Under what circumstances are __rmul__ called?

Question

Say I have a list l. Under what circumstance is l.__rmul__(self, other) called?

I basically understood the documentation, but I would also

Answer 1

Binary operators by their nature have two operands. Each operand may be on either the left or the right side of an operator. When you overload an operator for some type, you can specify for which side of the operator the overloading is done. This is useful when invoking the operator on two operands of different types. Here's an example:

class Foo(object):
    def __init__(self, val):
        self.val = val

    def __str__(self):
        return "Foo [%s]" % self.val


class Bar(object):
    def __init__(self, val):
        self.val = val

    def __rmul__(self, other):
        return Bar(self.val * other.val)

    def __str__(self):
        return "Bar [%s]" % self.val


f = Foo(4)
b = Bar(6)

obj = f * b    # Bar [24]
obj2 = b * f   # ERROR

Here, obj will be a Bar with val = 24, but the assignment to obj2 generates an error because Bar has no __mul__ and Foo has no __rmul__.

I hope this is clear enough.