[typing] Runtime protocols with ClassVar data members should support issubclass

[Fidget-Spinner]

BPO	44975
Nosy	@gvanrossum, @JelleZijlstra, @Fidget-Spinner
PRs	bpo-44975: [typing] Support issubclass for ClassVar data members #27883

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/Fidget-Spinner'
closed_at = None
created_at = <Date 2021-08-22.08:56:49.329>
labels = ['type-feature', 'library', '3.11']
title = '[typing] Runtime protocols with ClassVar data members should support issubclass'
updated_at = <Date 2022-01-21.21:12:18.991>
user = 'https://github.com/Fidget-Spinner'

bugs.python.org fields:

activity = <Date 2022-01-21.21:12:18.991>
actor = 'gvanrossum'
assignee = 'kj'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-08-22.08:56:49.329>
creator = 'kj'
dependencies = []
files = []
hgrepos = []
issue_num = 44975
keywords = ['patch']
message_count = 2.0
messages = ['400059', '411184']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'JelleZijlstra', 'kj']
pr_nums = ['27883']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44975'
versions = ['Python 3.11']

[Fidget-Spinner]

This is a feature request by a user at python/typing#822.

A copy of their request:
Currently issubclass cannot be used for runtime_checkable protocols with data members, because those attributes could be set in __init__. I propose to relax this restriction to allow protocols with ClassVar members, as those should be present in the class definition.

I'm unsure if I need to update PEP-544 too. I don't remember if PEPs can be updated when 'Accepted', or was it 'Final' PEPs that can't be updated anymore?

[gvanrossum]

IMO we shouldn't update PEP-544. PEPs reflect the historical proposal, they are not documentation for the current state of the interpreter.

[hauntsaninja]

I'm -1 on implementing this feature.

In general, I believe stdlib should try to get out of the business of runtime type checking. Even with isinstance on runtime checkable protocols I've seen complaints about performance and unsoundness and interactions with descriptors. I don't think it's particularly intuitive for a runtime type check to only check presence of a ClassVar and then not check the type (and we definitely don't want to check the type).

[gvanrossum]

So should we just deprecate @runtime_checkable?