Bad error message for covariant overriding of a mutable attribute

[mthuurne]

When I check this with mypy 0.711:

from typing import Callable, Optional

class Base:
    formatter1: Callable[[int], str]
    formatter2: Optional[Callable[[int], str]]
    formatter3: Optional[Callable[[int], str]]
    formatter4: Optional[Callable[[int], str]]

class Sub(Base):
    @staticmethod
    def formatter1(value: int) -> str:
        return str(value)
    @staticmethod
    def formatter2(value: int) -> str:
        return str(value)
    formatter3 = formatter2
    formatter4 = str

It reports:

14: error: Signature of "formatter2" incompatible with supertype "Base"

I would expect all four formatters to be accepted, but for some reason formatter2 is not.

It doesn't make a difference whether I use the old or new semantic analyzer.

[ilevkivskyi]

It looks like mypy already prohibits covariant overriding of mutable attributes if one of them is a method, unlike for variables (see #3208), which is quite funny.

Note however, this is not a bug, this is actually unsafe code:

y = Sub()
x: Base = y

x.formatter2 = None
y.formatter2(42)  # Oops, TypeError: NoneType not callable.

[ilevkivskyi]

Actually, on the second thought I think we should keep this open for cryptic error message. It should explain that the problem here is mutability in superclass.

[mthuurne]

Hmm, indeed this is unsafe in general. The way I use this pattern is to define optional operations, so these fields will never be modified at runtime, but there is no way for mypy to know that.

[ilevkivskyi]

It now works as expected (both without and with --enable-error-code=mutable-override)