>>> from abc import ABCMeta
>>> class AbstractMeta(type, metaclass=ABCMeta): pass
...
>>> class Meta1(AbstractMeta): pass
...
>>> class Meta2(AbstractMeta): pass
...
>>> class ClassA(metaclass=Meta1): pass
...
>>> isinstance(ClassA, Meta2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\miniconda\miniconda3\lib\abc.py", line 139, in __instancecheck__
    return _abc_instancecheck(cls, instance)
TypeError: __subclasscheck__() takes exactly one argument (0 given)


This is driven by the fact that the metaclass of AbstractMeta is ABCMeta; if I change its metaclass to something else that does not contain ABCMeta in the inheritance chain, isinstance returns False as expected.


Possibly related to Issue 2325?

I think this is related to how `__subclasscheck__` is implemented in `ABCMeta`. I just checked this also happens in Python 3.6 (i.e. it is not something specific to the C version introduced in Python 3.7).

PR27648 fixes this, but it also breaks test_abc.test_issubclass_bad_arguments so it's not quite right.


I don't know this area, but I think the problem is in the place the PR changes, and that the issue is due to this difference in the type of __subclasses__ between C and Meta2:

>>> class C: pass
... 
>>> from abc import ABCMeta
>>> class AbstractMeta(type, metaclass=ABCMeta): pass
... 
>>> class Meta2(AbstractMeta): pass
... 
>>> C.__subclasses__
<built-in method __subclasses__ of type object at 0x7f96bf5040b0>
>>> Meta2.__subclasses__
<method '__subclasses__' of 'type' objects>