My python version:

Python 2.3.3 (#1, Jan 25 2004, 21:45:01)
[GCC 3.2.3 20030422 (Gentoo Linux 1.4 3.2.3-r3,
propolice)] on linux2

Please try the example. That will explain a lot. The
problem is that not all new style class instances are
picklable. In my example a class is derived from a list
and a base class. The list-descendant contains an
instance of the base class, which has a reference to
the list containing it. with cPickle things work fine,
but not for the normal pickle routines

class subitem:
	def __init__(self, parent):
		self.parent = parent
		if parent != None:
			parent.append(self)

class group(subitem, list):
	def __init__(self, parent):
		subitem.__init__(self, parent)
		

g = group(None)
s = subitem(g)

import cPickle
print cPickle.dumps(g)

import pickle
print pickle.dumps(g)

Logged In: YES 
user_id=1188172

Verified in 2.5cvs.

Please assign this bug to me.

Note that neither cPickle or pickle is able to load the stream generated
by cPickle correctly:

   >>> g = group(None)
   >>> subitem(g)
   >>> g[0].parent is g
   True
   >>> gp = cPickle.loads(cPickle.dumps(g))
   >>> gp[0].parent is gp
   False

I don't think that will be easy to fix, but I will try to see what I can do.

Confirmed on trunk.

This is still a problem with 2.6 and 2.7.  I don't know how to test this with py3k as cPickle doesn't exist, can someone advise, thanks.