Should mypy raise errors on missing kwargs for __init__ on factory functions?

[fernandobperezm]

I have been trying to create a factory function for my classes and subclasses.

Most subclasses have common args and kwargs. However, my main problem is that some subclasses have kwargs that are unique for them.

An MRE of my situation is the following:

from typing import TypeVar, Type


class A:
    def __init__(self, b: int):
        ...


class B(A):
    def __init__(self, b: int, c: str):
        ...


class C(A):
    def __init__(self, b: int, d: int):
        ...


T = TypeVar('T', bound=A)


def foo(cls: Type[T], *args, **kwargs) -> T:
    return cls(*args, **kwargs)


foo(B)
foo(B, b=1, c="1")

foo(C)
foo(C, b=1, d=1)

I'm wondering if a type checker (specifically looking at mypy) should raise errors for both foo(B) and foo(C) since I'm not passing the required kwargs. Is this something that mypy (or another type-checker) is able to do?

My reasoning is that I want to be able to call that function and them mypy tell me if I'm missing any arg or kwarg. I know that I can get the errors by running the interpreter, however, when dealing with several kwargs, it can be time-consuming to execute the run the interpreter several times just to know that I missed a kwarg.

NOTE: This is my first discussion/question in this repo so I don't know if this is the best place to ask this question or even if it should be asked here. I very much welcome suggestions/corrections about this topic too 😊. Thanks!

[erictraut]

You can use ParamSpec (see PEP 612 for details).

Here's how that would be specified in your example use case.

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

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

[fernandobperezm]

Thanks! I see that ParamSpec is currently available in Python 3.10. I'm at Python 3.9, so I'm using typing_extensions, it should work in the same way, right?.

mypy is currently raising the following errors:

• Name "P.args" is not defined
• Name "P.kwargs" is not defined
• The first argument to Callable must be a list of types or "..."

And the snippet that I sent now is:

from typing import TypeVar, Callable
from typing_extensions import ParamSpec


class A:
    def __init__(self, b: int):
        ...


class B(A):
    def __init__(self, b: int, c: str):
        ...


class C(A):
    def __init__(self, b: int, d: int):
        ...


P = ParamSpec("P")
T = TypeVar('T', bound=A)


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


foo(B)
foo(B, b=1, c="1")

foo(C)
foo(C, b=1, d=1)

Idk if it is a bug in mypy or I'm missing something else?

Thanks for your quick reply!

[AlexWaygood]

What version of mypy are you using? This should be supported on the latest version of mypy.

[fernandobperezm]

Oh my, I'm sorry, I ran update before replying and it told me I was updated (prob sending an issue now to Poetry 😂 as I was not).

I was running mypy 0.910. Updated to 0.950 and now it works!

Thanks!!

[AlexWaygood]

Awesome -- no worries!

[fernandobperezm]

If I try to use this same approach on a classmethod decorated method inside A, mypy throws the following error: Mypy: The erased type of self "def (*P.args, **P.kwargs) -> T'-2" is not a supertype of its class "Type[types.A]".

To avoid repetitions, the only change is in the definition of A:

class A:
    def __init__(self, b: int):
        ...

    @classmethod
    def build(cls: Callable[P, T], *args: P.args, **kwargs: P.kwargs) -> T:
        return cls(*args, **kwargs)

I tried changing the function definition here and there but ended up in my original situation (so no error from mypy when args where missing) or with the error I just showed.

The semantics of this build function would be exactly the same as foo in my example. I would be use it in this way so subclasses have this build function defined.

[erictraut]

Yeah, mypy isn't happy with this usage, but it looks to me like it's valid. You might want to file a bug in the mypy tracker.