issubclass raising error when argument 1 is a typing._GenericAlias

[nickeldan]

This seems related to issue #89489 but I'm not sure if it's the same problem.

I have a metaclass that examines the __annotations__ of its instances. At one point, I check whether the type of an annotated field is a subclass of int (e.g., an enum.IntEnum subclass) by issubclass(field_type, int). It works fine unless the type is a typing._GenericAlias such as typing.Optional[bytes] in which case it raises an error:

TypeError: issubclass() arg 1 must be a class

Could issubclass just return False in this case?

This is on Python 3.8.10.

[sobolevn]

Well, there are several things to it:

1. python3.8 is quite old now, we can only add new feature to 3.12 at the moment
2. Typing API specifically states that you cannot use isinstance / issubclass with generic aliases. Otherwise, people would ask: why can't you use isinstance([1, 2], list[int])? I don't think this is going to change anytime soon
3. You can actually work around it by catching TypeError and return False in your own code

[JelleZijlstra]

Yes, this is documented and intentional behavior. The recommendation is to call isinstance(field_type, type) before trying to use issubclass().