On Tue, Apr 19, 2011 at 4:52 PM, andrew cooke <and...@acooke.org> wrote: > 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())
[You've already figured out the issue, but since I spent a while composing this, and for the benefit for the archives, I'll post anyway.] Makes sense after a little thought. http://docs.python.org/reference/datamodel.html#customizing-instance-and-subclass-checks "Note that [ __instancecheck__() is ] looked up on the type (metaclass) of a class. [It] cannot be defined as [a classmethod] in the actual class. This is consistent with the lookup of special methods that are called on instances, only in this case the instance is itself a class." Recall from http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes that lookup of __special__ methods never consults instance dictionaries, instead skipping directly to the type's namespace; as the quote says, in this case, the instance (of ABCMeta) is itself a class/type (namely A). Your two assert statements are therefore almost precisely equivalent in this case; and since the latter involves A.__class__ (a.k.a. ABCMeta) rather than A itself, it's understandable that that A's namespace is not consulted. > class B(type): > def foo(self): return 42 > class C(metaclass=B): > @classmethod > def foo(cls): return 7 > # override > assert C().__class__.foo() == 7 More simply: assert C.foo() == 7 "foo" is not a __special__ method name; therefore we look in the instance dictionary of the receiver (i.e. C) before consulting the receiver's type (i.e. B). Our check in the instance dictionary is successful (we find C.foo), and therefore we don't even bother looking at C's type (i.e. B, where we would find B.foo). > 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. The difference is in the __special__-ness of the method names in question. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list