Hi all -- I seem to have discovered an inconsistency in the semantics of object comparison between plain old Python instances and ExtensionClass instances. (I've cc'd python-dev because it looks as though one *could* blame Python for the inconsistency, but I don't really understand the guts of either Python or ExtensionClass enough to know.) Here's a simple script that shows the difference: class Simple: def __init__ (self, data): self.data = data def __repr__ (self): return "<%s at %x: %s>" % (self.__class__.__name__, id(self), `self.data`) def __cmp__ (self, other): print "Simple.__cmp__: self=%s, other=%s" % (`self`, `other`) return cmp (self.data, other) if __name__ == "__main__": v1 = 36 v2 = Simple (36) print "v1 == v2?", (v1 == v2 and "yes" or "no") print "v2 == v1?", (v2 == v1 and "yes" or "no") print "v1 == v2.data?", (v1 == v2.data and "yes" or "no") print "v2.data == v1?", (v2.data == v1 and "yes" or "no") If I run this under Python 1.5.2, then all the comparisons come out true and my '__cmp__()' method is called twice: v1 == v2? Simple.__cmp__: self=<Simple at 1b5148: 36>, other=36 yes v2 == v1? Simple.__cmp__: self=<Simple at 1b5148: 36>, other=36 yes v1 == v2.data? yes v2.data == v1? yes The first one and the last two are obvious, but the second one only works thanks to a trick in PyObject_Compare(): if (PyInstance_Check(v) || PyInstance_Check(w)) { ... if (!PyInstance_Check(v)) return -PyObject_Compare(w, v); ... } However, if I make Simple an ExtensionClass: from ExtensionClass import Base class Simple (Base): Then the "swap v and w and use w's comparison method" no longer works. Here's the output of the script with Simple as an ExtensionClass: v1 == v2? no v2 == v1? Simple.__cmp__: self=<Simple at 1b51c0: 36>, other=36 yes v1 == v2.data? yes v2.data == v1? yes It looks as though ExtensionClass would have to duplicate the trick in PyObject_Compare() that I quoted, since Python has no idea that ExtensionClass instances really should act like instances. This smells to me like a bug in ExtensionClass. Comments? BTW, I'm using the ExtensionClass provided with Zope 2.1.4. Mostly tested with Python 1.5.2, but also under the latest CVS Python and we observed the same behaviour. Greg