FAQ
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«CMeta):
@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

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  • Andrew cooke at Apr 19, 2011 at 11:59 pm
    Also, there's something strange about the number of arguments (they're not consistent between the two examples - the "A" to __instancecheck__ should not be needed). Yet it compiles and runs like that. Very confused :o(
  • Andrew cooke at Apr 20, 2011 at 12:20 am
    OK, sorry, I see the mistake. I'm confusing __class__ on the instance and on te class (the latter being the metaclass). Sorry again, Andrew
  • Chris Rebert at Apr 20, 2011 at 12:45 am

    On Tue, Apr 19, 2011 at 4:52 PM, andrew cooke 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«CMeta):
    ? ? ? ? ? ?@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
  • Andrew cooke at Apr 20, 2011 at 12:11 pm

    Thanks for finding that reference in the data model docs! I was about to post a bug report because in PEP 3119 it says otherwise:

    The primary mechanism proposed here is to allow overloading the built-in
    functions isinstance() and issubclass(). The overloading works as follows:
    The call isinstance(x, C) first checks whether C.__instancecheck__ exists,
    and if so, calls C.__instancecheck__(x) instead of its normal implementation.
    http://www.python.org/dev/peps/pep-3119/

    But that's now what's implemented in Issue http://bugs.python.org/issue1708353 which behaves as you quoted (only on the metaclass).

    Cheers,
    Andrew
  • Andrew cooke at Apr 20, 2011 at 12:13 pm
    I didn't phrase that very well. I do see the point about this being "an instance lookup on a class"...

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