The other day I found a bug in a python file I had written. I ordered a list by two attributes descending, when I really wanted to order the list by one attribute descending and the other ascending.

For reference, I started off with something similar to the following code. I'm using tuples here instead of attributes off of an object, but the premise remains.

lst = [(2, 3), (2, 2), (2, 5), (1, 6), (1, 4), (50, 1)]
lst.sort(reverse=True)
# if using an object the line would look similar to this lst.sort(key=lambda x: (x.first_attribute, x.second_attribute), reverse=True)

This is all fine, great, and wonderful, but I really needed to sort by the first attribute descending and the second ascending. So how do we accomplish this? We use cmp on the sort. A compare function takes 2 parameters and returns an integer. If the integer is negative, the sort knows that the first item is should be favored, if 0 it should favor neither, and if positive the second item should be favored.

lst = [(2, 3), (2, 2), (2, 5), (1, 6), (1, 4), (50, 1)]
lst.sort(cmp=lambda x, y: x[1] - y[1] if x[0] == y[0] else y[0] - x[0])

If you aren't familiar with lambda's they are just functions that take in parameters and no return statement is needed. Everything right of the colon is returned. So what's going on here? Well if the first values are equal and x is less than y (x[1]-y[1] < 0) we favor x (ascending). Otherwise we just compare y[0] and x[0].

What about Python 3.0? It doesn't support cmp, but don't worry there's a workaround. If you are in Python 3.2 you can utilize a method called functools.cmp_to_key(). If you are between 3.0 and 3.2, you will have to implement the method yourself. See https://docs.python.org/3/howto/sorting.html.

import functools
lst = [(2, 3), (2, 2), (2, 5), (1, 6), (1, 4), (50, 1)]
lst.sort(key=functools.cmp_to_key(lambda x, y: x[1] - y[1] if x[0] == y[0] else y[0] - x[0]))

The key variable is also available in Python 2, but there you will probably just use it as it's meant to be used: to tell what the key of your sort is.

So how would you compare strings? You should use locale.strcoll

import functools
import locale
lst = [("b", 3), ("b", 2), ("b", 5), ("a", 6), ("a", 4), ("z", 1)]
lst.sort(key=functools.cmp_to_key(lambda x, y:  x[1] - y[1] if x[0] == y[0] else locale.strcoll(y[0], x[0])))

In C# it's really easy to do this kind of thing. Linq has an OrderBy, OrderByDescending, ThenBy, and ThenByDescending methods. But even though it's not as easy to do this in python, it's not really that difficult.