18
Enter something : When the work is done
Length of the string is 21
Enter something : if you wanna make your work also fun:
Length of the string is 37
Enter something : use Python!
Length of the string is 11
Enter something : quit
Done
Programming is fun
When the work is done
if you wanna make your work also fun:
    use Python!
while True:
    s = raw_input('Enter something : ')
    if s == 'quit':
        break
    if len(s) < 3:
        print 'Too small'
        continue
    print 'Input is of sufficient length'
    # Do other kinds of processing here...
$ python continue.py
Enter something : a
Too small
Enter something : 12
Too small
Enter something : abc
Input is of sufficient length
Enter something : quit
def say_hello():
    # block belonging to the function
    print 'hello world'
# End of function

say_hello() # call the function
say_hello() # call the function again
$ python function1.py
hello world
hello world
def print_max(a, b):
    if a > b:
        print a, 'is maximum'
    elif a == b:
        print a, 'is equal to', b
    else:
        print b, 'is maximum'

# directly pass literal values
print_max(3, 4)

x = 5
y = 7

# pass variables as arguments
print_max(x, y)
$ python function_param.py
4 is maximum
7 is maximum
x = 50

def func(x):
    print 'x is', x
    x = 2
    print 'Changed local x to', x

func(x)
print 'x is still', x
$ python function_local.py
x is 50
Changed local x to 2
x is still 50
x = 50

def func():
    global x

    print 'x is', x
    x = 2
    print 'Changed global x to', x

func()
print 'Value of x is', x
$ python function_global.py
x is 50
Changed global x to 2
Value of x is 2
def say(message, times=1):
    print message * times

say('Hello')
say('World', 5)
$ python function_default.py
Hello
WorldWorldWorldWorldWorld
def func(a, b=5, c=10):
    print 'a is', a, 'and b is', b, 'and c is', c

func(3, 7)
func(25, c=24)
func(c=50, a=100)
$ python function_keyword.py
a is 3 and b is 7 and c is 10
a is 25 and b is 5 and c is 24
a is 100 and b is 5 and c is 50
def total(initial=5, *numbers, **keywords):
    count = initial
    for number in numbers:
        count += number
    for key in keywords:
        count += keywords[key]
    return count

print total(10, 1, 2, 3, vegetables=50, fruits=100)
$ python function_varargs.py
166
def maximum(x, y):
    if x > y:
        return x
    elif x == y:
        return 'The numbers are equal'
    else:
        return y

print maximum(2, 3)
$ python function_return.py
3
def some_function():
    pass
def print_max(x, y):
    '''Prints the maximum of two numbers.

    The two values must be integers.'''
    # convert to integers, if possible
    x = int(x)
    y = int(y)

    if x > y:
        print x, 'is maximum'
    else:
        print y, 'is maximum'

print_max(3, 5)
print print_max.__doc__
$ python function_docstring.py
5 is maximum
Prints the maximum of two numbers.

    The two values must be integers.
import sys

print('The command line arguments are:')
for i in sys.argv:
    print i

print '\n\nThe PYTHONPATH is', sys.path, '\n'
$ python module_using_sys.py we are arguments
The command line arguments are:
module_using_sys.py
we
are
arguments

The PYTHONPATH is ['/tmp/py',
# many entries here, not shown here
'/Library/Python/2.7/site-packages',
'/usr/local/lib/python2.7/site-packages']
from math import sqrt
print "Square root of 16 is", sqrt(16)
if __name__ == '__main__':
    print 'This program is being run by itself'
else:
    print 'I am being imported from another module'
$ python module_using_name.py
This program is being run by itself

$ python
>>> import module_using_name
I am being imported from another module
>>>
def say_hi():
    print 'Hi, this is mymodule speaking.'

