Clustering the keys of a dict according to its values

[bearophileHUGS at lycos.com]

Alternative version:

def cluster(data):
    d = defaultdict(list)
    pairs = enumerate(data) if isinstance(data, list) else
data.iteritems()
    for k, v in pairs:
        d[v].append(k)
    return d

Bye,
bearophile