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

Ian Bicking ianb at colorstudy.com
Tue Apr 4 18:54:09 CEST 2006

Alex Martelli wrote:

It's a bit late for 2.5, of course, but, I thought I'd propose it anyway -- I noticed it on c.l.py.

In 2.3/2.4 we have many ways to generate and process iterators but few "accumulators" -- functions that accept an iterable and produce some kind of "summary result" from it. sum, min, max, for example. And any, all in 2.5. The proposed function tally accepts an iterable whose items are hashable and returns a dict mapping each item to its count (number of times it appears). This is quite general and simple at the same time: for example, it was proposed originally to answer some complaint about any and all giving no indication of the count of true/false items: tally(bool(x) for x in seq) would give a dict with two entries, counts of true and false items. Just like the other accumulators mentioned above, tally is simple to implement, especially with the new collections.defaultdict: import collections def tally(seq): d = collections.defaultdict(int) for item in seq: d[item] += 1 return dict(d)

Or:

import collections bag = collections.Bag([1, 2, 3, 2, 1]) assert bag.count(1) == 2 assert bag.count(0) == 0 assert 3 in bag

etc...

-- Ian Bicking / ianb at colorstudy.com / http://blog.ianbicking.org

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