Get *args and **kwargs autocompletion but with modified types (wrapped in generic containers)

[Vince-LD]

Hello,

I aplogize for the bad title formulation but I don't know how to phrase what I am looking for. I would be happy to change the title if you have suggestions.

I have a small value container class (Field) that allow me to get and set the value at multiple places/time the object as a reference to the value. These objects are passed to Step objects that have a run method.

The issue i am having is that I would like to create FuncStep class that wraps around a given function, exposes its *args and **kwargs but as a TypeVarTuple[Field, ...] instead of TypeVarTuple. I know this syntax is wrong, sorry for the abomination haha. The idea is to allow a user to have autocompletion for the arguments aswell as type checking. To make it clearer, I created a simple version of what It could look like. The code snippet is long, the intersting part is at the end.

from typing import Callable, Generic, Optional, ParamSpec, TypeVar

T = TypeVar("T")
R = TypeVar("R")
P = ParamSpec("P")


class Field(Generic[T]):
    def __init__(self, value: T) -> None:
        self._value = value

    def get(self) -> T:
        return self._value

    def set(self, value: T):
        self._value = value


class Step:
    def run(self):
        ...

class FuncStep(Step, Generic[P]):
    def __init__(
        self,
        func: Callable[P, R],
        output: Field[Optional[R]],
        *args: P.args, # This should allow to use `func` arg names but wrapped in Field objects
        **kwargs: P.kwargs, # This should allow to use `func` arg names but wrapped in Field objects
    ) -> None:
        self.func = func
        self.output = output
        self.args = args 
        self.kwargs = kwargs
        super().__init__()

    def run(self):
        args = [arg.get() for arg in self.args]
        kwargs = {key: arg.get() for key, arg in self.kwargs.items()}
        self.output.set(self.func(*args, **kwargs))


def test_func(a: int, b: int) -> int:
    return a + b


if __name__ == "__main__":
    # These fields are Field[int]
    a_field, b_field, out_field = Field(1), Field(2), Field[Optional[int]](None)

    #I would like this FuncStep to await the parameters output: Field[int], a: Field[int] and b: Field[int]
    wrap_obj = FuncStep(test_func, output=out_field, a=a_field, b=b_field)
    wrap_obj.run() # This runs without error
    print(wrap_obj.output.get()) # We get the the right value

    # This is correct for the typechecker but doesn't work and doesn't allow me to 
    # modify the after the init for anywhere else in the program
    wrap_obj = FuncStep(test_func, output=out_field, a=8, b=7)
    wrap_obj.run()
    print(wrap_obj.output.get())

I guess a workaround solution would be to "cast" the Field[T] into the underlying T type so the type checker accepts Field[int] as input as it if was an int but I have no idea how to do that... As an example, I modified the Field class to have a cast property.

class Field(Generic[T]):
    def __init__(self, value: T) -> None:
        self._value = value

    def get(self) -> T:
        return self._value

    def set(self, value: T):
        self._value = value

    @property
    def cast(self) -> T:
        return cast(T, self)

...

if __name__ == "__main__":
    # This works but is ugly, I would like not to have to use  the `.cast` property
    wrap_obj = FuncStep(test_func, output=out_field, a=a_field.cast, b=b_field.cast)

Thank you for your help!

[Daverball]

This sort of thing requires something like the previously proposed Map construct. Unfortunately no concrete proposal or PEP has come out of it yet, so the thing you're doing right now is pretty much the only thing you can do. SQLAlchemy used a very similar workaround before TypeVarTuple was available for their Row type, which had a value property that just returned self but cast it from Row[T] to T.

[Vince-LD]

Thank your for your answer. It's unfortunate there is no solution. However, I'm happy to find out my solution is somewhat standard. What I did is created a property called T that returns self.
I really hope something like Map becomes available as it is a common pattern, especially in wrappers...