Python: multiplication override

Question

So, I've got a custom class that has a __mul__ function which works with ints. However, in my program (in libraries), it's getting called the other way around, i.e., 2 * x where x is of my class. Is there a way I can have it use my __mul__ function for this?

score 60 · Accepted Answer · answered Jul 31 '11 at 22:05

60

Just add the following to the class definition and you should be good to go:

__rmul__ = __mul__

answered Jul 31 '11 at 22:05

Karl Knechtel

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Note that this won't always be what you want. Suppose you have vector and matrix classes, and you want to define multiplication between them by treating the vector as a 1xN matrix. Matrix multiplication isn't commutative, so you'd need to re-switch the order of arguments. – Karl Knechtel Aug 01 '11 at 04:24

score 37 · Answer 2 · answered Jul 31 '11 at 22:05

37

Implement __rmul__ as well.

class Foo(object):
    def __mul__(self, other):
        print '__mul__'
        return other
    def __rmul__(self, other):
        print '__rmul__'
        return other

x = Foo()
2 * x # __rmul__
x * 2 # __mul__

answered Jul 31 '11 at 22:05

Cat Plus Plus

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But `x * x` gives: `<__main__.Foo at 0x7f8604be1e50>` How to handle this? – P i Mar 22 '21 at 16:33
@Pi also overload `__repr__`. See https://stackoverflow.com/questions/1535327. – Karl Knechtel Apr 15 '23 at 05:35

Python: multiplication override

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