# 6 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
    >>>

- 6.1 More on Modules
  - 6.1.1 The Module Search Path
  - 6.1.2 ``Compiled'' Python files
- 6.2 Standard Modules
- 6.3 The `dir()` function