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Fri Aug 8 01:01:49 CEST 2014 · https://mail.python.org/pipermail/python-dev/2014-August/135681.html

On 08/07/2014 03:06 PM, Chris Barker wrote:

[snip timings, etc.]

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.

--> timeit.Timer("for string in ['booya'] * 100000: blah = blah + string", "blah = ''").repeat(3, 1)
[0.021117210388183594, 0.013692855834960938, 0.00768280029296875]

--> timeit.Timer("for string in ['booya'] * 100000: blah = string + blah", "blah = ''").repeat(3, 1)
[15.301048994064331, 15.343288898468018, 15.268463850021362]

--
~Ethan~

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