Re: My stupidity / strange inconsistency overriding class methods
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 -- http://mail.python.org/mailman/listinfo/python-list
Re: My stupidity / strange inconsistency overriding class methods
I didn't phrase that very well. I do see the point about this being an instance lookup on a class... -- http://mail.python.org/mailman/listinfo/python-list
My stupidity / strange inconsistency overriding class methods
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 -- http://mail.python.org/mailman/listinfo/python-list
Re: My stupidity / strange inconsistency overriding class methods
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( -- http://mail.python.org/mailman/listinfo/python-list
Re: My stupidity / strange inconsistency overriding class methods
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 -- http://mail.python.org/mailman/listinfo/python-list
Re: My stupidity / strange inconsistency overriding class methods
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