__version__ = '0.1'
import mymodule

mymodule.say_hi()
print 'Version', mymodule.__version__
$ python mymodule_demo.py
Hi, this is mymodule speaking.
Version 0.1
from mymodule import say_hi, __version__

say_hi()
print 'Version', __version__
from mymodule import *
$ python
>>> import sys

# sys 모듈 내에 선언된 속성들의 식별자 이름 목록
>>> dir(sys)
['__displayhook__', '__doc__',
'argv', 'builtin_module_names',
'version', 'version_info']
# 실제로는 너무 길어 여기에는 몇 개만 적었음

# 현재 모듈에 선언된 속성들의 식별자 이름 목록
>>> dir()
['__builtins__', '__doc__',
'__name__', '__package__']

# 새로운 변수 'a' 생성
>>> a = 5

>>> dir()
['__builtins__', '__doc__', '__name__', '__package__', 'a']

# 식별자 이름 제거
>>> del a

>>> dir()
['__builtins__', '__doc__', '__name__', '__package__']
- <some folder present in the sys.path>/
    - world/
        - __init__.py
        - asia/
            - __init__.py
            - india/
                - __init__.py
                - foo.py
        - africa/
            - __init__.py
            - madagascar/
                - __init__.py
                - bar.py
# This is my shopping list
shoplist = ['apple', 'mango', 'carrot', 'banana']

print 'I have', len(shoplist), 'items to purchase.'

print 'These items are:',
for item in shoplist:
    print item,

print '\nI also have to buy rice.'
shoplist.append('rice')
print 'My shopping list is now', shoplist

print 'I will sort my list now'
shoplist.sort()
print 'Sorted shopping list is', shoplist

print 'The first item I will buy is', shoplist[0]
olditem = shoplist[0]
del shoplist[0]
print 'I bought the', olditem
print 'My shopping list is now', shoplist
$ python ds_using_list.py
I have 4 items to purchase.
These items are: apple mango carrot banana
I also have to buy rice.
My shopping list is now ['apple', 'mango', 'carrot', 'banana', 'rice']
I will sort my list now
Sorted shopping list is ['apple', 'banana', 'carrot', 'mango', 'rice']
The first item I will buy is apple
I bought the apple
My shopping list is now ['banana', 'carrot', 'mango', 'rice']
# I would recommend always using parentheses
# to indicate start and end of tuple
# even though parentheses are optional.
# Explicit is better than implicit.
zoo = ('python', 'elephant', 'penguin')
print 'Number of animals in the zoo is', len(zoo)

new_zoo = 'monkey', 'camel', zoo
print 'Number of cages in the new zoo is', len(new_zoo)
print 'All animals in new zoo are', new_zoo
print 'Animals brought from old zoo are', new_zoo[2]
print 'Last animal brought from old zoo is', new_zoo[2][2]
print 'Number of animals in the new zoo is', \
    len(new_zoo)-1+len(new_zoo[2])
$ python ds_using_tuple.py
Number of animals in the zoo is 3
Number of cages in the new zoo is 3
All animals in new zoo are ('monkey', 'camel', ('python', 'elephant', 'penguin'))
Animals brought from old zoo are ('python', 'elephant', 'penguin')
Last animal brought from old zoo is penguin
Number of animals in the new zoo is 5
# 'ab' is short for 'a'ddress'b'ook

ab = {  'Swaroop'   : 'swaroop@swaroopch.com',
        'Larry'     : 'larry@wall.org',
        'Matsumoto' : 'matz@ruby-lang.org',
        'Spammer'   : 'spammer@hotmail.com'
    }

print "Swaroop's address is", ab['Swaroop']

# Deleting a key-value pair
del ab['Spammer']

print '\nThere are {} contacts in the address-book\n'.format(len(ab))

for name, address in ab.items():
    print 'Contact {} at {}'.format(name, address)

# Adding a key-value pair
ab['Guido'] = 'guido@python.org'

if 'Guido' in ab:
    print "\nGuido's address is", ab['Guido']
$ python ds_using_dict.py
Swaroop's address is swaroop@swaroopch.com

