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 |