gistfile1.txt

def categorize(fn, coll):
    """Like group_by, but fn returns multiple keys"""
 
    acc = {}
    for e in coll:
        for key in fn(e):
            acc.setdefault(key, []).append(e)
    return acc
 
def group_by(fn, l):
    return categorize(lambda e: [fn(e)], l)