[Tutor] __mul__ for different variable types?

Rich Lovely roadierich at googlemail.com
Sun Oct 4 23:31:53 CEST 2009

2009/10/4 Warren <warren at wantonhubris.com>:
> I'm a little confused on this one.
> I have a Vector class that I want to be able to multiply by either another
> vector or by a single float value.  How would I implement this in my
> override of __mul__ within that class?
> Do you check the variable type with a stack of "if isinstance" statements or
> something?  What is the preferred Python way of doing this?
> - Warren
> (warren at wantonhubris.com)
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I think isinstance is probably the most pythonic way of doing this,
and don't forget to add __rmul__ as well, for floats:

    from numbers import Real
except NameError:
    Real = (int, long, float)

class Vector(object):
    #__init__ and other methods ommitted.
    def __mul__(self, other):
        """self * other"""
        if isinstance(other, Vector):
            # code for Vector * Vector
        elif isinstance(other, Number):
            # code for Vector * number
            return NotImplemented
    def __rmul__(self, other):
        """other * self"""
        return self.__mul__(other)

Note that I've got no type checking in __rmul__, because if only
Vector * Vector has a different value if the terms are swapped, and
other * self will use other.__mul__ if other is a Vector.

Also note the import at the top:  numbers.Real is an Abstract Base
Class use for all real number types, introduced in Python 2.6, so
simplifies testing types.  See
http://docs.python.org/library/numbers.html for info.  If it's not
there, the code demos another feature of isinstance most people don't
notice: the type argument can be a sequence of types,  so I set Real
to a tuple of all the builtin real number types (that I can remember
off the top of my head) if the import fails.

If you want multiplication to be defined for complex numbers, change
all occurances of Real to Number, and add complex to the tuple.

Rich "Roadie Rich" Lovely

There are 10 types of people in the world: those who know binary,
those who do not, and those who are off by one.

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