False positive error on subclass overriding a ClassVar

[carljm]

This seems to be a bug.

With the following code:

from typing import ClassVar

class Base:
    name: ClassVar[str]

class Sub(Base):
    name = "Sub"

mypy complains that :7: error: Cannot override class variable (previously declared on base class "Base") with instance variable.

But the assignment on the subclass is clearly at the class level. Assigning a value to a class attribute that has already been declared as one should not result in an error. I'd expect this error only if the assignment took place e.g. inside __init__.

Explicitly annotating the subclass assignment (name: ClassVar[str] = "Sub") gets rid of the error, but that repeated annotation should not be necessary.

Reproduced on mypy 0.580-dev-9374acf2d808905d392b2ec8ea52bfce03f9ecad under Python 3.6.4, with no flags or options.

I realize that inability to distinguish instance vs class attributes is a known mypy limitation (#1097), but in general it just results in false negatives. This seems worth a distinct issue since it appears that mypy is attempting to make the distinction, but doing so incorrectly, resulting in a false positive.

[gvanrossum]

Agreed this is fishy. Do you feel up to submitting a PR to fix this?

[carljm]

I can try to find time to look at it soon.

[lokhman]

I'm getting this error in the recent version of MyPy. Was the merged fix overridden or rolled back?

[AlexWaygood]

@lokhman, commenting on an issue that's been closed for 4 years isn't the best way to discuss a problem ;)

I'd recommend opening a new issue with a minimal, reproducible example of the code that doesn't type check. The code in the original example in this issue type-checks fine for me on mypy 0.950, even using mypy --strict, so I suspect that your problem is a slightly different one to the one that was discussed in this issue.