[Python-Dev] Incorrect length of collections.Counter objects / Multiplicity function (original) (raw)

Gustavo Narea me at gustavonarea.net
Wed May 19 00:00:20 CEST 2010

Hello, everyone.

I've checked the new collections.Counter class and I think I've found a bug:

>>> from collections import Counter >>> c1 = Counter([1, 2, 1, 3, 2]) >>> c2 = Counter([1, 1, 2, 2, 3]) >>> c3 = Counter([1, 1, 2, 3]) >>> c1 == c2 and c3 not in (c1, c2) True >>> # Perfect, so far. But... There's always a "but": ... >>> len(c1) 3

The length of a Counter is the amount of unique elements. But the length must be the cardinality, and the cardinality of a multiset is the total number of elements (including duplicates) [1] [2]. The source code mentions that the recipe on ActiveState [3] was one of the references, but that recipe has this right.

Also, why is it indexed? The indexes of a multiset call to mind the position of its elements, but there's no such thing in sets. I think this is inconsistent with the built-in set. I would have implemented the multiplicity function as a method instead of the indexes: c1.get_multiplicity(element) # instead of c1[element]

Is this the intended behavior? If so, I'd like to propose a proper multiset implementation for the standard library (preferably called "Multiset"; should I create a PEP?). If not, I can write a patch to fix it, although I'm afraid it'd be a backwards incompatible change.

Cheers,

[1] http://en.wikipedia.org/wiki/Multiset#Overview [2] http://preview.tinyurl.com/smalltalk-bag [3] http://code.activestate.com/recipes/259174/

Gustavo Narea <xri://=Gustavo>. | Tech blog: =Gustavo/(+blog)/tech ~ About me: =Gustavo/about |

More information about the Python-Dev mailing list