Vector Class - emulating numeric type

Question

I have a python class

class Vector2D(object):

    def __init__(self, x, y):
        self.x = float(x)
        self.y = float(y)

    def mag(self):
        return sqrt(self.x**2 + self.y**2)

    ...

I want to be able to multiply vectors together like vector1 * vector2, so I added

    def __mul__(self, v):
        return Vector2D(self.x * v.x, self.y * v.y)

But I also want to use new_vector = some_vector * 2 and return a new vector like so

    def __mul__(self, factor):
        return Vector2D(self.x * factor, self.y * factor)

How do I do both?

Answer 1

Check to see if v is a Vector2D, and if not pass it to float() and multiply appropriately.

Answer 2

There is no function overload in Python, you have to do it manually.

class Vector(object):
  def __init__(self, x, y):
    self.x = x
    self.y = y
  def __mul__(self, k):
    if type(k) == float or type(k) == int:
      return Vector(self.x * k, self.y * k)
    if type(k) == Vector:
      return Vector(self.x * k.x, self.y * k.y)
    raise "What the hell!?"
  def __str__(self):
    return "<%f, %f>" % (self.x, self.y)


print Vector(1, 2) * Vector(3, 4)
print Vector(1, 2) * 5

Answer 3

I'm not incredible with Python, but I'm pretty sure you just write one function and check the type of the second argument.

By the way, why are you multiplying vectors by just multiplying their components? That seems entirely useless.

Answer 4

Consider that you might want to use numpy for this sort of data.