[Python-Dev] sum(...) limitation (original) (raw)

Chris Barker chris.barker at noaa.gov
Fri Aug 8 17:23:51 CEST 2014

On Thu, Aug 7, 2014 at 4:01 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

I don't remember where, but I believe that cPython has an optimization built in for repeated string concatenation, which is probably why you aren't seeing big differences between the + and the sum().

Indeed -- clearly so.

A little testing shows how to defeat that optimization:

blah = ''

for string in ['booyah'] * 100000: blah = string + blah

Note the reversed order of the addition.

thanks -- cool trick.

Oh, and the join() timings:

--> timeit.Timer("blah = ''.join(['booya'] * 100000)", "blah = ''").repeat(3, 1) [0.0014629364013671875, 0.0014190673828125, 0.0011930465698242188] So, + is three orders of magnitude slower than join.

only one if if you use the optimized form of + and not even that if you need to build up the list first, which is the common use-case.

So my final question is this:

repeated string concatenation is not the "recommended" way to do this -- but nevertheless, cPython has an optimization that makes it fast and efficient, to the point that there is no practical performance reason to prefer appending to a list and calling join()) afterward.

So why not apply a similar optimization to sum() for strings?

-Chris

--

Christopher Barker, Ph.D. Oceanographer

Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception

Chris.Barker at noaa.gov -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20140808/7428da91/attachment.html>

More information about the Python-Dev mailing list