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

Mark Dickinson dickinsm at gmail.com
Thu May 20 23🔞56 CEST 2010

On Tue, May 18, 2010 at 11:00 PM, Gustavo Narea <me at gustavonarea.net> wrote:

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

This is the intended behaviour; it also agrees with what you get when you iterate over a Counter object:

list(c1) [1, 2, 3]

As I understand it, there are other uses for Counter objects besides treating them as multisets; I think the choices for len() and iter() reflected those other uses.

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?).

Feel free! The proposal should probably go to python-list or python-ideas rather than here, though.

See also this recent thread on python-list, and in particular the messages from Raymond Hettinger in that thread:

http://mail.python.org/pipermail/python-list/2010-March/thread.html

Mark

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