reverse second sort characteristic in python sorted()

Question

I\'m trying to sort a list and I can\'t figure out to reverse the order of a second sorting characteristic.

import math
ps = {1:(1,1),2:(3,2),3:(3,-3),4:(-3,4),         


        

Answer 1

If your key-function get's longish and/or complicated you can also use a custom class that compares like you want. It's a bit slower but it can be more readable, especially because you can comment the code:

import math

class Angle_Distance(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y
        self.angle = math.atan2(self.y, self.x)
        self.distance = math.hypot(self.x, self.y)

    def __repr__(self):
        return '{self.__class__.__name__}(x={self.x}, y={self.y})'.format(self=self)

    def __eq__(self, other):
        # this method is not strictly necessary, but makes the class more generally useful.
        return self.x == other.x and self.y == other.y

    def __lt__(self, other):
        if self.angle < other.angle:
            return True
        elif self.angle == other.angle:  # break ties
            return self.distance > other.distance
        else:
            return False

And this can be applied on the list:

>>> ps = {1:(1,1),2:(3,2),3:(3,-3),4:(-3,4),5:(-2,-2),6:(3,3),7:(1,-1)} 
>>> l = [Angle_Distance(i, j) for i, j in ps.values()]
>>> sorted(l, reverse=True)
[Angle_Distance(x=-3, y=4),
 Angle_Distance(x=1, y=1),
 Angle_Distance(x=3, y=3),
 Angle_Distance(x=3, y=2),
 Angle_Distance(x=1, y=-1),
 Angle_Distance(x=3, y=-3),
 Angle_Distance(x=-2, y=-2)]

But you could also use it as key-function:

>>> ps = {1:(1,1),2:(3,2),3:(3,-3),4:(-3,4),5:(-2,-2),6:(3,3),7:(1,-1)}
>>> sorted(ps.values(), key=lambda x: Angle_Distance(x[0], x[1]), reverse=True)
[(-3, 4), (1, 1), (3, 3), (3, 2), (1, -1), (3, -3), (-2, -2)]