[FIXED] Static typing of the parameters of a Generic type

Issue

If I have "factory" method, taking a class type and the relevant parameters to create a resource, is there a way to have static analysis of the parameters?

from typing import Type

from pydantic import BaseModel

class Res(BaseModel):
    f: int
    class Config:
        extra = "forbid"


def factory[T](resource_type: Type[T], **kwargs) -> T:
    return resource_type(**kwargs)


factory(Res, f=3) # no problem!
factory(Res, f=3, g=4) # ValidationError, g is not allowed.

This works as expected at run time. I would love for pyright to notice statically that g is not a valid parameter to Res.

Is it at all possible? I am looking in the direction of ParamSpecs, but it does not help yet.

def factory[T, **P](resource_type: Type[T], *args: P.args, **kwargs: P.kwargs) -> T:
    return resource_type(**kwargs)
factory(Res, f=3) # says: Param spec "P@factory" has no bound value

Solution

typing.ParamSpec should be the way to go. The error message says it all: the ParamSpec is not bound to any callable. Change the signature from type[T] to Callable[P, T].

from typing import Callable, ParamSpec, TypeVar, reveal_type

from pydantic import BaseModel

T = TypeVar("T", bound=BaseModel)
P = ParamSpec("P")


class Res(BaseModel):
    f: int

    class Config:
        extra = "forbid"


def factory(resource_type: Callable[P, T], *args: P.args, **kwargs: P.kwargs) -> T:
    return resource_type(*args, **kwargs)


reveal_type(factory(Res, f=3))  # Revealed type is "Res"

Answered By - Paweł Rubin