[Python-Dev] tally (and other accumulators)

Tue Apr 4 19:35:50 CEST 2006

Alex Martelli wrote:
> On Apr 4, 2006, at 8:01 AM, Jeremy Hylton wrote:
>>> On 4/4/06, Alex Martelli <aleaxit at gmail.com> wrote:
>>> import collections
>>> def tally(seq):
>>> d = collections.defaultdict(int)
>>> for item in seq:
>>> d[item] += 1
>>> return dict(d)
> ...
>> Putting it somewhere in collections seems like a great idea.
>> defaultdict is a bit odd, because the functionality doesn't have
>> anything to do with defaults, just dicts. maybe a classmethod on
>> regular dicts would make more sense?
>> Good points: it should probably be a classmethod on dict, or a 
> function in module collections.
>>> I write this function regularly, so I'd be happy to have it 
>> available directly.
>> Heh, same here -- soon as I saw it proposed on c.l.py I recognized an 
> old friend and it struck me that, simple but widely used, it should 
> be somewhere in the standard library.

Why not make it collections.bag, like the following:
class bag(dict):
 def __init__(self, iterable=None):
 dict.__init__(self)
 if iterable: self.update(iterable)
 def update(self, iterable):
 for item in iterable:
 self.add(item)
 def add(self, item):
 self[item] = self.get(item, 0) + 1
 def remove(self, item):
 if self[item] == 1:
 del self[item]
 else:
 self[item] -= 1
 def count(self, item):
 return self[item]
(etc.)
Georg