There are 3 contacts in the address-book

Contact Swaroop at swaroop@swaroopch.com
Contact Matsumoto at matz@ruby-lang.org
Contact Larry at larry@wall.org

Guido's address is guido@python.org
shoplist = ['apple', 'mango', 'carrot', 'banana']
name = 'swaroop'

# Indexing or 'Subscription' operation #
print 'Item 0 is', shoplist[0]
print 'Item 1 is', shoplist[1]
print 'Item 2 is', shoplist[2]
print 'Item 3 is', shoplist[3]
print 'Item -1 is', shoplist[-1]
print 'Item -2 is', shoplist[-2]
print 'Character 0 is', name[0]

# Slicing on a list #
print 'Item 1 to 3 is', shoplist[1:3]
print 'Item 2 to end is', shoplist[2:]
print 'Item 1 to -1 is', shoplist[1:-1]
print 'Item start to end is', shoplist[:]

# Slicing on a string #
print 'characters 1 to 3 is', name[1:3]
print 'characters 2 to end is', name[2:]
print 'characters 1 to -1 is', name[1:-1]
print 'characters start to end is', name[:]
$ python ds_seq.py
Item 0 is apple
Item 1 is mango
Item 2 is carrot
Item 3 is banana
Item -1 is banana
Item -2 is carrot
Character 0 is s
Item 1 to 3 is ['mango', 'carrot']
Item 2 to end is ['carrot', 'banana']
Item 1 to -1 is ['mango', 'carrot']
Item start to end is ['apple', 'mango', 'carrot', 'banana']
characters 1 to 3 is wa
characters 2 to end is aroop
characters 1 to -1 is waroo
characters start to end is swaroop
>>> shoplist = ['apple', 'mango', 'carrot', 'banana']
>>> shoplist[::1]
['apple', 'mango', 'carrot', 'banana']
>>> shoplist[::2]
['apple', 'carrot']
>>> shoplist[::3]
['apple', 'banana']
>>> shoplist[::-1]
['banana', 'carrot', 'mango', 'apple']
>>> bri = set(['brazil', 'russia', 'india'])
>>> 'india' in bri
True
>>> 'usa' in bri
False
>>> bric = bri.copy()
>>> bric.add('china')
>>> bric.issuperset(bri)
True
>>> bri.remove('russia')
>>> bri & bric # OR bri.intersection(bric)
{'brazil', 'india'}
print 'Simple Assignment'
shoplist = ['apple', 'mango', 'carrot', 'banana']
# mylist is just another name pointing to the same object!
mylist = shoplist

# I purchased the first item, so I remove it from the list
del shoplist[0]

print 'shoplist is', shoplist
print 'mylist is', mylist
# Notice that both shoplist and mylist both print
# the same list without the 'apple' confirming that
# they point to the same object

print 'Copy by making a full slice'
# Make a copy by doing a full slice
mylist = shoplist[:]
# Remove first item
del mylist[0]

print 'shoplist is', shoplist
print 'mylist is', mylist
# Notice that now the two lists are different
$ python ds_reference.py
Simple Assignment
shoplist is ['mango', 'carrot', 'banana']
mylist is ['mango', 'carrot', 'banana']
Copy by making a full slice
shoplist is ['mango', 'carrot', 'banana']
mylist is ['carrot', 'banana']
# This is a string object
name = 'Swaroop'

if name.startswith('Swa'):
    print 'Yes, the string starts with "Swa"'

if 'a' in name:
    print 'Yes, it contains the string "a"'

if name.find('war') != -1:
    print 'Yes, it contains the string "war"'

delimiter = '_*_'
mylist = ['Brazil', 'Russia', 'India', 'China']
print delimiter.join(mylist)
$ python ds_str_methods.py
Yes, the string starts with "Swa"
Yes, it contains the string "a"
Yes, it contains the string "war"
Brazil_*_Russia_*_India_*_China
import os
import time

# 1. The files and directories to be backed up are
# specified in a list.
# Example on Windows:
# source = ['"C:\\My Documents"', 'C:\\Code']
# Example on Mac OS X and Linux:
source = ['/Users/swa/notes']
# Notice we had to use double quotes inside the string
# for names with spaces in it.

