Support for variadiac arguments for inherited classes?

[adewes]

I'm unsure if this might be a bug, a feature request or if I just failed to find the right spot in the documentation.

What I want to do: I have an abstract base class that defines an interface which derived classes need to implement. There's an abstract method run that requires several mandatory arguments, and may accept additional optional arguments (to override defaults). Currently I'm not sure how I can model this using mypy. Here's some example code that reproduces the issue (ran with Python 3.7 and mypy 0.720:

import abc
  
from typing import Any

class Base(abc.ABC):

    @abc.abstractmethod
    def run(self, model: Any, **kwargs: int) -> None:
        pass

class Child(Base):
    def run(self, model: Any, foo : int) -> None:
        pass

This produces the following error:

mypy_test.py:12: error: Signature of "run" incompatible with supertype "Base"

I sued the **kwargs annotation in reference to this PEP but I realize the use case is different. I couldn't find anything in the mypy documentation about annotating this correctly.

I could of course use **kwargs in the Child class and parse optional parameters from there, but I don't think that's a good solution. Any help to get this working would be highly appreciated!

[JukkaL]

The override is unsafe (violates Liskov substitutability), so mypy is acting as designed here. Mypy requires that method overrides have more general signatures than the original method. You can add a # type: ignore comment on the line with error to silence this.

[adewes]

Ok thanks for the feedback, I'll used the kwargs route then for now as I don't want to ignore the type information.

[bpshaver]

For anyone reading this in the future dealing with a similar problem, it appears that the following modification to the example code does pass mypy:

import abc
  
from typing import Any

class Base(abc.ABC):

    @abc.abstractmethod
    def run(self, model: Any) -> None:
        pass

class Child(Base):
    def run(self, model: Any, foo : int = 5) -> None:
        pass

As far as I can tell, this works since it doesn't violate the Liskov substitution principle. Since foo now has a default value, instances of Child work where instances of any subclass Base might be expected.