`pathlib.PurePath.__class_getitem__` does not return `GenericAlias`

[sobolevn]

BPO	46483
Nosy	@asvetlov, @serhiy-storchaka, @JelleZijlstra, @miss-islington, @isidentical, @sobolevn, @Fidget-Spinner, @AlexWaygood
PRs	bpo-46483: change PurePath.__class_getitem__ to return GenericAlias #30822 bpo-46483: remove __class_getitem__ from pathlib.PurePath #30848 bpo-46483: [doc] Remove pathlib classes from list of stdlib classes that can be parameterized at runtime #31281 [3.9] bpo-46483: [doc] pathlib classes no longer support parameterized generics (GH-31281) #31286 [3.10] bpo-46483: [doc] pathlib classes no longer support parameterized generics (GH-31281) #31287

^{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 2022-02-03.09:24:10.997>
created_at = <Date 2022-01-23.08:16:30.578>
labels = ['type-bug', 'library', '3.11']
title = '`pathlib.PurePath.__class_getitem__` does not return `GenericAlias`'
updated_at = <Date 2022-02-11.21:04:05.289>
user = 'https://github.com/sobolevn'

bugs.python.org fields:

activity = <Date 2022-02-11.21:04:05.289>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-03.09:24:10.997>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2022-01-23.08:16:30.578>
creator = 'sobolevn'
dependencies = []
files = []
hgrepos = []
issue_num = 46483
keywords = ['patch']
message_count = 12.0
messages = ['411354', '411373', '411374', '411376', '411460', '411518', '411569', '412146', '412430', '413096', '413099', '413100']
nosy_count = 8.0
nosy_names = ['asvetlov', 'serhiy.storchaka', 'JelleZijlstra', 'miss-islington', 'BTaskaya', 'sobolevn', 'kj', 'AlexWaygood']
pr_nums = ['30822', '30848', '31281', '31286', '31287']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46483'
versions = ['Python 3.11']

[sobolevn]

After reviewing #30777 I had a chance to look through other definitions of def __class_getitem__. And I found that the only one left is: pathlib.PurePath.__class_getitem__

All other definitions already have this form: __class_getitem__ = classmethod(GenericAlias).

I don't think that there's anything special about PurePath in this regard. So, I propose to make __class_getitem__ to return GenericAlias as all other types do.

Initial PR: #17498

PR is on its way.

[isidentical]

New changeset 1f715d5 by Nikita Sobolev in branch 'main':
bpo-46483: change PurePath.__class_getitem__ to return GenericAlias (GH-30822)
1f715d5

[serhiy-storchaka]

Why __class_getitem__ was added in PurePath at first place? PurePath should not be a generic class, unlike to os.PathLike. For os.PathLike the type parameters represent the returning type of os.fspath() (either str or bytes), but the pathlib module only supports paths as strings, so no parametrization is needed.

I think PurePath.__class_getitem__ should be removed.

[AlexWaygood]

It looks like __class_getitem__ was added to PurePath at @asvetlov's request in PR 17498. But, I'm also not quite sure why. The class is not generic in typeshed, so type-checkers will raise an error if you do PurePath[str], even if it is possible to do so at runtime.

Typeshed stub for PurePath: https://github.com/python/typeshed/blob/dff461fc6adef1ec3a4b58af5abdf0cdebf2429a/stdlib/pathlib.pyi#L20

[asvetlov]

IIRC it was added to reflect the presence of __class_getitem__ method in typeshed.
Please feel free to drop it the method is not really needed.