# 2. The backup must be stored in a
# main backup directory
# Example on Windows:
# target_dir = 'E:\\Backup'
# Example on Mac OS X and Linux:
target_dir = '/Users/swa/backup'
# Remember to change this to which folder you will be using

# 3. The files are backed up into a zip file.
# 4. The name of the zip archive is the current date and time
target = target_dir + os.sep + \
         time.strftime('%Y%m%d%H%M%S') + '.zip'

# Create target directory if it is not present
if not os.path.exists(target_dir):
    os.mkdir(target_dir) # make directory

# 5. We use the zip command to put the files in a zip archive
zip_command = "zip -r {0} {1}".format(target,
                                      ' '.join(source))

# Run the backup
print "Zip command is:"
print zip_command
print "Running:"
if os.system(zip_command) == 0:
    print 'Successful backup to', target
else:
    print 'Backup FAILED'
$ python backup_ver1.py
Zip command is:
zip -r /Users/swa/backup/20140328084844.zip /Users/swa/notes
Running:
  adding: Users/swa/notes/ (stored 0%)
  adding: Users/swa/notes/blah1.txt (stored 0%)
  adding: Users/swa/notes/blah2.txt (stored 0%)
  adding: Users/swa/notes/blah3.txt (stored 0%)
Successful backup to /Users/swa/backup/20140328084844.zip
import os
import time

# 1. The files and directories to be backed up are
# specified in a list.
# Example on Windows:
# source = ['"C:\\My Documents"', 'C:\\Code']
# Example on Mac OS X and Linux:
source = ['/Users/swa/notes']
# Notice we had to use double quotes inside the string
# for names with spaces in it.

# 2. The backup must be stored in a
# main backup directory
# Example on Windows:
# target_dir = 'E:\\Backup'
# Example on Mac OS X and Linux:
target_dir = '/Users/swa/backup'
# Remember to change this to which folder you will be using

# Create target directory if it is not present
if not os.path.exists(target_dir):
    os.mkdir(target_dir) # make directory

# 3. The files are backed up into a zip file.
# 4. The current day is the name of the subdirectory
# in the main directory.
today = target_dir + os.sep + time.strftime('%Y%m%d')
# The current time is the name of the zip archive.
now = time.strftime('%H%M%S')

# The name of the zip file
target = today + os.sep + now + '.zip'

# Create the subdirectory if it isn't already there
if not os.path.exists(today):
    os.mkdir(today)
    print 'Successfully created directory', today

# 5. We use the zip command to put the files in a zip archive
zip_command = "zip -r {0} {1}".format(target,
                                      ' '.join(source))

# Run the backup
print "Zip command is:"
print zip_command
print "Running:"
if os.system(zip_command) == 0:
    print 'Successful backup to', target
else:
    print 'Backup FAILED'
$ python backup_ver2.py
Successfully created directory /Users/swa/backup/20140329
Zip command is:
zip -r /Users/swa/backup/20140329/073201.zip /Users/swa/notes
Running:
  adding: Users/swa/notes/ (stored 0%)
  adding: Users/swa/notes/blah1.txt (stored 0%)
  adding: Users/swa/notes/blah2.txt (stored 0%)
  adding: Users/swa/notes/blah3.txt (stored 0%)
Successful backup to /Users/swa/backup/20140329/073201.zip
import os
import time

# 1. The files and directories to be backed up are
# specified in a list.
# Example on Windows:
# source = ['"C:\\My Documents"', 'C:\\Code']
# Example on Mac OS X and Linux:
source = ['/Users/swa/notes']
# Notice we had to use double quotes inside the string
# for names with spaces in it.

