Bug report

Bug description:

During construction of a class Foo, all objects in the class dictionary are checked to see if they have a __set_name__ attribute. If an object in the dictionary does have a __set_name__ attribute, it is called as part of the construction of Foo:

>>> class Vanilla:
...     def __set_name__(self, owner, name):
...         print('set_name called')
...
>>> class Foo:
...     attr = Vanilla()
...
set_name called

But what if an object in a class namespace is an instance of class that has a metaclass that raises a non-AttributeError exception if __set_name__ is looked up on the class object? In this case, the exception is silently ignored when __set_name__ is looked up:

>>> class Meta(type):
...     def __getattribute__(self, attr):
...         if attr == "__set_name__":
...             raise SystemExit('NO')
...         return object.__getattribute__(self, attr)
...
>>> class Problematic(metaclass=Meta): pass
...
>>> try:
...     Problematic.__set_name__
... except SystemExit:
...     print('exception was raised')
...
exception was raised
>>> class Bar:
...     attr = Problematic()
...
>>>

__set_name__, like all dunders, is looked up on the Problematic class object itself, rather than the instance of Problematic found in the class dictionary of Foo, so SystemExit should be raised during the creation of the Bar class when the interpreter tries to determine whether or not the Problematic class has a __set_name__ attribute. Instead, however, the SystemExit is silently swallowed.

I think the issue lies in this section of code here:

I'm interested in working on this.

CPython versions tested on:

3.8, CPython main branch

Operating systems tested on:

Windows