ContextVar[str] should return ContextVar class, not None

[asvetlov]

BPO	38979
Nosy	@asvetlov, @miss-islington, @amiremohamadi
PRs	bpo-38979: fix ContextVar "__class_getitem__" method #17497 [3.8] bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497) #17505 [3.7] bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497) #17506 [3.8] bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497) #17507

^{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 = None
closed_at = <Date 2019-12-08.12:49:21.791>
created_at = <Date 2019-12-05.13:04:18.933>
labels = ['easy', 'type-bug', '3.8', '3.9', 'extension-modules', '3.7']
title = 'ContextVar[str] should return ContextVar class, not None'
updated_at = <Date 2019-12-08.12:49:21.790>
user = 'https://github.com/asvetlov'

bugs.python.org fields:

activity = <Date 2019-12-08.12:49:21.790>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2019-12-08.12:49:21.791>
closer = 'asvetlov'
components = ['Extension Modules']
creation = <Date 2019-12-05.13:04:18.933>
creator = 'asvetlov'
dependencies = []
files = []
hgrepos = []
issue_num = 38979
keywords = ['patch', 'easy (C)']
message_count = 8.0
messages = ['357850', '357877', '357887', '357963', '357964', '357997', '357998', '358001']
nosy_count = 3.0
nosy_names = ['asvetlov', 'miss-islington', 'Amir']
pr_nums = ['17497', '17505', '17506', '17507']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38979'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[asvetlov]

The issue is minor, I suspect nobody wants to derive from ContextVar class.

The generic implementation for __class_getitem__ is returning unmodified self argument. Yuri, is there a reason to behave differently in the case of ContextVar?

If no, we can mark the issue as easy(C) and wait for a volunteer, the fix seems trivial.

[1st1]

The issue is minor, I suspect nobody wants to derive from ContextVar class.

I don't think that's allowed, actually.

The generic implementation for __class_getitem__ is returning unmodified self argument. Yuri, is there a reason to behave differently in the case of ContextVar?

No, just an oversight, probably.

[DahlitzFlorian]

I'd be happy to work on this!

[amiremohamadi]

@asvetlov I have a question! Should "contextvar_cls_getitem" function be changed? And can you please help me figure out how to change it?

[asvetlov]

I think it should look like the corresponding function from multidict: https://github.com/aio-libs/multidict/blob/master/multidict/_multidict.c#L803-L808

Please note, the method definition also should be updated, see https://github.com/aio-libs/multidict/blob/master/multidict/_multidict.c#L923-L928 for inspiration.

[miss-islington]

New changeset 28c9163 by Miss Islington (bot) (AMIR) in branch 'master':
bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497)
28c9163

[miss-islington]

New changeset 9baa870 by Miss Islington (bot) in branch '3.7':
bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497)
9baa870

[miss-islington]

New changeset 960fca1 by Miss Islington (bot) in branch '3.8':
bpo-38979: fix ContextVar "__class_getitem__" method (GH-17497)
960fca1