# 2. The backup must be stored in a
# main backup directory
# Example on Windows:
# target_dir = 'E:\\Backup'
# Example on Mac OS X and Linux:
target_dir = '/Users/swa/backup'
# Remember to change this to which folder you will be using

# Create target directory if it is not present
if not os.path.exists(target_dir):
    os.mkdir(target_dir) # make directory

# 3. The files are backed up into a zip file.
# 4. The current day is the name of the subdirectory
# in the main directory.
today = target_dir + os.sep + time.strftime('%Y%m%d')
# The current time is the name of the zip archive.
now = time.strftime('%H%M%S')

# Take a comment from the user to
# create the name of the zip file
comment = raw_input('Enter a comment --> ')
# Check if a comment was entered
if len(comment) == 0:
    target = today + os.sep + now + '.zip'
else:
    target = today + os.sep + now + '_' +
        comment.replace(' ', '_') + '.zip'

# Create the subdirectory if it isn't already there
if not os.path.exists(today):
    os.mkdir(today)
    print 'Successfully created directory', today

# 5. We use the zip command to put the files in a zip archive
zip_command = "zip -r {0} {1}".format(target,
                                      ' '.join(source))

# Run the backup
print "Zip command is:"
print zip_command
print "Running:"
if os.system(zip_command) == 0:
    print 'Successful backup to', target
else:
    print 'Backup FAILED'
$ python backup_ver3.py
  File "backup_ver3.py", line 39
    target = today + os.sep + now + '_' +
                                        ^
SyntaxError: invalid syntax
import os
import time

# 1. The files and directories to be backed up are
# specified in a list.
# Example on Windows:
# source = ['"C:\\My Documents"', 'C:\\Code']
# Example on Mac OS X and Linux:
source = ['/Users/swa/notes']
# Notice we had to use double quotes inside the string
# for names with spaces in it.

# 2. The backup must be stored in a
# main backup directory
# Example on Windows:
# target_dir = 'E:\\Backup'
# Example on Mac OS X and Linux:
target_dir = '/Users/swa/backup'
# Remember to change this to which folder you will be using

# Create target directory if it is not present
if not os.path.exists(target_dir):
    os.mkdir(target_dir) # make directory

# 3. The files are backed up into a zip file.
# 4. The current day is the name of the subdirectory
# in the main directory.
today = target_dir + os.sep + time.strftime('%Y%m%d')
# The current time is the name of the zip archive.
now = time.strftime('%H%M%S')

# Take a comment from the user to
# create the name of the zip file
comment = raw_input('Enter a comment --> ')
# Check if a comment was entered
if len(comment) == 0:
    target = today + os.sep + now + '.zip'
else:
    target = today + os.sep + now + '_' + \
        comment.replace(' ', '_') + '.zip'

# Create the subdirectory if it isn't already there
if not os.path.exists(today):
    os.mkdir(today)
    print 'Successfully created directory', today

# 5. We use the zip command to put the files in a zip archive
zip_command = "zip -r {0} {1}".format(target,
                                      ' '.join(source))

# Run the backup
print "Zip command is:"
print zip_command
print "Running:"
if os.system(zip_command) == 0:
    print 'Successful backup to', target
else:
    print 'Backup FAILED'
$ python backup_ver4.py
Enter a comment --> added new examples
Zip command is:
zip -r /Users/swa/backup/20140329/074122_added_new_examples.zip /Users/swa/notes
Running:
  adding: Users/swa/notes/ (stored 0%)
  adding: Users/swa/notes/blah1.txt (stored 0%)
  adding: Users/swa/notes/blah2.txt (stored 0%)
  adding: Users/swa/notes/blah3.txt (stored 0%)
Successful backup to /Users/swa/backup/20140329/074122_added_new_examples.zip
class Person:
    pass # An empty block

p = Person()
print(p)
$ python oop_simplestclass.py
<__main__.Person instance at 0x10171f518>
class Person:
    def say_hi(self):
        print('Hello, how are you?')

p = Person()
p.say_hi()
# The previous 2 lines can also be written as
# Person().say_hi()
$ python oop_method.py
Hello, how are you?
class Person:
    def __init__(self, name):
        self.name = name
    def say_hi(self):
        print 'Hello, my name is', self.name

p = Person('Swaroop')
p.say_hi()
# The previous 2 lines can also be written as
# Person('Swaroop').say_hi()
$ python oop_init.py
Hello, my name is Swaroop
class Robot:
    """Represents a robot, with a name."""

