functools.partial should use PEP612 ParamSpec (depends on #8708) #8703

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1. AlexWaygood commented on Sep 7, 2022

   

   AlexWaygood

   on Sep 7, 2022

   Last edited by AlexWaygood

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   You can nearly get there with a combination of PEP 612 and PEP 646:

   import sys
   from types import GenericAlias
   from typing import Any, Callable, Concatenate, Generic, ParamSpec, TypeVar, TypeVarTuple, overload
   
   _P1 = ParamSpec("_P1")
   _P2 = ParamSpec("_P2")
   _T = TypeVar("_T")
   _R_co = TypeVar("_R_co", covariant=True)
   _Ts = TypeVarTuple("_Ts")
   
   class partial(Generic[_P1, _P2, _T, _R_co, *_Ts]):
       @overload
       def __new__(cls, __func: Callable[_P1, _R_co]) -> partial[_P1, _P1, Any, _R_co]: ...
       @overload
       def __new__(cls, __func: Callable[Concatenate[*_Ts, _P2], _R_co], *args: *_Ts) -> partial[Concatenate[*_Ts, _P2], _P2, Any, _R_co, *_Ts]: ...
       @overload
       def __new__(cls, __func: Callable[_P1, _R_co], *args: *_Ts, **kwargs: _T) -> partial[_P1, ..., _T, _R_co, *_Ts]: ...
       def __call__(self, *args: _P2.args, **kwargs: _P2.kwargs) -> _R_co: ...
       @property
       def func(self) -> Callable[_P1, _R_co]: ...
       @property
       def args(self) -> tuple[*_Ts]: ...
       @property
       def keywords(self) -> dict[str, _T]: ...
       if sys.version_info >= (3, 9):
           def __class_getitem__(cls, item: Any) -> GenericAlias: ...

   Mypy doesn't support PEP 646 yet, however. And even if it did, there would still be some corner cases that don't work (PyCQA/flake8-pyi#150 (comment))
2. benbariteau commented on Sep 8, 2022

   

   benbariteau

   on Sep 8, 2022

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   Hmmm, I think that's probably the best we're gonna get for now. It's a strict improvement in my opinion.
3. srittau commented on Sep 8, 2022

   

   srittau

   on Sep 8, 2022

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   Cf. #8708
4. AlexWaygood

   added

   status: deferredIssue or PR deferred until some precondition is fixedIssue or PR deferred until some precondition is fixed

   on Sep 14, 2022
5. AlexWaygood

   changed the title [-]functools.partial should use PEP612 ParamSpec[/-] [+]functools.partial should use PEP612 ParamSpec (depends on #8708)[/+] on Sep 28, 2022
6. andersk commented on Dec 11, 2023

   

   andersk

   on Dec 11, 2023

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   #8708 has been closed. Can this be un-deferred yet?
7. AlexWaygood commented on Dec 11, 2023

   

   AlexWaygood

   on Dec 11, 2023

   Last edited by AlexWaygood

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   #8708 has been closed. Can this be un-deferred yet?

   No, pytype still crashes (meaning our CI would fail) if we make a class Generic over a TypeVarTuple, which is what the solution outlined in #8703 (comment) would require.
8. Dreamsorcerer commented on Dec 11, 2023

   

   Dreamsorcerer

   on Dec 11, 2023

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   pytype still crashes

   Is that the one that has apparently been fixed a couple of days ago: google/pytype#1525 (comment)
   Or is there another issue to track?
9. AlexWaygood commented on Dec 11, 2023

   

   AlexWaygood

   on Dec 11, 2023

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   Is that the one that has apparently been fixed a couple of days ago

   no
10. erikkemperman commented on Jul 18, 2024

    

    erikkemperman

    on Jul 18, 2024

    Last edited by erikkemperman

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    @AlexWaygood
    It would certainly be fantastic if the type annotations for partial captured something about the arguments as well the return type. But are you sure that the approach you outlined above would work, in principle? I didn't think it would be legal to unnpack a TypeVarTuple within Concatenate? I'm not sure if this is just a matter of the checkers not yet implementing it, or if there is some fundamental reason that this could not work, but currently both pyright and mypy disallow this.