Allow a type of Zero

[LivInTheLookingGlass]

I am requesting a feature. Often people want to be able to sum() their value types together. This is made easiest if their value types can accept an addition of 0. But the signature on such a method implies that it can accept any integer, which is false. The smallest working example would be:

from typing import Union


class Value(object):
    def __init__(self, val: int) -> None:
        self._value = val

    def __add__(self, other: Union[int, 'Value']) -> 'Value':
        if isinstance(other, Value):
            return Value(self._value, other._value)
        elif other == 0:
            return self
        else:
            return NotImplemented

sum((Value(1), Value(3))

It would be nice if this code could instead be written as:

from typing import Union, Zero


class Value(object):
    def __init__(self, val: int) -> None:
        self._value = val

    def __add__(self, other: Union[Zero, 'Value']) -> 'Value':
        if isinstance(other, Value):
            return Value(self._value, other._value)
        elif other == 0:
            return self
        else:
            return NotImplemented

sum((Value(1), Value(3))

This more clearly communicates the intent of the method.

[ilevkivskyi]

This is a duplicate of #3062 which is already on our short/medium term roadmap. With the literal types you will be able to just write Zero = Literal[0].

[gvanrossum]

I'm also curious where the convention to just start with 0 comes from. Do you have any examples of real code that uses that? (I understand the principle, I am just wondering who would do such a thing.)

[LivInTheLookingGlass]

I dont have a real example off the top of my head, except for code I was working on for a book club (Test Driven Development by Example). The book was written with Java in mind, and we decided that we should each implement it in our favorite language.

[gvanrossum]

OK, thanks.

[pawelswiecki]

I think Union[Literal[0], Value] or Union[Literal[False], Value] (or even Union[Literal[""], Value], depending on the use-case) is analogous to Union[None, Value], that is to Optional[Value].