class Bar(object):
    ...

reveal_type(Bar.__slots__)
reveal_type(Bar().__slots__)

>>> class Bar(object):
...     ...
... 
>>> Bar.__slots__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'Bar' has no attribute '__slots__'. Did you mean: '__class__'?
>>> Bar().__slots__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Bar' object has no attribute '__slots__'. Did you mean: '__class__'?