Incompatible overrides

It’s unsafe to override a method with a more specific argument type, as it violatesthe Liskov substitution principle. For return types, it’s unsafe to override a method with a more general return type.

Here is an example to demonstrate this

from typing import Sequence, List, Iterable
 
class A:
    def test(self, t: Sequence[int]) -> Sequence[str]:
        pass
 
# Specific argument type doesn't work
class OverwriteArgumentSpecific(A):
    def test(self, t: List[int]) -> Sequence[str]:
        pass
 
# Specific return type works
class OverwriteReturnSpecific(A):
    def test(self, t: Sequence[int]) -> List[str]:
        pass
 
# Generic return type doesn't work
class OverwriteReturnGeneric(A):
    def test(self, t: Sequence[int]) -> Iterable[str]:
        pass

mypy won’t report an error for OverwriteReturnSpecific but it does for OverwriteReturnGeneric and OverwriteArgumentSpecific.

We can use # type: ignore[override] to silence the error (add it to the line that genreates the error) if type safety is not needed.