Python

>>> import sys, pprint

>>> pprint.pprint(sys.path)

This returns a list of places where python will look for modules, so you can save a module in any of these places(although you should create your own folder and add this to your path for any modules you create)  and import a module  using import

['',

 'C:\\Python27\\Lib\\idlelib',

 'C:\\my_python_modules',

 'C:\\WINDOWS\\system32\\python27.zip',

 'C:\\Python27\\DLLs',

 'C:\\Python27\\lib',

 'C:\\Python27\\lib\\plat-win',

 'C:\\Python27\\lib\\lib-tk',

 'C:\\Python27',

 'C:\\Python27\\lib\\site-packages',

 'C:\\Python27\\lib\\site-packages\\wx-2.8-msw-unicode']

Type help() for interactive help, or help(object) for help about object.
>>> help()
Welcome to Python 2.7!  This is the online help utility.
 
If this is your first time using Python, you should definitely check out
the tutorial on the Internet at http://docs.python.org/tutorial/.
 
Enter the name of any module, keyword, or topic to get help on writing
Python programs and using Python modules.  To quit this help utility and
return to the interpreter, just type "quit".
 
To get a list of available modules, keywords, or topics, type "modules",
"keywords", or "topics".  Each module also comes with a one-line summary
of what it does; to list the modules whose summaries contain a given word
such as "spam", type "modules spam".
 
To find out information on built-in function, type the function name after the  help> prompt. Here we will find out about the function open().
help> open
Help on built-in function open in module __builtin__:
 
open(...)
    open(name[, mode[, buffering]]) -> file object
   
    Open a file using the file() type, returns a file object.  This is the
    preferred way to open a file.  See file.__doc__ for further information.

An exception is what python uses to flag a exceptional behavior in a program. These are objects that if not dealt with by the program will terminate the program with an error or traceback.

>>> 1/0
Traceback (most recent call last):
File "", line 1, in
1/0
ZeroDivisionError: integer division or modulo by zero
>>>

As you can see a ZeroDivisionError has been raised. To see a list of all the exception available to you use:

>>> import exceptions
>>> dir(exceptions)
['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BufferError', 'BytesWarning', 'DeprecationWarning', 'EOFError', 'EnvironmentError', 'Exception', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'NameError', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'ReferenceError', 'RuntimeError', 'RuntimeWarning', 'StandardError', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'WindowsError', 'ZeroDivisionError', '__doc__', '__name__', '__package__']
>>>


Exception handling allows the programmer to catch the error and do something about it, rather than letting the program fail. We will use the divide by 0 error above to show you how to catch an exception.

# catch_exception.py
num1 = input('Enter first number: ')
num2 = input('Enter second number: ')
try:
result = num1/num2
print result
except ZeroDivisionError:
print "You can't divide by zero"


Polymorphism comes from two Greek words meaning ‘many shapes’ and is best explained by a real world example. The feet of a human are a very different shape to that of a centipede, but they have basically the same function, walking. So we can refer to the feet of a human, centipede, horse, or frog without confusion. Polymorphism allows the programmer to give the same names to methods and attributes that play similar roles in distinctive classes.
As an example we will use a built-in operator, function or method on an object without knowing what kind of object it is.

Here we use the count method on a string.

a = 'This is a string'
print a.count('s')
>>>
3

Now we will use the count method on a list.

b = [1,2,3,4,5,'s','i','n']
print b.count('s')
>>>
1


This is where a function calls itself. Let’s look at a classic example of recursion, the power of something. First we define the problem:

power(x, 0) is 1 for all numbers x.
power(x, n) for n > 0 is the product of x and power(x, n-1).


def power(x, n):
if n == 0:
return 1
else:
return x * power(x, n-1)


Takes two lists as parameters and returns a list of tuples where the i-th tuple contains the i-th items from each sequence. This is best explained with an example.

>>> x = ['a','b','c']
>>> y = ['d','e','f']
>>> zipped = zip(x,y)
>>> zipped
[('a', 'd'), ('b', 'e'), ('c', 'f')]
>>>


Concatenate two lists

>>> list1 = [1,2,3,4,5,6,7,8]
>>> list2 = [9,10,11]
>>> list1.extend(list2)
>>> list1
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 9, 10, 11]
>>> list2
[9, 10, 11]
>>>


Maps one sequence into another. map(function, iterable, ...) Apply function to every item of iterable and return a list of the results.

>>> lst
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
>>> map(lambda n: n *2, lst)
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38]
>>>

Sequences are numbered from 0 upward so you can access them like this

>>> myString = "This is a string"
>>> string[0]
'T'
>>> string[0:4]
'This'
>>>


To find out what the built in module math contains first import it using
import math
Then perform a dir() on it
dir(math)
This returns a list of all the functions available in that module
['__doc__', '__name__', '__package__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'hypot', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc']

To filter out all the functions in the list that begin with an _ as we are not supposed to use these outside the module, we could use:
[funct for funct in dir(math)if funct[0] != '_']
['acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'hypot', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc']

If you don’t want to change the environment variables on your machine you can run your scripts from the command prompt by specifying the full path to the python.exe like this;
C:\>C:\Python24\python hello.py
If you want to change it then Start and right-click My Computer and click on Properties then click Advanced and click Environment Variables. Now scroll down along system variables until you see a variable called PATH. Click this and click edit. Now go to the end of the string and add ;C:\Python27\ or the path to your version of python. Now click OK and OK and OK again and now you’re done. The path depends on your python version installed. To find out what your path should be you could open up IDEL interpreter and type

import sys, pprint
pprint.pprint(sys.path)