Clustering the keys of a dict according to its values

[Mark Wooding]

Florian Brucker <torf at torfbold.com> wrote:

> That is, generate a new dict which holds for each value of the old
> dict a list of the keys of the old dict that have that very value.
> Another requirement is that it should also work on lists, in that case
> with indices instead of keys. We may assume that all values in the
> original dict/list can be used as dict keys.

import itertools

def invert(d):
    def hack(d):
        try:
            return d.iteritems()
        except AttributeError:
            return itertools.izip(itertools.count(), d)
    dd = {}
    for k, v in hack(d):
        dd.setdefault(v, []).append(k)
    return dd

(It's the setdefault trick which is the important part.)

-- [mdw]