[Python-Dev] tally (and other accumulators) (original) (raw)

Scott David Daniels Scott.Daniels at Acm.Org
Wed Apr 5 21:56:14 CEST 2006

Jess Austin wrote:

Alex wrote:

On Apr 4, 2006, at 10:53 PM, Jess Austin wrote:

Alex wrote:

import collections def tally(seq): d = collections.defaultdict(int) for item in seq: d[item] += 1 return dict(d) [Jess again] def tally(seq): return dict((group[0], len(tuple(group[1]))) for group in itertools.groupby(sorted(seq)))

In general itertools.groupby() seems like a very clean way to do this sort of thing, whether you want to end up with a dict or not. I'll go so far as to suggest that the existence of groupby() obviates the proposed tally(). Maybe I've just coded too much SQL and it has warped my brain...

You're right in that it won't raise an exception on an iterator, but the sorted() means that it's much less memory efficient than your version for iterators. Another reason to avoid sorted() for this application, besides the big-O. Anyway, I still like groupby() for this sort of thing, with the aforementioned caveats. Functional code seems a little clearer to me, although I realize that preference is not held universally.

However, sorted requires ordering. Try seq = [1, 1j, -1, -1j] * 5 Alex's tally works, but yours does not.

-- -- Scott David Daniels Scott.Daniels at Acm.Org

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