Question

So I was messing around writing a vector class when I discovered this interesting fact.

>>> e = int(3)
>>> e.__mul__(3.0)
NotImplemented

Can anyone explain why this is and subsequently, how to fix my vector class?

class Vector(tuple):
    '''A vector representation.'''
    def __init__(self, iterable):
        super(Vector, self).__init__(iterable)

    def __add__(self, other):
        return Vector(map(operator.add, self, other))

    def __sub__(self, other):
        return Vector(map(operator.sub, self, other))

    def __mul__(self, scalar):
        return Vector(map(scalar.__mul__, self))

    def __rmul__(self, scalar):
         return Vector(map(scalar.__mul__, self))

    def __div__(self, scalar):
        return Vector(map(scalar.__rdiv__, self))

EDIT: To be a little more clear:

>>> a = Vector([10, 20])
>>> a
(10, 20)
>>> b = a / 2.0
>>> b
(5.0, 10.0)
>>> 2 * b
(NotImplemented, NotImplemented)

Answer 1

That’s because when you do 3 * 3.0 the interpreter is calling (3.0).__rmul__(3) after realizing that (3).__mul__(3.0) is not implemented

Float’s __mul__ and __rmul__ functions do cast integers to float but that should not happen with the int class.

Otherwise 3 * 3.5 would be 9 instead of 10.5

The second question:

Why do people insist on map when list comprehensions (and generator expressions) are much better?

try that:

def __mul__(self, scalar):
    return Vector(scalar * j for j in self)

You should do that to every other function on the class.