Using:

ActivePython 2.4.2 Build 10 (ActiveState Corp.) based 
on
Python 2.4.2 (#67, Jan 17 2006, 15:36:03) [MSC v.1310 
32 bit (Intel)] on win32

For reasons I do not understand, the following class 
leaks itself continuously:

class AttrDict(dict):
   def __init__(self, *args, **kw):
      dict.__init__(self, *args, **kw)
      self.__dict__ = self

Whereas this version does not:

class AttrDict(dict):
   def __init__(self, *args, **kw):
      dict.__init__(self, *args, **kw)
   
   def __getattr__(self, key):
      return self[key]
   
   def __setattr__(self, key, value):
      self[key] = value

My test looks like this:

for n in xrange(1000000):
   import gc
   gc.collect()
   ad = AttrDict()
   ad['x'] = n
   ad.y = ad.x
   print n, ad.x, ad.y

And I sit and watch in the windows task manager while 
the process grows and grows.  With the __getattr__ 
version, it doesn't grow.