How to define when a class is empty

Question

I wrote my own vector class:

#! /usr/bin/env python3

class V:
    """Defines a 2D vector"""
    def __init__(self,x,y):
        self.x = x
        self.y = y
    def __add__(self,other):
        newx = self.x + other.x
        newy = self.y + other.y
        return V(newx,newy)
    def __sub__(self,other):
        newx = self.x - other.x
        newy = self.y - other.y
        return V(newx,newy)
    def __str__(self):
        return "V({x},{y})".format(x=self.x,y=self.y)

I want to define that V(0,0) is an empty vector, such that this would work: (The first case should return "Vector is empty")

v = V(0,0)
u = V(1,2)

if u:
    print (u)
else:
    print("Vector is empty")

if v:
    print(v)
else:
    print("Vector is empty")

Answer 1

You can implement the special method __bool__:

def __bool__ (self):
    return self.x != 0 or self.y != 0

Note that in Python 2, the special method is named __nonzero__.

Alternatively, because you have a vector, it might make even more sense to implement __len__ and provide the actual vector length instead. If __bool__ is not defined, Python will automatically try to use the __len__ method to get the length and evaluate if it’s not zero.

Answer 2

Define __bool__, like this:

class V:
    """Defines a 2D vector"""
    def __init__(self,x,y):
        self.x = x
        self.y = y

    def __bool__(self):
        return self.x != 0 or self.y != 0

    # Python 2 compatibility
    __nonzero__ = __bool__