What is the difference between __mul__ and __rmul__?

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In Python, __mul__ and __rmul__ are both used for the * operator, but they are called in different situations.

__mul__(self, other) is tried when your object is on the left side:

Python
a * b

__rmul__(self, other) is tried when your object is on the right side and the left object does not know how to multiply with it:

Python
b * a

Example:

Python
class Box:
    def __mul__(self, other):
        return "Box __mul__"

    def __rmul__(self, other):
        return "Box __rmul__"

x = Box()

print(x * 3)   # calls __mul__
print(3 * x)   # calls __rmul__

Python’s rule is roughly:

1. Try left operand’s __mul__

2. If it returns NotImplemented, try right operand’s __rmul__

This is especially useful for supporting expressions where your custom class appears on the right side of a built-in type, like 2 * custom_object.

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