[Python-Dev] tally (and other accumulators)

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