mypy doesn't handle property setters correctly

[WyattBlue]

Consider this example:

class A:
    def __init__(self, val: int) -> None:
        self.val = 1

    @property
    def my_property(self) -> int:
        return self.val
    
    @my_property.setter
    def my_property(self, value: int | str) -> None:
        self.val = int(value)

a = A(1)
a.my_property = "2"  # I expect this to be allowed

but instead I get this error:

test.py:14: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]

[JelleZijlstra]

This is python/mypy#3004.

[WyattBlue]

If mypy doesn't support getters/setters being different types (fair enough), why isn't this line def my_property(self, value: int | str) -> None: causing an error because right now, it does nothing and misleading.

[erictraut]

The Python typing spec doesn't say anything about asymmetric descriptors or properties, but other type checkers like pyright handle them fine. As Jelle noted, this is a long-standing open issue with mypy. Interestingly, mypy does handle asymmetric descriptors if they are not properties.