Question

It feels like such a simple thing but I can’t seem to find the info I need.
Say I define a class Matrix:

class Matrix():
    def __mul__(self, other):
    if isinstance(other, Matrix):
        #Matrix multiplication.
    if isinstance(other, int): #or float, or whatever
        #Matrix multiplied cell by cell.

This work fine if I multiply a matrix by an int, but since int doesn’t know how to deal with matrices, 3*Matrix raises a TypeError.
How do I deal with this?

Answer 1

Define __rmul__ to override the calling of int()‘s __mul__ method:

class Matrix():
    # code

    def __rmul__(self, other):
        #define right multiplication here.

        #As Ignacio said, this is the ideal
        #place to define matrix-matrix multiplication as __rmul__() will
        #override __mul__().

    # code

Note that you can do this with all of the numeric operators.

Also note that it’s better to use new style classes, so define your class as:

class Matrix(object):

This will allow you to do things like:

if type(other) == Matrix: ...