__key__ parameter for classes in Python

Question

I have an array of vectors and I want to sort them by length:

class Vector:

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

     def __add__(a, b):
       return Vector(a.x + b.x, a.y + b.y)

     def __str__(a):
       return str(a.x) + ' ' + str(a.y) + '\n'

     def __key__(self):
       return self.x * self.x + self.y * self.y


a = []
a.append(Vector(1,2))
a.append(Vector(1, 1))
a.sort()
print("".join(map(str,a)))

It says: "unorderable types: Vector() < Vector()" It wants me to create lt, gt.. methods. But I want to sort without using cmp. Is it possible?

Answer 1

I would implement __lt__ and __eq__ and then use the functools.total_ordering class decorator to get the rest of the comparison methods.

If it doesn't make sense to have your vectors ordered like that, then you can always just use the key keyword to sort (or sorted for that matter):

mylist.sort(key = lambda v: v.x**2 + v.y**2)

Answer 2

You have two variants here: implement __cmp__ function in Vector class or perform sorting this way:

...
a.sort(key=Vector.__key__)