[Python-Dev] counterintuitive behavior (bug?) in Counter with += (original) (raw)

INADA Naoki songofacandy at gmail.com
Mon Oct 3 13:57:45 CEST 2011

+1

Because Counter is mutable object, I think += should mutate left side object.

On Mon, Oct 3, 2011 at 7:12 PM, Lars Buitinck <L.J.Buitinck at uva.nl> wrote:

Hello,

[First off, I'm not a member of this list, so please Cc: me in a reply!] I've found some counterintuitive behavior in collections.Counter while hacking on the scikit-learn project [1]. I wanted to use a bunch of Counters to do some simple term counting in a set of documents, roughly as follows:  counttotal = Counter()  for doc in documents:  countcurrent = Counter(analyze(doc))  counttotal += countcurrent  countperdoc.append(countcurrent) Because we target Python 2.5+, I implemented a lightweight replacement with just the functionality we need, including iadd, but then my co-developer ran the above code on Python 2.7 and performance was horrible. After some digging, I found out that Counter [2] does not have iadd and += copies the entire left-hand side in add! I also figured out that I should use the update method instead, which I will, but I still find that uglier than +=. I would submit a patch to implement iadd, but I first want to know if that's considered the right behavior, since it changes the semantics of +=:  >>> from collections import Counter  >>> a = Counter([1,2,3])  >>> b = a  >>> a += Counter([3,4,5])  >>> a is b  False would become  # snip  >>> a is b  True TIA, Lars

[1] https://github.com/scikit-learn/scikit-learn/commit/de6e93094499e4d81b8e3b15fc66b6b9252945af [2] http://hg.python.org/cpython/file/tip/Lib/collections/init.py#l399 -- Lars Buitinck Scientific programmer, ILPS University of Amsterdam

Python-Dev mailing list Python-Dev at python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/songofacandy%40gmail.com

-- INADA Naoki  <songofacandy at gmail.com>

More information about the Python-Dev mailing list