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Using pydoc
Pages: 1, 2

Exploiting pydoc

Alongside standalone pydoc, accessed as a separate process launched from a graphical desktop or command line, a third interesting interactive way to exploit pydoc is within the Python shell. Suppose you're at the interpreter's >>> prompt, you request from pydoc import help. Now all the standard Python documentation is one keyword away. help is a function that knows how to retrieve documentation on other functions, classes, and modules. If you type "help(int)", for example, you'll see

Help on built-in function int:

int (no arg info)
  int(x) -> integer

  Convert a string or number to an integer, 
  if possible. A floating point argument will 
  be truncated towards zero.

Another way to reach the same point is simply to command "help" at the interpreter's prompt. Without any arguments, this launches the interactive help browser we've already seen.

Experienced Python users will recognize that int.__doc__ holds the same content. help() has several advantages over direct __doc__ retrieval, though. The former is easier to type, has the intelligence to display docstrings with proper formatting, and knows how to navigate to information that doesn't fit in the function.__doc__ syntax. Among other things, pydoc knows to use sys.path to reach documentation that corresponds to the programming modules the current interpreter is using, and it knows how to display information about inherited modules. Early 2.1 users laud help() as a significant benefit to their work.

An even more powerful form of pydoc is available through a 'Net connection. The publicly available benefits from a special index that knows all docstrings of all objects in all modules, not just the one-line module synopses.


Interactive help() and Web-accessibility don't exhaust pydoc's flexibility. To appreciate the final way of using it, think about pydoc itself as a project. It has three elements:

  • the documentation set;
  • the pydoc module's class definitions about general-purpose formatting for display of internal information;
  • wrappers and shortcuts to make the programming interface available as "stand-alone" processes.

Each of these is simple and completely available as source in the Python distribution. It follows that pydoc is also available as a programmable base. By substituting, subclassing, and rewriting any of the three elements above, programmers can build their own, special-purpose help systems.

The first such substantial project to be built on top of pydoc is Yee's own cgitb module, also available at his Python site. cgitb is a tiny traceback printer for CGI fault management. Here's how Yee describes cgitb:

it uses pydoc and inspect together to produce an elaborate and nicely-formatted display of a traceback whenever an error occurs in a CGI script.

I honestly believe that if "cgitb" gets into the Python distribution or gets otherwise distributed, Python will be so vastly much better an environment for CGI programming that hardly anyone will choose any other language any more. That's because most other languages just quit silently when there's an error in your CGI script, leaving you no information about what went wrong. When you debug scripts with the cgitb facility, the detailed exception dump shows you all the function names and argument values, and that makes finding and squishing bugs extremely fast and easy.


When you install your copy of Python 2.1, one of the first things you should do is exercise pydoc. Learning pydoc will repay you with more efficient ways to learn everything else there is to know about Python. Then when the time comes to implement a help system for one of your own projects, pydoc will make a great starting point.

Cameron Laird is the vice president of Phaseit, Inc. and frequently writes for the O'Reilly Network and other publications.

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