Alexey Izbyshev <izbys...@ispras.ru> added the comment:
Regarding status quo (expanding the examples of @inada.naoki and @jab):
>>> import typing
>>> import collections.abc as cabc
>>> issubclass(typing.Mapping, cabc.Mapping)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/izbyshev/workspace/cpython/Lib/abc.py", line 143, in
__subclasscheck__
return _abc_subclasscheck(cls, subclass)
TypeError: issubclass() arg 1 must be a class
>>> from abc import ABC
>>> issubclass(typing.Mapping, ABC)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/izbyshev/workspace/cpython/Lib/abc.py", line 143, in
__subclasscheck__
return _abc_subclasscheck(cls, subclass)
File "/home/izbyshev/workspace/cpython/Lib/abc.py", line 143, in
__subclasscheck__
return _abc_subclasscheck(cls, subclass)
File "/home/izbyshev/workspace/cpython/Lib/contextlib.py", line 30, in
__subclasshook__
return _collections_abc._check_methods(C, "__enter__", "__exit__")
File "/home/izbyshev/workspace/cpython/Lib/_collections_abc.py", line 73, in
_check_methods
mro = C.__mro__
File "/home/izbyshev/workspace/cpython/Lib/typing.py", line 706, in
__getattr__
raise AttributeError(attr)
AttributeError: __mro__
>>> ABC.register(int)
<class 'int'>
>>> issubclass(typing.Mapping, ABC)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/izbyshev/workspace/cpython/Lib/abc.py", line 143, in
__subclasscheck__
return _abc_subclasscheck(cls, subclass)
TypeError: issubclass() arg 1 must be a class
>>> typing.Mapping.__mro__ = ()
>>> issubclass(typing.Mapping, ABC)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/izbyshev/workspace/cpython/Lib/abc.py", line 143, in
__subclasscheck__
return _abc_subclasscheck(cls, subclass)
TypeError: issubclass() arg 1 must be a class
>>> typing.Mapping.__bases__ = ()
>>> issubclass(typing.Mapping, ABC)
False
Can't say that I'm OK with it :)
I'm for forbidding non-types in ABCMeta.__subclasscheck__, but if we are to add
clean support for "class-likes" instead, I think that "class-like" objects
should be clearly defined, for example, that they must have __mro__ and
__bases__ (and probably support weakrefs unless we want to skip caching if they
don't). ABCMeta.__subclasscheck__ is not standalone: it relies on issubclass()
and __subclasshook__, and both of them have some expectations in practice.
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue33018>
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