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(python2.1-tut.info)Modules

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Modules
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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

More on Modules

Standard Modules

dir Function

Packages