I had the bright idea of defining a mixin class whose methods I thought I could use in my regular Zope product classes: class AMixin: def __init__(self): blah, blah blah class B(AMixin, Persistent, Folder, ....): def __init__(self): # do some of my own stuff AMixin.__init__(self) This fails with the warning (as I recall) that the unbound method AMixin.__init__ needs to be called with an instance as its first argument. (I've stripped the example down to give what I think are the essentials). self in the context of B is an Extension class, and I'm guessing this is making AMixin unhappy because it's not. I assume I'll have the same problem with other methods that I attempt to access using AMixin.foo(self). Can anyone confirm this diagnosis? And what's the best solution? I have redefined things class AMixin: def _mixin_init(self): #stuff class B(AMixin, .....): def __init__(self): #my own stuff self._mixin_init(self) and this seems to work, but I wonder if there's a way to get the classes to play together better (maybe "class AMixin(ExtensionClass):"? though since ExtensionClass is a type I guess that's not exactly it). This was after already discovering that isinstance doesn't work with ExtensionClass (which I see confirmed on the list, along with the fact that ExtensionClass generally has a lot of rough spots and may be on the way out). This is with Zope 2.5.1.