Description

AlexWaygood

opened on Jun 2, 2023

Issue body actions

Bug report

On Python 3.11, you can do this:

>>> import typing
>>> issubclass(object, typing.Protocol)
False

But 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 protocols

The 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.

Linked PRs

• gh-105237: Allow calling issubclass(X, typing.Protocol) again #105239
• [3.12] gh-105237: Allow calling issubclass(X, typing.Protocol) again (GH-105239) #105316

Reacted by sunmy2019

Activity

1. AlexWaygood

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   topic-typing

   3.12only security fixesonly security fixes

   3.13only security fixesonly security fixes

   on Jun 2, 2023
2. AlexWaygood

   self-assigned this

   on Jun 2, 2023
3. JelleZijlstra

   added a commit that references this issue on Jun 5, 2023

   gh-105237: Allow calling `issubclass(X, typing.Protocol)` again (#105239

   cdfb201
4. miss-islington

   added a commit that references this issue on Jun 5, 2023

   pythongh-105237: Allow calling `issubclass(X, typing.Protocol)` again (…

   f45b5d7
5. AlexWaygood

   added a commit that references this issue on Jun 5, 2023

   [3.12] gh-105237: Allow calling `issubclass(X, typing.Protocol)` again (

   5175026
6. AlexWaygood commented on Jun 5, 2023

   

   AlexWaygood

   on Jun 5, 2023

   MemberAuthor

   More actions

   While working on fixing the bug, we discovered another bug, where issubclass(object, Protocol) can sometimes evaluate to True. Unlike the original bug, this one reproduces all the way back to Python 3.8. It's now fixed on main and 3.12, but it might be good to see if we can fix it on 3.11 as well.
7. AlexWaygood commented on Jun 5, 2023

   

   AlexWaygood

   on Jun 5, 2023

   MemberAuthor

   More actions

   While working on fixing the bug, we discovered another bug, where issubclass(object, Protocol) can sometimes evaluate to True. Unlike the original bug, this one reproduces all the way back to Python 3.8. It's now fixed on main and 3.12, but it might be good to see if we can fix it on 3.11 as well.

   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.)
8. AlexWaygood

   closed this as completedon Jun 5, 2023

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