Fix Python – Custom Python list sorting

Question 1

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?

Question 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.

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Answer