type: ignore for “incompatible with supertype” error in subclass should suffice for subsubclasses too

class A:
    def foo(self, s: str) -> str: ...

class B(A):
    def foo(self, n: float) -> float: ... # type: ignore

class C(B):
    def foo(self, n: float) -> float: ...

Mypy playground link

Mypy is correct to report an error for B.foo that necessitated a type: ignore comment. However, having added that type: ignore for B.foo, mypy should not also report an error for C.foo. Although C.foo is inconsistent with A.foo, it’s consistent with B.foo, and I have already asked mypy to ignore the inconsistency between B.foo and A.foo.

To put this another way, I recognize that mypy would fail to catch the runtime error in a: A = C(); a.foo("str"), but this would still be a runtime error if C.foo did not override B.foo, so the blame should be assigned solely to B.foo and not to C.foo.

This comes up in the typeshed stubs for markdown.inlinepatterns:

class Pattern:
    def handleMatch(self, m: Match) -> Optional[Union[str, Element]]: ...

class InlineProcessor(Pattern):
    def handleMatch(self, m: Match, data) -> Union[Tuple[Element, int, int], Tuple[None, None, None]]: ...  # type: ignore

These stubs correctly reflect the upstream API which uses handleMatch with a different signature in subclasses of InlineProcessor than in other subclasses of Pattern. That wasn’t a great API design decision, but now that it’s made and the type: ignore was added to InlineProcessor.handleMatch, ideally I shouldn’t have to repeat the same type: ignore in order to write a custom subclass of InlineProcessor whose handleMatch satisfies the signature of InlineProcessor.handleMatch.

Riproduci il comportamento di controllo delle sottoclassi nel mypy playground collegato, quindi confrontalo con gli stub di typeshed in stubs/Markdown/markdown/inlinepatterns.pyi e con l’implementazione upstream in markdown/treeprocessors.py. Il lavoro è completato quando un type: ignore su InlineProcessor.handleMatch impedisce anche un errore non necessario incompatible-with-supertype per un override compatibile di una sottoclasse, mentre la diagnostica originale di B.foo rimane soppressa.