Concatenate is not covered to be a return type in test_typing

[sobolevn]

BPO	46395
Nosy	@gvanrossum, @sobolevn, @Fidget-Spinner
PRs	bpo-46395: cover Concatenate as return type in test_typing #30618

^{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 2022-01-16.10:01:17.698>
created_at = <Date 2022-01-16.08:27:37.365>
labels = ['invalid', 'type-bug', 'tests', '3.11']
title = '`Concatenate` is not covered to be a return type in `test_typing`'
updated_at = <Date 2022-01-16.10:01:17.697>
user = 'https://github.com/sobolevn'

bugs.python.org fields:

activity = <Date 2022-01-16.10:01:17.697>
actor = 'sobolevn'
assignee = 'none'
closed = True
closed_date = <Date 2022-01-16.10:01:17.698>
closer = 'sobolevn'
components = ['Tests']
creation = <Date 2022-01-16.08:27:37.365>
creator = 'sobolevn'
dependencies = []
files = []
hgrepos = []
issue_num = 46395
keywords = ['patch']
message_count = 5.0
messages = ['410681', '410685', '410686', '410689', '410691']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'sobolevn', 'kj']
pr_nums = ['30618']
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46395'
versions = ['Python 3.11']

[sobolevn]

Right now all Concatenate tests do not cover Callable[..., Concatenate] case.

Link:

cpython/Lib/test/test_typing.py

Lines 4862 to 4877 in 09087b8

	P = ParamSpec('P')
	T = TypeVar('T')
	C1 = Callable[Concatenate[int, P], int]
	self.assertEqual(C1.__args__, (Concatenate[int, P], int))
	self.assertEqual(C1.__parameters__, (P,))
	C2 = Callable[Concatenate[int, T, P], T]
	self.assertEqual(C2.__args__, (Concatenate[int, T, P], T))
	self.assertEqual(C2.__parameters__, (T, P))
	# Test collections.abc.Callable too.
	C3 = collections.abc.Callable[Concatenate[int, P], int]
	self.assertEqual(C3.__args__, (Concatenate[int, P], int))
	self.assertEqual(C3.__parameters__, (P,))
	C4 = collections.abc.Callable[Concatenate[int, T, P], T]
	self.assertEqual(C4.__args__, (Concatenate[int, T, P], T))
	self.assertEqual(C4.__parameters__, (T, P))

I think, that this should be improved. It is an important case.
PR is on its way.

[Fidget-Spinner]

Sorry I'm confused: Callable[..., Concatenate] is invalid usage. So I don't think we need to test it.

https://docs.python.org/3/library/typing.html#typing.Concatenate. "Concatenate is currently only valid when used as the first argument to a Callable."

[sobolevn]

Sorry I'm confused: Callable[..., Concatenate] is invalid usage. So I don't think we need to test it.

Yes, my bad :)

In this case, do we need to make this a TypeError? Right now you can use Concatenate as the return type.

[Fidget-Spinner]

In this case, do we need to make this a TypeError? Right now you can use Concatenate as the return type.

Last I recall, we stopped checking anything in the param args of Callable since 3.10 because it has too many problems with inter-version typing and typing_extensions usage. E.g. in Python 3.8

Callable[typing_extensions.Concatenate[...], ...]

was extremely difficult to implement in typing_extensions since Callable checks were too strict back then (they only allowed list of args). So to avoid such problems in the future, we ceased all param args checking.

For the return arg, I'm not too keen on adding more checks for the same reason of allowing future additions. Also the static type checker should warn the user and mypy correctly does that.

[sobolevn]

Thank you for the explanation, Ken Jin.

Going to close this issue as invalid.