[issue11610] Improved support for abstract base classes with descriptors

[Darren Dale]

Darren Dale <dsdale24 at gmail.com> added the comment:

[...]
>> Traceback (most recent call last):
>>  File "<stdin>", line 1, in <module>
>> TypeError: Can't instantiate abstract class D with abstract methods foo.__func__
>
> You still need to use @abc.abstractstaticmethod.

Thinking about this some more, I think the error you found should be
considered a problem. The issue is with the descriptor access itself:
In class C we inspect the staticmethod instance in the namespace dict
passed to ABCMeta and find foo's abstract __func__. But later, ABCMeta
identifies C as a base of D, identifies C.foo.__func__ from
C.__abstractmethods__, then does a getattr on the formative D class.
D.__dict__['foo'] is a descriptor, which when accessed as D.foo
returns D.__dict__['foo'].__func__ (as opposed to the other
descriptors we have been discussing, where descriptor "get" access is
only invoked by an instance of the class). ABCMeta needs to inspect
the descriptor itself, not whatever the descriptor wants to return
when accessed. We can't use D.__dict__ to access the foo descriptor,
since some other base class of D may have provided the concrete
implementation of foo. Does anyone know another way to search the MRO
and return the foo descriptor?

You helped bring up another point: all functions are descriptors,
since they all have __get__ methods. That means every method is going
to be inspected for abstract members. Is this a problem?

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11610>
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