More precise type checking of NotImplemented and operator overloading

[JukkaL]

NotImplemented should be implicitly a valid return value of operator methods such as __lt__, __add__, but it should be flagged as an error to return it elsewhere (unless, maybe, the return type includes a suitable type such as typing.NotImplementedType).

Currently NotImplemented has type Any which allows it to be used in any context, which is wrong.

This should be fine:

def __lt__(self, x: int) -> bool:
    if isinstance(x, int):
        return ...
    return NotImplemented

This should not be fine:

def f() -> bool:
    return NotImplemented

[mthuurne]

(unless, maybe, the return type includes a suitable type such as typing.NotImplementedType).

There is no typing.NotImplementedType at the moment though. I think Literal[NotImplemented] would be more elegant anyway, but that is not accepted by mypy 0.790.

[brianrodri]

Could we please get some official guidance on how to workaround this issue in the short term?

I understand the complexity at stake here after reading the referenced/referencing issues, but I think it'd be helpful to have an "official workaround" until this gets resolved.

From my understanding, the problem is:

def __eq__(self, other: Any) -> bool: ... # Doesn't work.
def __eq__(self, other: Any) -> Union[bool, NotImplemented]: ... # Doesn't work.
def __eq__(self, other: Any) -> Any: ... # Only option?

[0dminnimda]

Are there any updates on this?