Properties, setter, and Incompatible types in assignment

[engnadeau]

Expected Behaviour

• Ability to use property setter with Union[]
• The intention is to overload the setter

Actual Behaviour & Repro Case

Python Code

from typing import Union


class A:
    def __init__(self) -> None:
        self._foo = 0

    @property
    def foo(self) -> int:
        return self._foo

    @foo.setter
    def foo(self, value: Union[int, float]) -> None:
        self._foo = int(value)


if __name__ == '__main__':
    a = A()
    print(a.foo)

    a.foo = 123
    print(a.foo)

    a.foo = 456.789 # error thrown here
    print(a.foo)

Stdout

0
123
456

(i.e., valid Python code)

Mypy CLI

mypy --strict

Result

error: Incompatible types in assignment (expression has type "float", variable has type "int")

Notes

• Mypy seems to be type checking the assignment using the type declared in def foo(self) -> int: instead of the declared types in the setter: def foo(self, value: Union[int, float]) -> None:

Current Solution

• Silence the error with # type: ignore

[gvanrossum]

Your hunch is correct: for properties, mypy uses the type declared in the initial @property definition as the final type for the attribute, regardless of what the @property.setter does.

I suppose we could fix this though it might require a bunch of one-off changes in odd corners of the code since properties are unique in this respect.

[srittau]

This seems similar to issue #3004, which has more discussion.

[gvanrossum]

Oh, yeah, it's a duplicate.