ABCs with bogus __subclasses__ break instance/subclass checks for other ABCs

Bug report

Bug description:

Consider this evil class:

import abc

class EvilABC(abc.ABC):
    __subclasses__ = ...

Now all instance/subclass checks for other ABCs fail:

>>> isinstance(..., abc.ABC)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    isinstance(..., abc.ABC)
    ~~~~~~~~~~^^^^^^^^^^^^^^
  File "<frozen abc>", line 119, in __instancecheck__
  File "<frozen abc>", line 123, in __subclasscheck__
  File "<frozen abc>", line 123, in __subclasscheck__
TypeError: attribute of type 'ellipsis' is not callable

This happens because the recursive call to __subclasses__ in https://github.com/python/cpython/blob/v3.13.0a5/Lib/_py_abc.py#L141 trusts that __subclasses__ will always be callable and return an iterable (or a list in the C implementation).

You might argue that the EvilABC class is evil and whoever does that is doomed to be punished. However, it can easily be done by a mistake. This class has a broken __subclasses__ without explicitly defining it:

class Crossbreed(abc.ABCMeta, os.PathLike):
    ...

>>> isinstance("/", os.PathLike)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    isinstance("/", os.PathLike)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 119, in __instancecheck__
  File "<frozen abc>", line 123, in __subclasscheck__
  File "<frozen abc>", line 123, in __subclasscheck__
TypeError: unbound method type.__subclasses__() needs an argument

It is very surprising that by defining a class we can break checks for other classes and hence third party code, even from the standard library:

$ python3 -c 'import abc, os, subprocess
> class A(abc.ABCMeta, os.PathLike): pass
> subprocess.run(("echo",))'
Traceback (most recent call last):
  File "<string>", line 3, in <module>
  File "/usr/lib64/python3.12/subprocess.py", line 548, in run
    with Popen(*popenargs, **kwargs) as process:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/subprocess.py", line 1026, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib64/python3.12/subprocess.py", line 1802, in _execute_child
    elif isinstance(args, os.PathLike):
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 119, in __instancecheck__
  File "<frozen abc>", line 123, in __subclasscheck__
  File "<frozen abc>", line 123, in __subclasscheck__
TypeError: unbound method type.__subclasses__() needs an argument

In my opinion, the __subclasses__ call should be guarded by a try-except and either:

• skip broken subclasses, or
• skip broken subclasses and warn about them, or
• raise a better exception message, saying which subclass is broken

See also https://discuss.python.org/t/abcmeta-change-isinstancecheck-of-additional-parent-class/19908

CPython versions tested on:

3.8, 3.9, 3.10, 3.11, 3.12, 3.13, CPython main branch

Operating systems tested on:

No response

Lib/_py_abc.py の再帰的な subclasses 呼び出しから始め、issue の 2 つの例を再現します。次に、その経路を報告で言及されている C 実装と比較します。壊れた subclasses 定義によって、無関係な ABC のインスタンスチェックまたはサブクラスチェックが失敗することがなくなり、報告されたケースのリグレッションカバレッジが追加されていれば完了です。