User defined __mul__ method is not commutative

Question

I wrote a class to represent vectors in Python (as an exercise) and I'm having problems with extending the built-in operators.

I defined a __mul__ method for the vector class. The problem is that in the expression x * y the interpreter calls the __mul__ method of x, not y.

So vector(1, 2, 3) * 2 returns a vector <2, 4, 6> just like it should; but 2 * vector(1, 2, 3) creates a TypeError because the built-in int class does not support multiplication by my user-defined vectors.

I could solve this problem by simply writing a new multiplication function

def multiply(a, b):
    try:
        return a * b
    except TypeError:
        return b * a

but this would require redefining every function that I want to use with my user-defined classes.

Is there a way to make the built-in function handle this correctly?

Answer 1

If you want commutativity for different types you need to implement __rmul__(). If implemented, it is called, like all __r*__() special methods, if the operation would otherwise raise a TypeError. Beware that the arguments are swapped:

class Foo(object):
    def __mul_(self, other):
        ''' multiply self with other, e.g. Foo() * 7 '''
    def __rmul__(self, other):
        ''' multiply other with self, e.g. 7 * Foo() '''