A Byte of Python

Chapter 8. Modules

Table of Contents

Using the sys module
Byte-compiled .pyc files
The from..import statement
A module's __name__
Using a module's __name__
Making your own Modules
Creating your own Modules
The dir() function
Using the dir function


You have seen how you can reuse code in your program by defining functions once. What if you wanted to reuse a number of functions in other programs that you write? As you might have guessed, the answer is modules. A module is basically a file containing all your functions and variables that you have defined. To reuse the module in other programs, the filename of the module must have a .py extension.

A module can be imported by another program to make use of its functionality. This is how we can use the Python standard library as well. First, we will see how to use the standard library modules.

Using the sys module

Example 8.1. Using the sys module

# Filename: using_sys.py

import sys

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

print '\n\nThe PYTHONPATH is', sys.path, '\n'


$ python using_sys.py we are arguments
The command line arguments are:

The PYTHONPATH is ['/home/swaroop/byte/code', '/usr/lib/python23.zip',
'/usr/lib/python2.3', '/usr/lib/python2.3/plat-linux2',
'/usr/lib/python2.3/lib-tk', '/usr/lib/python2.3/lib-dynload',
'/usr/lib/python2.3/site-packages', '/usr/lib/python2.3/site-packages/gtk-2.0']

How It Works

First, we import the sys module using the import statement. Basically, this translates to us telling Python that we want to use this module. The sys module contains functionality related to the Python interpreter and its environment.

When Python executes the import sys statement, it looks for the sys.py module in one of the directores listed in its sys.path variable. If the file is found, then the statements in the main block of that module is run and then the module is made available for you to use. Note that the initialization is done only the first time that we import a module. Also, 'sys' is short for 'system'.

The argv variable in the sys module is referred to using the dotted notation - sys.argv - one of the advantages of this approach is that the name does not clash with any argv variable used in your program. Also, it indicates clearly that this name is part of the sys module.

The sys.argv variable is a list of strings (lists are explained in detail in later sections). Specifically, the sys.argv contains the list of command line arguments i.e. the arguments passed to your program using the command line.

If you are using an IDE to write and run these programs, look for a way to specify command line arguments to the program in the menus.

Here, when we execute python using_sys.py we are arguments, we run the module using_sys.py with the python command and the other things that follow are arguments passed to the program. Python stores it in the sys.argv variable for us.

Remember, the name of the script running is always the first argument in the sys.argv list. So, in this case we will have 'using_sys.py' as sys.argv[0], 'we' as sys.argv[1], 'are' as sys.argv[2] and 'arguments' as sys.argv[3] . Notice that Python starts counting from 0 and not 1.

The sys.path contains the list of directory names where modules are imported from. Observe that the first string in sys.path is empty - this empty string indicates that the current directory is also part of the sys.path which is same as the PYTHONPATH environment variable. This means that you can directly import modules located in the current directory. Otherwise, you will have to place your module in one of the directories listed in sys.path .