TopologicalSorter is not Generic at runtime (but is in typeshed)

[JacobHayes]

BPO	45359
Nosy	@asvetlov, @AlexWaygood, @JacobHayes
PRs	bpo-45359: Support TopologicalSorter type subscript #28714

^{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 2021-12-09.11:36:14.329>
created_at = <Date 2021-10-03.22:18:50.416>
labels = ['type-feature', 'library', '3.11']
title = 'TopologicalSorter is not Generic at runtime (but is in typeshed)'
updated_at = <Date 2021-12-10.14:59:57.694>
user = 'https://github.com/JacobHayes'

bugs.python.org fields:

activity = <Date 2021-12-10.14:59:57.694>
actor = 'JacobHayes'
assignee = 'none'
closed = True
closed_date = <Date 2021-12-09.11:36:14.329>
closer = 'asvetlov'
components = ['Library (Lib)']
creation = <Date 2021-10-03.22:18:50.416>
creator = 'JacobHayes'
dependencies = []
files = []
hgrepos = []
issue_num = 45359
keywords = ['patch']
message_count = 7.0
messages = ['403115', '408041', '408109', '408110', '408117', '408177', '408211']
nosy_count = 3.0
nosy_names = ['asvetlov', 'AlexWaygood', 'JacobHayes']
pr_nums = ['28714']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45359'
versions = ['Python 3.11']

[JacobHayes]

Reproduction:

from graphlib import TopologicalSorter

TopologicalSorter[str]({"a": {}, "b": {"a"}})

$ mypy /tmp/toposort.py
Success: no issues found in 1 source file
$ python3 /tmp/toposort.py
Traceback (most recent call last):
  File "/tmp/toposort.py", line 3, in <module>
    TopologicalSorter[str]({"a": {}, "b": {"a"}})
TypeError: 'type' object is not subscriptable

I opened the issue here (rather than typeshed) because we'd presumably like to support this at runtime too.

Typeshed link: https://github.com/python/mypy/blob/0a830481980bfc554ded61a3eaaaecde384a21e4/mypy/typeshed/stdlib/graphlib.pyi#L6

[asvetlov]

New changeset 3cb9731 by Jacob Hayes in branch 'main':
bpo-45359: Support TopologicalSorter type subscript (GH-28714)
3cb9731

[asvetlov]

The new feature is applied to Python 3.11 only

[JacobHayes]

Thanks for merging!

Should typeshed be updated for <3.11 in the meantime or do you suggest if TYPE_CHECKING blocks on user side? Perhaps it's a non-issue if no one else has noticed this. :)

[asvetlov]

`if TYPE_CHECKING:` is a good choice, it works fine just now.
`from __future__ import annotations` is another alternative.

I don't see a reason for REMOVING already existing and correct annotations from typeshed.

[AlexWaygood]

Another option for code using Python <3.11, that will work without the from __future__ import annotations import, is to do something like this:

from graphlib import TopologicalSorter
x: 'TopologicalSorter[str]' = TopologicalSorter({"a": {}, "b": {"a"}})

By using a string as the annotation, we give mypy the specificity it needs, but the expression will never need to be resolved at runtime.

[JacobHayes]

Thanks for the tips! I've been using this patch in my own code in a early imported __init__.py:

from graphlib import TopologicalSorter
from types import GenericAlias

if not hasattr(TopologicalSorter, "__class_getitem__"):  # pragma: no cover
    TopologicalSorter.__class_getitem__ = classmethod(GenericAlias)  # type: ignore

Certainly a bit hacky, but aside from no-op on >=3.11 and satisfying mypy, it supports runtime type inspection via get_type_hints before 3.11 (which otherwise still errors for stringized versions). The stringized versions are probably a bit simpler/safer for most folks though :)