    # A class variable, counting the number of robots
    population = 0

    def __init__(self, name):
        """Initializes the data."""
        self.name = name
        print "(Initializing {})".format(self.name)

        # When this person is created, the robot
        # adds to the population
        Robot.population += 1

    def die(self):
        """I am dying."""
        print "{} is being destroyed!".format(self.name)

        Robot.population -= 1

        if Robot.population == 0:
            print "{} was the last one.".format(self.name)
        else:
            print "There are still {:d} robots working.".format(
                Robot.population)

    def say_hi(self):
        """Greeting by the robot.

        Yeah, they can do that."""
        print "Greetings, my masters call me {}.".format(self.name)

    @classmethod
    def how_many(cls):
        """Prints the current population."""
        print "We have {:d} robots.".format(cls.population)

droid1 = Robot("R2-D2")
droid1.say_hi()
Robot.how_many()

droid2 = Robot("C-3PO")
droid2.say_hi()
Robot.how_many()

print "\nRobots can do some work here.\n"

print "Robots have finished their work. So let's destroy them."
droid1.die()
droid2.die()

Robot.how_many()
$ python oop_objvar.py
(Initializing R2-D2)
Greetings, my masters call me R2-D2.
We have 1 robots.
(Initializing C-3PO)
Greetings, my masters call me C-3PO.
We have 2 robots.

Robots can do some work here.

Robots have finished their work. So let's destroy them.
R2-D2 is being destroyed!
There are still 1 robots working.
C-3PO is being destroyed!
C-3PO was the last one.
We have 0 robots.
how_many = classmethod(how_many)
class SchoolMember:
    '''Represents any school member.'''
    def __init__(self, name, age):
        self.name = name
        self.age = age
        print '(Initialized SchoolMember: {})'.format(self.name)

    def tell(self):
        '''Tell my details.'''
        print 'Name:"{}" Age:"{}"'.format(self.name, self.age),

class Teacher(SchoolMember):
    '''Represents a teacher.'''
    def __init__(self, name, age, salary):
        SchoolMember.__init__(self, name, age)
        self.salary = salary
        print '(Initialized Teacher: {})'.format(self.name)

    def tell(self):
        SchoolMember.tell(self)
        print 'Salary: "{:d}"'.format(self.salary)

class Student(SchoolMember):
    '''Represents a student.'''
    def __init__(self, name, age, marks):
        SchoolMember.__init__(self, name, age)
        self.marks = marks
        print '(Initialized Student: {})'.format(self.name)

    def tell(self):
        SchoolMember.tell(self)
        print 'Marks: "{:d}"'.format(self.marks)

t = Teacher('Mrs. Shrividya', 40, 30000)
s = Student('Swaroop', 25, 75)

# prints a blank line
print

members = [t, s]
for member in members:
    # Works for both Teachers and Students
    member.tell()
$ python oop_subclass.py
(Initialized SchoolMember: Mrs. Shrividya)
(Initialized Teacher: Mrs. Shrividya)
(Initialized SchoolMember: Swaroop)
(Initialized Student: Swaroop)

Name:"Mrs. Shrividya" Age:"40" Salary: "30000"
Name:"Swaroop" Age:"25" Marks: "75"
def reverse(text):
    return text[::-1]

def is_palindrome(text):
    return text == reverse(text)

something = raw_input("Enter text: ")
if is_palindrome(something):
    print "Yes, it is a palindrome"
else:
    print "No, it is not a palindrome"
$ python io_input.py
Enter text: sir
No, it is not a palindrome

