On Thu, Aug 7, 2014 at 4:01 PM, Ethan Furman email@example.com 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?