collections.Counter surprisingly slow

Joshua Landau joshua at landau.ws
Mon Jul 29 14:07:48 CEST 2013


On 29 July 2013 12:46, Stefan Behnel <stefan_ml at behnel.de> wrote:

> Steven D'Aprano, 28.07.2013 22:51:
> > Calling Counter ends up calling essentially this code:
> >
> > for elem in iterable:
> >     self[elem] = self.get(elem, 0) + 1
> >
> > (although micro-optimized), where "iterable" is your data (lines).
> > Calling the get method has higher overhead than dict[key], that will also
> > contribute.
>
> It comes with a C accelerator (at least in Py3.4dev), but it seems like
> that stumbles a bit over its own feet. The accelerator function special
> cases the (exact) dict type, but the Counter class is a subtype of dict and
> thus takes the generic path, which makes it benefit a bit less than
> possible.
>
> Look for _count_elements() in
>
> http://hg.python.org/cpython/file/tip/Modules/_collectionsmodule.c
>
> Nevertheless, even the generic C code path looks fast enough in general. I
> think the problem is just that the OP used Python 2.7, which doesn't have
> this accelerator function.
>

# _count_elements({}, items), _count_elements(dict_subclass(), items),
Counter(items), defaultdict(int) loop with exception handling
# "items" is always 1m long with varying levels of repetition

>>> for items in randoms:
... helper.timeit(1), helper_subclass.timeit(1), counter.timeit(1),
default.timeit(1)
...
(0.18816172199876746, 0.4679023139997298, 0.9684444869999425,
0.33518486200046027)
(0.2936601179990248, 0.6056111739999324, 1.1316078849995392,
0.46283868699902087)
(0.35396358400066674, 0.685048443998312, 1.2120939880005608,
0.5497965239992482)
(0.5337620789996436, 0.8658702100001392, 1.4507492869997805,
0.7772859329998028)
(0.745282343999861, 1.1455801379997865, 2.116569702000561,
1.3293145009993168)

:(

I have the helper but Counter is still slow. Is it not getting used for
some reason? It's not even as fast as helper on a dict's (direct, no
overridden methods) subclass.
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