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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().

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.

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