On Thu, Sep 21, 2000 at 08:00:29AM -0500, John D. Heintz wrote:
Thanks for the reply Martijn,
I do want the objects of Test2 class to be compared by identity, like I'm assuming Test1 objects are.
If I have to define __cmp__ and __hash__ then I will basically be making them up because the object in question are mutable - except for their identity.
I don't think that in this case __cmp__ has to be implemented; you just use the default behaviour of object identity comparison. All you need to do is then implent a __hash__() method that will return a 32-bit integer that is unique to the object. You could base this on repr(self).
Why do the Python class instances naturally act as dictionary keys while the ExtensionClass instances don't?
ExtensionClasses pretend to be Python classes, but are not succeeding everywhere. I am not that deeply knowledgable about Extension Classes, so I cannot tell you why exactly you see this difference.
Martijn Pieters wrote:
From the Python Library Reference:
"""A dictionary's keys are almost arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity."""
<...>
So, if you want to be able to use a Persistent based object as keys to a dictionary, implement __cmp__ and __hash__ methods on that class:
import ZODB from Persistence import Persistent class Test1: ... pass ... class Test2(Persistent): ... def __cmp__(self): return 1 ... def __hash__(self): return 1 ... dict = {} t1 = Test1() t2 = Test2() dict[t1] = 1 dict[t2] = 2 dict {<Test2 instance at 80b3aa0>: 2, <__main__.Test1 instance at 80a3e78>: 1}
-- John D. Heintz DataChannel, Inc. Senior Engineer 512-633-1198 jheintz@isogen.com
-- Martijn Pieters | Software Engineer mailto:mj@digicool.com | Digital Creations http://www.digicool.com/ | Creators of Zope http://www.zope.org/ ---------------------------------------------