Confusing error message for property with @overloaded function

[3rdcycle]

The following does not seem to work:

from typing import overload


class Example:

	@overload
	def fun(self, p: int) -> int: ...
	@overload
	def fun(self, p: str) -> str: ...

	def fun(self, p=1):
		if isinstance(p, int):
			return 'foo'
		else:
			return 'bar'

	prop = property(fun)

Argument 1 to "property" has incompatible type overloaded function; expected "Optional[Callable[[Any], Any]]"

Is this a bug or expected?
Also, fun(0) returns a string instead of an int as specified in the stub, but mypy doesn't seem to catch this. Again, bug or expected?

[JelleZijlstra]

The property part seems like a real bug, if a low-priority one (I doubt it's common to turn an overloaded function into a property).

As for not catching the incorrect return type, mypy doesn't type check the body of the overloaded function according to the overloads, only according to the annotations on the body itself. Since your function body has no annotation, mypy doesn't typecheck the body at all.

[JukkaL]

But property should be called with a function that takes one argument, not two arguments like fun? The problem here seems to be a confusing error message.

[3rdcycle]

Just to be sure, you're saying it's not enough that the implementation provides a default argument for p? Indeed, if I change the first overload to

def fun(self, p: int = 1) -> int: ...

It seems to work.

[Bibo-Joshi]

Hi. I'd like to come back to this old discussion with a new use case that's maybe a bit better. In short, the return value of the overloaded property depends on the generic attribute of the class.

import datetime as dtm
from typing import overload


class Example[T: (dtm.date, None)]:

    def __init__(self, date: T) -> None:
        self.date: T = date

    @overload
    def day(self: "Example[dtm.date]") -> int: ...

    @overload
    def day(self: "Example[None]") -> None: ...

    @property
    def day(self) -> int | None:
        if self.date is None:
            return None
        return self.date.day


def foo(example: Example) -> int:
    if example.day is None:
        return 0
    return example.day  # this is line 26


if __name__ == "__main__":
    print(foo(Example(dtm.date.today())))
    print(foo(Example(None)))

running mypy 1.11.2 with --enable-incomplete-feature=NewGenericSyntax gives

foo.py:26: error: Incompatible return value type (got overloaded function, expected "int")  [return-value]