Handling "default" methods in class hierarchies

[rpgoldman]

feature request

See the directory test-data/default-methods/ in my branch: https://github.com/rpgoldman/mypy/tree/default-method-example

cd mypy/test-data/default-methods/code
mypy example.py

Complains that the SubClass method signature contradicts the one at SuperClass (which has a vacuous parameter list):

../stubs/example_classes.pyi:13: error: Signature of "example_method" incompatible with supertype "SuperClass"

Ideally, mypy should recognize that a method definition whose signature is:

(self, *args: Any, **kwargs: Any)

does not constrain any subclass's implementations of that method.

• What are the versions of mypy and Python you are using?

mypy 0.640+dev-b923c587a3eaa6cb55d9908be5344b2d9a574b33

Python 3.6

• What are the mypy flags you are using? (For example --strict-optional)

none

[rpgoldman]

P.S. in the PyMC3 code base I see these vacuously-parameterized methods used for delegation and for raising exceptions where a method is not implemented, but should be. I think there is probably a better mechanism for the latter, but I'm not as sure about the former.

[ilevkivskyi]

As a random suggestion, we can maybe make it a strictness flag, like in #5670 instead of adding a special decorator or similar.

[hauntsaninja]

Modern mypy is more unsound in its handling of *args: Any, **kwargs: Any