overloading __new__

[Rouslan]

I propose allowing the use of @overload on __new__ to specify the return type. E.g.:

T = TypeVar('T')

class MyClass(Generic[T]):
    @overload
    def __new__(cls,val: str) -> 'MyClass[int]':
        pass
    @overload
    def __new__(cls,val: int) -> int:
        pass
           
    def __new__(cls,val):
        if isinstance(val,int):
            return val
        r = super().__init__(cls)
        if isinstance(val,str):
            r.val = int(val)
        r.val = val
        return r

This is useful on its own, but if @overload is allowed to have return types that only differ by the generic type argument (e.g. MyClass[int] vs MyClass[str]), then this would also supersede issue #307 and allow more complicated type argument deduction.

[gvanrossum]

This sounds like something to be filed in the mypy tracker. What's a realistic use case for allowing __new__ to return something that's not an instance of the containing class?

[ilevkivskyi]

What's a realistic use case for allowing __new__ to return something that's not an instance of the containing class?

To be fair typing uses this few times :-) but I agree this is quite esoteric, and this feature proposal belongs to https://github.com/python/mypy/issues