mypy infer paramspec of function based on paramspec of first input

Question

I'm creating a function which will act as a factory for a class, here's how it will look:

T = TypeVar("T")
def app_factory(app: Type[T], *args, **kwargs) -> T:
    ...
    return app(*args, **kwargs)

class App:
    def __init__(self, a: str, b: bool):
        self.a = a
        self.b = b

app_instance = app_factory(app=App, a="string", b=True)

Would it be possible to dynamically give app_factory the typehints a: str, b: bool based on the first input arg app? In other words, is there any way to dynamically set ParamSpec of a function to be equal to one of the function input callables?

Answer 1

Yes, this is doable because type-checker implementations should recognise type[T] as a subtype of Callable[P, T], where P is the parameters type of T's __new__ or __init__.

from __future__ import annotations
import typing as t

if t.TYPE_CHECKING:
    import collections.abc as cx
    T = t.TypeVar("T")
    P = t.ParamSpec("P")

def app_factory(app: cx.Callable[P, T], *args: P.args, **kwargs: P.kwargs) -> T:
    return app(*args, **kwargs)

class App:
    def __init__(self, a: str, b: bool) -> None:
        self.a = a
        self.b = b

>>> app_factory(app=App, a="string", b=True)  # OK
>>> app_factory(app=App, a="string", b=1) # mypy: Argument "b" to "app_factory" has incompatible type "int"; expected "bool" [arg-type]