Standard Library

The Python Standard Library contains a huge number of useful modules and is part of every standard Python installation. It is important to become familiar with the Python Standard Library since many problems can be solved quickly if you are familiar with the range of things that these libraries can do.

We will explore some of the commonly used modules in this library. You can find complete details for all of the modules in the Python Standard Library in the 'Library Reference' section of the documentation that comes with your Python installation.

Let us explore a few useful modules.

CAUTION: If you find the topics in this chapter too advanced, you may skip this chapter. However, I highly recommend coming back to this chapter when you are more comfortable with programming using Python.

sys module

The sys module contains system-specific functionality. We have already seen that the sys.argv list contains the command-line arguments.

Suppose we want to check the version of the Python software being used, the sys module gives us that information.

>>> import sys
>>> sys.version_info
sys.version_info(major=3, minor=6, micro=0, releaselevel='final', serial=0)
>>> sys.version_info.major == 3

How It Works

The sys module has a version_info tuple that gives us the version information. The first entry is the major version. We can pull out this information to use it.

logging module

What if you wanted to have some debugging messages or important messages to be stored somewhere so that you can check whether your program has been running as you would expect it? How do you "store somewhere" these messages? This can be achieved using the logging module.

Save as

import os
import platform
import logging

if platform.platform().startswith('Windows'):
    logging_file = os.path.join(os.getenv('HOMEDRIVE'),
    logging_file = os.path.join(os.getenv('HOME'),

print("Logging to", logging_file)

    format='%(asctime)s : %(levelname)s : %(message)s',

logging.debug("Start of the program")"Doing something")
logging.warning("Dying now")


$ python
Logging to /Users/swa/test.log

$ cat /Users/swa/test.log
2014-03-29 09:27:36,660 : DEBUG : Start of the program
2014-03-29 09:27:36,660 : INFO : Doing something
2014-03-29 09:27:36,660 : WARNING : Dying now

The cat command is used in the command line to read the 'test.log' file. If the cat command is not available, you can open the test.log file in a text editor instead.

How It Works

We use three modules from the standard library - the os module for interacting with the operating system, the platform module for information about the platform i.e. the operating system and the logging module to log information.

First, we check which operating system we are using by checking the string returned by platform.platform() (for more information, see import platform; help(platform)). If it is Windows, we figure out the home drive, the home folder and the filename where we want to store the information. Putting these three parts together, we get the full location of the file. For other platforms, we need to know just the home folder of the user and we get the full location of the file.

We use the os.path.join() function to put these three parts of the location together. The reason to use a special function rather than just adding the strings together is because this function will ensure the full location matches the format expected by the operating system. Note: the join() method we use here that's part of the os module is different from the string method join() that we've used elsewhere in this book.

We configure the logging module to write all the messages in a particular format to the file we have specified.

Finally, we can put messages that are either meant for debugging, information, warning or even critical messages. Once the program has run, we can check this file and we will know what happened in the program, even though no information was displayed to the user running the program.

Module of the Week Series

There is much more to be explored in the standard library such as debugging, handling command line options, regular expressions and so on.

The best way to further explore the standard library is to read Doug Hellmann's excellent Python Module of the Week series (also available as a book) and reading the Python documentation.


We have explored some of the functionality of many modules in the Python Standard Library. It is highly recommended to browse through the Python Standard Library documentation to get an idea of all the modules that are available.

Next, we will cover various aspects of Python that will make our tour of Python more complete.

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