Infer type argument of an instance from arguments to a generic method called on it

I think this is a feature request to allow specifying typing for the following. See the following example:

from abc import *
from fractions import *
from typing import *

Input = TypeVar('Input')
Output = TypeVar('Output')

class Base(Generic[Input, Output]):
    def do(self, input: Input) -> Output:
        # In reality do a bit more than just call "_operator".
        return self._operator(input)

    @abstractmethod
    def _operator(self, input: Input) -> Output:
        pass

class IntAdd(Base[int, int]):
    def _operator(self, input: int) -> int:
        return input + 1

class UnionAdd(Base[Union[int, Fraction], Union[int, Fraction]]):
    def _operator(self, input: Union[int, Fraction]) -> Union[int, Fraction]:
        return input + 1

reveal_type(IntAdd().do(1))
reveal_type(UnionAdd().do(1))

A = TypeVar('A', bound=Union[int, Fraction])

def do(input: A) -> A:
    return input + 1

reveal_type(do(1))
reveal_type(do(Fraction(2, 3)))

class SuperUnionAdd(Base[A, A]):
    def _operator(self, input: A) -> A:
        return input + 1

reveal_type(SuperUnionAdd().do(1))
reveal_type(SuperUnionAdd().do(Fraction(2, 3)))

The output I get is:

demo.py:25: error: Revealed type is 'builtins.int*'
demo.py:26: error: Revealed type is 'Union[builtins.int, fractions.Fraction]'
demo.py:31: error: Incompatible return value type (got "Union[int, Any]", expected "A")
demo.py:33: error: Revealed type is 'builtins.int*'
demo.py:34: error: Revealed type is 'fractions.Fraction*'
demo.py:38: error: Incompatible return value type (got "Union[int, Any]", expected "A")
demo.py:40: error: Revealed type is '<nothing>'
demo.py:40: error: Argument 1 to "do" of "Base" has incompatible type "int"; expected <nothing>
demo.py:41: error: Revealed type is '<nothing>'
demo.py:41: error: Argument 1 to "do" of "Base" has incompatible type "Fraction"; expected <nothing>

I am running mypy 0.701 on Python 3.7.1. I am on purpose use Fraction here so that we do not get automatic compatibility between numbers into effect.

What I would like to define is a UnionAdd class where if I pass to do an int, the return type would also be an int. And if I pass in Fraction, return type would be Fraction. But at the same time, I would like to use a Generic base class.

I know that I could define something like the do function I defined at the end. That one returns correctly the revealed type on lines 33 and 34 based on the input type. (Not sure what is the Incompatible return value type on line 31 about and how to fix it). But what I do not see is a way to combine the effect what I am doing for function do with the UnionAdd class and generic Base class. The example SuperUnionAdd where I blindly combine A and Base does not work.

Inizia riproducendo l’esempio demo.py fornito e analizza il modo in cui mypy deduce le variabili di tipo per i metodi generici su istanze generiche. Confronta i tipi rivelati e la diagnostica attuali con l’inferenza richiesta per argomento per UnionAdd e SuperUnionAdd. Il lavoro è completo quando i metodi della classe deducono int o Fraction dai loro argomenti, mantenendo al contempo il design generico di Base.