[Python-Dev] tally (and other accumulators)

Tue Apr 4 16:52:28 CEST 2006

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)
Nevertheless, simplicity and generality make it advisable to supply 
it as part of the standard library (location TBD).
A good alternative would be a classmethod tally within 
collections.defaultdict, building and returning a defaultdict as 
above (with a .factory left to int, for further possible use as a 
'bag'/multiset data structure); this would solve the problem of where 
to locate tally if it were to be a function. defaultdict.tally would 
be logically quite similar to dict.fromkeys, except that keys 
happening repeatedly get counted (and so each associated to a value 
of 1 and upwards) rather than "collapsed".
Alex