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.