Can someone please explain the best way for me to map from one instance to another? In the following code either: (1) I am using dictionaries wrong, or (2) I'm missing something with ExtensionClass in general. I have tried searching Zope and Python mailing lists and resources for an answer to this question but have become confused by different conflicting messages. On the one hand I have heard that dictionary keys are supposed to be immutable. On the other, I've heard that dictionaries are used to simulate (key is object, value is ignored) some aspects of sets. Which is correct? What I really want is to be able to map a Persistent object to a string with fast lookup. Thanks for any help. John Heintz import ZODB from Persistence import Persistent class Test1: pass class Test2(Persistent): pass dict = {} t1 = Test1() t2 = Test2() try: dict[t1] = 1 except: print "Using instance of Test1 as key failed!" try: dict[t2] = 2 except: print "Using instance of Test2(Persistent) as key failed!" -- John D. Heintz DataChannel, Inc. Senior Engineer 512-633-1198 jheintz@isogen.com