`isinstance` check against `collection.abc` classes fails if value has no `__class__`

[pekkaklarck]

Bug report

Bug description:

The problem can be reproduced with the following class. It succeeds when tested with isinstance() against base types like int, but fails when tested against collections.abc classes like Mapping.

>>> from collections.abc import Mapping
>>> 
>>> class NoClass:
...     def __getattribute__(self, attr):
...         if attr == "__class__":
...             raise AttributeError(attr)
...         return super().__getattribute__(attr)
...         
>>> 
>>> isinstance(NoClass(), int)
False
>>> isinstance(NoClass(), Mapping)
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    isinstance(NoClass(), Mapping)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 119, in __instancecheck__
  File "<python-input-0>", line 4, in __getattribute__
    raise AttributeError(attr)
AttributeError: __class__

The example class is arguably broken, but it seems objects without __class__ actually exist. For a real life example see robotframework/robotframework#5699 where the problematic object apparently is a Qt widget.

This is a very annoying case, because handling these problematic objects requires changing all isinstance usages where the type can be collections.abc class from something like

if isinstance(value, Mapping):
    ...

to

try:
    is_mapping = isinstance(value, Mapping)
except AttributeError:
    is_mapping = False
if is_mapping:
    ...

I'll do that in our code in cases where there problematic objects are likely to occur, but wrapping all isinstance usage isn't practical so I'm worried there still can be crashes.

I hope collections.abc types would be fixed so that they return False with objects without __class__ instead of failing with AttributeError. Alternatively, if objects without __class__ are considered to be totally broken, the interpreter should fail/warn when they are instantiated and not only with isinstance().

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[picnixz]

Alternatively, if objects without __class__ are considered to be totally broken, the interpreter should fail/warn when they are instantiated and not only with isinstance().

Objercts without a __class__ may be considered broken but it depends on how you break them. Here, it's because you raised an exception. But I think we enter the realm of when you mess with dunders, you're on your own. With ints, it may be because of other fast paths here and there that actually bypass the __class__ query or whatever else (I don't remember and I'm not sure about it).

However, docs say that the attribute should exist (__dict__, OTOH, may be lacking).

cc @serhiy-storchaka @JelleZijlstra