[mod_python] Subclassing Session

Jim Gallacher jpg at jgassociates.ca
Tue Jul 18 13:12:58 EDT 2006

Thomas J. Schirripa wrote:
> Before stating my problem, let me say that I am running apache version 2.0.52, mod_python 3.2.8, python 2.3, all on Redhat Enterprise Linux WS release 4.
> I am making some basic web utilities that require that I create sessions. Essentially, I want to use session ids created by mod_python to create a session folder where I can temporarily dump files  and manipulate stuff so I can generate some output for the client. Since I would be using this feature a lot, I thought it would be a good idea to make a subclass of BaseSession that added this functionality of making a folder with the session name. I made a file located somewhere else on my machine called SessDirectory.py, and it defines the class SessionDirectory. The file looks like this:
> from Session import BaseSession
> import os
> class SessionDirectory(BaseSession):
> 	def __init__(self, req, path=None, sid=None, secret=None, lock=1, timeout=0):
> 		BaseSession.__init__(self, req, sid, secret, lock, timeout)
> 		if path == None:
> 			self.absPath = os.path.join(os.getcwd(), sid)
> 		else:
> 			self.absPath = os.path.join(path, sid)
> 		os.mkdir(self.absPath, mode=0755)
> It can take an absolute path to create the session directory, or it can take no path, in which case it creates the session directory in the current directory. When I try to instantiate the SessionDirectory object, the following exception is raised:
> AttributeError: 'SessionDirectory' object has no attribute 'do_load'
> I did some fishing around and looked at the source code of the session module. Apparently, BaseSession has a method called "load()" that makes a call to "do_load()". BaseSession does not define "do_load()", HOWEVER, it seems that the built in sublcasses of BaseSession (ie- FileSession, DbmSession, etc) all define a "do_load()" method.
> My background is really in Java, and to me, it seems like BaseSession is an abstract class, but it has no signature for "do_load()" and I believe the exception would have been NotImplemented or something like that if there was a required implementation missing. 

This is a good suggestion, and it would make it easier to subclass
BaseSession if the following methods existed and raised NotImplemented.
 I suppose this info should be included in the docs as well and since I
have the magic powers required I shall make it so. :)

Offhand I'd say the 4 methods you'll need to implement are:


For your application do_delete and do_cleanup may be noop, as I'm sure
you'll want to preserve your data.

> Also, why does BaseSession inherit from "dict" and not "UserDict". I wanted to see if "dict" had a "do_load()" method, but if it did, then the instantiation of SessionDirectory should have called the "do_load()" from dict. 

I'm not sure I get your point about the superiority of UserDict.
According to the python docs, inheriting from dict is preferred:

UserDict -- Class wrapper for dictionary objects

Note: This module is available for backward compatibility only. If you
are writing code that does not need to work with versions of Python
earlier than Python 2.2, please consider subclassing directly from the
built-in dict type.


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