Custom Python list sorting

Question

I was refactoring some old code of mine and came across of this:

alist.sort(cmp_items)

def cmp_items(a, b):
    if a.foo > b.foo:
        return 1
    elif a.foo == b.foo:
        return 0
    else:
        return -1

The code works (and I wrote it some 3 years ago!) but I cannot find this thing documented anywhere in the Python docs and everybody uses sorted() to implement custom sorting. Can someone explain why this works?

Answer 1

As a side note, here is a better alternative to implement the same sorting:

alist.sort(key=lambda x: x.foo)

Or alternatively:

import operator
alist.sort(key=operator.attrgetter('foo'))

Check out the Sorting How To, it is very useful.

Answer 2

It's documented here.

The sort() method takes optional arguments for controlling the comparisons.

cmp specifies a custom comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument: cmp=lambda x,y: cmp(x.lower(), y.lower()). The default value is None.

Answer 3

Just like this example. You want sort this list.

[('c', 2), ('b', 2), ('a', 3)]

output:

[('a', 3), ('b', 2), ('c', 2)]

you should sort the tuples by the second item, then the first:

def letter_cmp(a, b):
    if a[1] > b[1]:
        return -1
    elif a[1] == b[1]:
        if a[0] > b[0]:
            return 1
        else:
            return -1
    else:
        return 1

Then convert it to a key function:

from functools import cmp_to_key
letter_cmp_key = cmp_to_key(letter_cmp))

Now you can use your custom sort order:

[('c', 2), ('b', 2), ('a', 3)].sort(key=letter_cmp_key)

Answer 4

This does not work in Python 3.

You can use functools cmp_to_key to have old-style comparison functions work though.

from functools import cmp_to_key

def cmp_items(a, b):
    if a.foo > b.foo:
        return 1
    elif a.foo == b.foo:
        return 0
    else:
        return -1

cmp_items_py3 = cmp_to_key(cmp_items)

alist.sort(cmp_items_py3)