$ python io_input.py
Enter text: madam
Yes, it is a palindrome

$ python io_input.py
Enter text: racecar
Yes, it is a palindrome
poem = '''\
Programming is fun
When the work is done
if you wanna make your work also fun:
    use Python!
'''

# Open for 'w'riting
f = open('poem.txt', 'w')
# Write text to file
f.write(poem)
# Close the file
f.close()

# If no mode is specified,
# 'r'ead mode is assumed by default
f = open('poem.txt')
while True:
    line = f.readline()
    # Zero length indicates EOF
    if len(line) == 0:
        break
    # The `line` already has a newline
    # at the end of each line
    # since it is reading from a file.
    print line,
# close the file
f.close()
$ python io_using_file.py
Programming is fun
When the work is done
if you wanna make your work also fun:
    use Python!
import pickle

# The name of the file where we will store the object
shoplistfile = 'shoplist.data'
# The list of things to buy
shoplist = ['apple', 'mango', 'carrot']

# Write to the file
f = open(shoplistfile, 'wb')
# Dump the object to a file
pickle.dump(shoplist, f)
f.close()

# Destroy the shoplist variable
del shoplist

# Read back from the storage
f = open(shoplistfile, 'rb')
# Load the object from the file
storedlist = pickle.load(f)
print storedlist
$ python io_pickle.py
['apple', 'mango', 'carrot']
>>> "hello world"
'hello world'
>>> type("hello world")
<type 'str'>
>>> u"hello world"
u'hello world'
>>> type(u"hello world")
<type 'unicode'>
# encoding=utf-8
import io

f = io.open("abc.txt", "wt", encoding="utf-8")
f.write(u"Imagine non-English language here")
f.close()

text = io.open("abc.txt", encoding="utf-8").read()
print text
>>> Print "Hello World"
  File "<stdin>", line 1
    Print "Hello World"
                      ^
SyntaxError: invalid syntax
>>> print "Hello World"
Hello World
>>> s = raw_input('Enter something --> ')
Enter something --> Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
EOFError
try:
    text = raw_input('Enter something --> ')
except EOFError:
    print 'Why did you do an EOF on me?'
except KeyboardInterrupt:
    print 'You cancelled the operation.'
else:
    print 'You entered {}'.format(text)
# Press ctrl + d
$ python exceptions_handle.py
Enter something --> Why did you do an EOF on me?

# Press ctrl + c
$ python exceptions_handle.py
Enter something --> ^CYou cancelled the operation.

$ python exceptions_handle.py
Enter something --> No exceptions
You entered No exceptions
class ShortInputException(Exception):
    '''A user-defined exception class.'''
    def __init__(self, length, atleast):
        Exception.__init__(self)
        self.length = length
        self.atleast = atleast

try:
    text = raw_input('Enter something --> ')
    if len(text) < 3:
        raise ShortInputException(len(text), 3)
    # Other work can continue as usual here
except EOFError:
    print 'Why did you do an EOF on me?'
except ShortInputException as ex:
    print ('ShortInputException: The input was ' + \
           '{0} long, expected at least {1}')\
          .format(ex.length, ex.atleast)
else:
    print 'No exception was raised.'
$ python exceptions_raise.py
Enter something --> a
ShortInputException: The input was 1 long, expected at least 3

$ python exceptions_raise.py
Enter something --> abc
No exception was raised.
import sys
import time

f = None
try:
    f = open("poem.txt")
    # Our usual file-reading idiom
    while True:
        line = f.readline()
        if len(line) == 0:
            break
        print line,
        sys.stdout.flush()
        print "Press ctrl+c now"
        # To make sure it runs for a while
        time.sleep(2)
except IOError:
    print "Could not find file poem.txt"
except KeyboardInterrupt:
    print "!! You cancelled the reading from the file."
finally:
    if f:
        f.close()
    print "(Cleaning up: Closed the file)"
$ python exceptions_finally.py
Programming is fun
Press ctrl+c now
^C!! You cancelled the reading from the file.
(Cleaning up: Closed the file)
with open("poem.txt") as f:
    for line in f:
        print line,
$ python
>>> import sys
>>> sys.version_info
sys.version_info(major=2, minor=7, micro=6, releaselevel='final', serial=0)
>>> sys.version_info.major == 2
True
import os, platform, logging

