On Thu, Aug 7, 2014 at 4:01 PM, Ethan Furman <ethan@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?



Christopher Barker, Ph.D.

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