https://docs.python.org/2.7/tutorial/modules.html

Modules

If you quit from the Python interpreter and enter it again, the definitions you have made (functions and variables) are lost. Therefore, if you want to write a somewhat longer program, you are better off using a text editor to prepare the input for the interpreter and running it with that file as input instead. This is known as creating a script. As your program gets longer, you may want to split it into several files for easier maintenance. You may also want to use a handy function that you’ve written in several programs without copying its definition into each program.

To support this, Python has a way to put definitions in a file and use them in a script or in an interactive instance of the interpreter. Such a file is called a module; definitions from a module can be imported into other modules or into the main module (the collection of variables that you have access to in a script executed at the top level and in calculator mode).

A module is a file containing Python definitions and statements. The file name is the module name with the suffix .py appended. Within a module, the module’s name (as a string) is available as the value of the global variable __name__. For instance, use your favorite text editor to create a file called fibo.py in the current directory with the following contents:

# Fibonacci numbers module

def fib(n):    # write Fibonacci series up to n

    a, b = 0, 1

    while b < n:

        print b,

        a, b = b, a+b

def fib2(n): # return Fibonacci series up to n

    result = []

    a, b = 0, 1

    while b < n:

        result.append(b)

        a, b = b, a+b

    return result

Now enter the Python interpreter and import this module with the following command:

>>>

>>> import fibo

This does not enter the names of the functions defined in fibo directly in the current symbol table; it only enters the module name fibo there. Using the module name you can access the functions:

>>>

>>> fibo.fib(1000)

1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

>>> fibo.fib2(100)

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

>>> fibo.__name__

'fibo'

If you intend to use a function often you can assign it to a local name:

>>>

>>> fib = fibo.fib

>>> fib(500)

1 1 2 3 5 8 13 21 34 55 89 144 233 377

http://www.zhihu.com/question/25577999