8000 overloading __new__ · Issue #421 · python/typing · GitHub
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Rouslan opened this issue May 2, 2017 · 2 comments
Closed

overloading __new__ #421

Rouslan opened this issue May 2, 2017 · 2 comments

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@Rouslan
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Rouslan commented May 2, 2017

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
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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
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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

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@Rouslan @gvanrossum @ilevkivskyi
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