Repository navigation
Cannot call issubclass() against typing.Protocol on Python 3.12 #105237
Description
Bug report
On Python 3.11, you can do this:
>>> import typing
>>> issubclass(object, typing.Protocol)
FalseBut on Python 3.12 (following the backport of c05c31d), this raises:
>>> import typing
>>> issubclass(object, typing.Protocol)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1797, in __subclasscheck__
raise TypeError(
TypeError: Instance and class checks can only be used with @runtime_checkable protocolsThe new behaviour doesn't make sense. That TypeError should only be triggered if a user is calling issubclass() against a subclass of typing.Protocol. typing.Protocol itself should be exempted.
Activity
gh-105237: Allow calling `issubclass(X, typing.Protocol)` again (#105239
pythongh-105237: Allow calling `issubclass(X, typing.Protocol)` again (…
[3.12] gh-105237: Allow calling `issubclass(X, typing.Protocol)` again (
AlexWaygood commentedon Jun 5, 2023
While working on fixing the bug, we discovered another bug, where
issubclass(object, Protocol)can sometimes evaluate toTrue. Unlike the original bug, this one reproduces all the way back to Python 3.8. It's now fixed onmainand 3.12, but it might be good to see if we can fix it on 3.11 as well.AlexWaygood commentedon Jun 5, 2023
Well, but on the other hand, I'm hesitant to touch 3.11 here, given that it's an edge-case bug, and it's hard to do a proper fix that's not somewhat invasive. So, I'll close this for now. (But if somebody else disagrees and wants to have a go at fixing it on 3.11, happy to reopen.)