attempt to add typing to _create

[jakkdl]

context: #3256 (comment)

@A5rocks

remaining errors:

src/trio/_core/_local.py:27: error: Argument 1 to "_create" of "NoPublicConstructor" has incompatible type "RunVar[T]"; expected "Never"  [arg-type]
src/trio/_channel.py:265: error: Argument 1 to "_create" of "NoPublicConstructor" has incompatible type "MemoryChannelState[SendType]"; expected "Never"  [arg-type]
src/trio/_channel.py:417: error: Argument 1 to "_create" of "NoPublicConstructor" has incompatible type "MemoryChannelState[ReceiveType]"; expected "Never"  [arg-type]

my hypothesis for these is that they all inherit from some other class which doesn't have this parameter, and _create sees the super class. idk how to fix that

src/trio/_core/_run.py:1907: error: Argument "coro" to "_create" of "NoPublicConstructor" has incompatible type "Coroutine[object, Never, object]"; expected "CoroutineType[Any, Outcome[object], Any]"  [arg-type]

idk who's in the wrong here

src/trio/_subprocess.py:430: error: Argument 1 to "_create" of "NoPublicConstructor" has incompatible type "Popen[str]"; expected "Popen[bytes]"  [arg-type]

this might be an actual bug? https://github.com/python/typeshed/blob/eec809d049d10a5ae9b88780eab15fe36a9768d7/stdlib/subprocess.pyi#L1464 for reference

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (0124394) to head (6ef891e).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3260   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19414        19414           
  Branches         1318         1318           
===============================================
  Hits            19414        19414           

Files with missing lines	Coverage Δ
src/trio/_util.py	100.00000% <100.00000%> (ø)
src/trio/testing/_fake_net.py	100.00000% <100.00000%> (ø)

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[TeamSpen210]

For the _create calls, I know Never often appears if mypy failed to solve the typevars, so could be an inference failure. Might be good to add reveal_type(TheClass._create) calls so we can see what mypy assumes it is.

[TeamSpen210]

What's going on here is the cls: Callable makes MyPy forget that it should require a class - NoPublicConstructor._create(lambda: 0) should function. It is useful to be able to widen self-types like that, but it means the super() call fails. I guess Mypy type-checks the 2-arg form of super() automatically?

[A5rocks]

Hmm, it's not wrong surprisingly:

>>> class Xs:
...     def f(self) -> None:
...         pass
...
>>> class X(Xs):
...     def f(self: object) -> None:
...         super().f()
...         return
...
>>> Xs().f()
>>> X().f()
>>> X.f(X())
>>> X.f(5)
Traceback (most recent call last):
  File "<python-input-7>", line 1, in <module>
    X.f(5)
    ~~~^^^
  File "<python-input-3>", line 3, in f
    super().f()
    ^^^^^^^^^
TypeError: super(type, obj): obj (instance of int) is not an instance or subtype of type (X).

[jakkdl]

For the _create calls, I know Never often appears if mypy failed to solve the typevars, so could be an inference failure. Might be good to add reveal_type(TheClass._create) calls so we can see what mypy assumes it is.

reveal_type(cls._create) in class RunVarToken(Generic[T], metaclass=NoPublicConstructor)

src/trio/_core/_local.py:27: note: Revealed type is "def (*args: Never, **kwargs: Never) -> trio._core._local.RunVarToken[T`1]"

reveal_type(MemorySendChannel._create)

src/trio/_channel.py:265: note: Revealed type is "def (*args: Never, **kwargs: Never) -> Never"

[jakkdl]

also if anybody wants to mess around with this PR you are very welcome, I'm not gonna put a lot of effort into it

[A5rocks]

Looks like this approach doesn't handle type vars (I think that's what you're seeing):

import dataclasses
from typing import Protocol, TypeVar, ParamSpec, Generic, Callable

T = TypeVar("T", covariant=True)
P = ParamSpec("P")

class Meta(type):
    def __call__(cls, *args: object, **kwargs: object) -> None:
        raise TypeError(
            f"{cls.__module__}.{cls.__qualname__} has no public constructor",
        )

    def _create(cls: Callable[P, T], *args: P.args, **kwargs: P.kwargs) -> type[T]:
        return super().__call__(*args, **kwargs)  # type: ignore

@dataclasses.dataclass
class X(Generic[T], metaclass=Meta):
    x: T


reveal_type(X._create)  # N: Revealed type is "def (*args: Never, **kwargs: Never) -> type[Never]"

Surprisingly now that I see it minimized like that, I think I know exactly the bug: python/mypy#18400. I haven't checked my solution PR to make sure it has the same cause though.