covariant overriding of fields considered unsafe

[gvanrossum]

I don't recall where I saw this but I believe we had some example where a base class would declare a field of type float and the subclass would redefine it as int. I would just like to insert a note in that thread referencing python/mypy#3208, which points out that this is not actually a safe thing to do unless the class is immutable.

[ilevkivskyi]

Yes, but mypy currently only rejects this for protocols (which are deliberately more strict in some aspects), so technically speaking the example will pass, but I agree Any (probably in supertype) may be better for covariant mutable overriding example.

[ericvsmith]

It was in a test.

        # Note that x from C1 replaces x in Base, but the order remains
        #  the same as defined in Base.
        @dataclass
        class Base:
            x: float = 15.0
            y: int = 0

        @dataclass
        class C1(Base):
            z: int = 10
            x: int = 15

I'm not sure we're using "super" and "sub" the same way here. Should the type of x be Any in base, or in C1?

[ilevkivskyi]

I think it would be better with Any in Base.