My stupidity / strange inconsistency overriding class methods

Tue Apr 19 19:52:57 EDT 2011

Hi,
I've been staring at this problem, in various forms, all day. Am I missing something obvious, or is there some strange hardwiring of isinstance? This is with Python 3.2.
 class A(metaclass=ABCMeta):
 @classmethod
 def __instancecheck__(cls, instance): return False
 # no override
 assert isinstance(A(), A)
 assert A.__class__.__instancecheck__(A, A())

 class B(type):
 def foo(self): return 42
 class C(metaclass=B):
 @classmethod
 def foo(cls): return 7
 # override
 assert C().__class__.foo() == 7
It seems to me that the above two cases are inconsistent. ABCMeta declares __instancecheck__ just like B declares foo. Yet C can override foo, but A is unable to override the instance check.
Please help!
Thanks,
Andrew