Python commutative operator override

Question

Hi I was wondering if there is a way to do a symmetric operator override in Python. For example, let's say I have a class:

class A:
    def __init__(self, value):
        self.value = value

    def __add__(self, other):
        if isinstance(other, self.__class__):
            return self.value + other.value
        else:
            return self.value + other

Then I can do:

a = A(1)
a + 1

But if I try:

1 + a

I get an error. Is there a way to override the operator add so that 1 + a will work?

Answer 1

Just implement an __radd__ method in your class. Once the int class can't handle the addition, the __radd__ if implemented, takes it up.

class A(object):
    def __init__(self, value):
        self.value = value

    def __add__(self, other):
        if isinstance(other, self.__class__):
            return self.value + other.value
        else:
            return self.value + other

    def __radd__(self, other):
        return self.__add__(other)


a = A(1)
print a + 1
# 2
print 1 + a
# 2

For instance, to evaluate the expression x - y, where y is an instance of a class that has an __rsub__() method, y.__rsub__(x) is called if x.__sub__(y) returns NotImplemented.

Same applies to x + y.

On a side note, you probably want your class to subclass object. See What is the purpose of subclassing the class "object" in Python?