gistfile1.txt

def categorize(fn, coll):
    """Like group_by, but fn returns multiple keys"""

    acc = {}
    for e in coll:
        keys = fn(e)
        for key in keys:
            if key not in acc:
                acc[key] = []
            acc[key].append(e)
    return acc

def group_by(fn, l):
    return categorize(lambda e: [fn(e)], l)