Python two way operator overloading?

Question

Is it possible in python to overload operators for a class so that the order of variables in using the operator does not matter?

For example, is there a way to do the following, without getting a TypeError?

class A:
    def __init__(self, x: int):
        self.x = x

    def __add__(self, other: int):
        return self.x + other

a = A(1) + 4  # a = 5

# what about this:
a = 4 + A(1)

Asked By: Blackgaurd

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Source

Answer 1

This what the __rxxx__ special methods are for. You implement the forward methods to be type strict (expect specific concrete types) (returning NotImplemented when a recognized type was not provided), and implement the reverse methods to be more liberal (accepting ABC interfaces, because if you’re called, you’re the last chance to get it right). The __rxxx__ methods pass the left hand side as their second argument so you know which in which direction the computation is being performed.

For your class, a properly written pair of operators would be something like:

import numbers

class A:
    def __init__(self, x: int):
        self.x = x

    def __add__(self, other: int):
        if isinstance(other, int):
            return self.x + other
        if isinstance(other, A):
            return type(self)(self.x + other.x)
        return NotImplemented

    def __radd__(self, other: numbers.Integral):
        if isinstance(other, numbers.Integral):  # Accept broader ABC rather than just int
            return other + self.x
        if isinstance(other, A):
            return type(self)(other.x + self.x)
        return NotImplemented

Implementing each of these in this way when you have a lot of them can get pretty ugly, so frequently you want to refactor to generate the methods using operator module utilities; you can refer to the source of the fractions.Fraction class for a more complete/complex example (it was intentionally written to provide such an example to people implementing their own numeric types)

Answered By: ShadowRanger

Python two way operator overloading?

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