if platform.platform().startswith('Windows'):
    logging_file = os.path.join(os.getenv('HOMEDRIVE'),
                                os.getenv('HOMEPATH'),
                                'test.log')
else:
    logging_file = os.path.join(os.getenv('HOME'),
                                'test.log')

print "Logging to", logging_file

logging.basicConfig(
    level=logging.DEBUG,
    format='%(asctime)s : %(levelname)s : %(message)s',
    filename = logging_file,
    filemode = 'w',
)

logging.debug("Start of the program")
logging.info("Doing something")
logging.warning("Dying now")
$ python stdlib_logging.py
Logging to /Users/swa/test.log

$ cat /Users/swa/test.log
2014-03-29 09:27:36,660 : DEBUG : Start of the program
2014-03-29 09:27:36,660 : INFO : Doing something
2014-03-29 09:27:36,660 : WARNING : Dying now
>>> def get_error_details():
...     return (2, 'details')
...
>>> errnum, errstr = get_error_details()
>>> errnum
2
>>> errstr
'details'
>>> a = 5; b = 8
>>> a, b
(5, 8)
>>> a, b = b, a
>>> a, b
(8, 5)
>>> flag = True
>>> if flag: print 'Yes'
...
Yes
points = [ { 'x' : 2, 'y' : 3 },
           { 'x' : 4, 'y' : 1 } ]
points.sort(key=lambda i : i['y'])
print points
$ python more_lambda.py
[{'y': 1, 'x': 4}, {'y': 3, 'x': 2}]
listone = [2, 3, 4]
listtwo = [2*i for i in listone if i > 2]
print listtwo
$ python more_list_comprehension.py
[6, 8]
>>> def powersum(power, *args):
...     '''Return the sum of each argument raised to the specified power.'''
...     total = 0
...     for i in args:
...         total += pow(i, power)
...     return total
...
>>> powersum(2, 3, 4)
25
>>> powersum(2, 10)
100
>>> mylist = ['item']
>>> assert len(mylist) >= 1
>>> mylist.pop()
'item'
>>> assert len(mylist) >= 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
from time import sleep
from functools import wraps
import logging
logging.basicConfig()
log = logging.getLogger("retry")

def retry(f):
    @wraps(f)
    def wrapped_f(*args, **kwargs):
        MAX_ATTEMPTS = 5
        for attempt in range(1, MAX_ATTEMPTS + 1):
            try:
                return f(*args, **kwargs)
            except:
                log.exception("Attempt %s/%s failed : %s",
                              attempt,
                              MAX_ATTEMPTS,
                              (args, kwargs))
                sleep(10 * attempt)
        log.critical("All %s attempts failed : %s",
                     MAX_ATTEMPTS,
                     (args, kwargs))
    return wrapped_f

counter = 0

@retry
def save_to_database(arg):
    print "Write to a database or make a network call or etc."
    print "This will be automatically retried if exception is thrown."
    global counter
    counter += 1
    # This will throw an exception in the first call
    # And will work fine in the second call (i.e. a retry)
    if counter < 2:
        raise ValueError(arg)

if __name__ == '__main__':
    save_to_database("Some bad value")
$ python more_decorator.py
Write to a database or make a network call or etc.
This will be automatically retried if exception is thrown.
ERROR:retry:Attempt 1/5 failed : (('Some bad value',), {})
Traceback (most recent call last):
  File "more_decorator.py", line 14, in wrapped_f
    return f(*args, **kwargs)
  File "more_decorator.py", line 39, in save_to_database
    raise ValueError(arg)
ValueError: Some bad value
Write to a database or make a network call or etc.
This will be automatically retried if exception is thrown.