[Python-Dev] collections.Counter add implementation quirk (original) (raw)

Michael Selik mike at selik.org
Tue Nov 24 03:19:36 EST 2015

Raymond, I think you made a typographical error in your Counter.update example.

>>> from collections import Counter
>>> c = Counter(a=4, b=2, c=0, d=-2)
>>> d = Counter(a=1, b=-5, c=-2, d=6)
>>> c.update(d)
>>> c
Counter({'a': 5, 'd': 4, 'c': -2, 'b': -3})

Pair programming ;-)

On Tue, Nov 24, 2015 at 1:02 AM Raymond Hettinger < raymond.hettinger at gmail.com> wrote:

> On Nov 23, 2015, at 10:43 AM, Vlastimil Brom <vlastimil.brom at gmail.com> wrote: > >> Is there any particular reason counters drop negative values when you add >> them together? I definitely expected them to act like ints do when you add >> negatives, and had to subclass it to get what I think is the obvious >> behavior. _>> ________________________ >> Python-Dev mailing list > ... > Hi, > this is probably more appropriate for the general python list rathere > then this developers' maillist, however, as I asked a similar question > some time ago, I got some detailed explanations for the the current > design decissions from the original developer; cf.: > https://mail.python.org/pipermail/python-list/2010-March/570618.html > > (I didn't check possible changes in Counter since that version (3.1 at > that time).) In Python3.2, Counter grew a subtract() method: >>> c = Counter(a=4, b=2, c=0, d=-2) >>> d = Counter(a=1, b=2, c=3, d=4) >>> c.subtract(d) >>> c Counter({'a': 3, 'b': 0, 'c': -3, 'd': -6}) The update() method has been around since the beginning: >>> from collections import Counter >>> c = Counter(a=4, b=2, c=0, d=-2) >>> d = Counter(a=1, b=-5, c=-2, d=6) >>> c.update(d) >>> d Counter({'d': 6, 'a': 1, 'c': -2, 'b': -5})

So, you have two ways of doing counter math: 1. Normal integer arithmetic using update() and subtract() does straight addition and subtraction, either starting with or ending-up with negative values. 2. Saturating arithmetic using the operators: + - & | excludes non-positive results. This supports bag-like behavior (c.f. smalltalk) and multiset operations (https://en.wikipedia.org/wiki/Multiset). Raymond

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