[Python-Dev] Usage of += on strings in loops in stdlib (original) (raw)

Lennart Regebro regebro at gmail.com
Wed Feb 13 15:53:27 CET 2013

On Wed, Feb 13, 2013 at 3:27 PM, Amaury Forgeot d'Arc <amauryfa at gmail.com> wrote:

2013/2/13 Lennart Regebro <regebro at gmail.com>

On Wed, Feb 13, 2013 at 1:10 PM, Serhiy Storchaka <storchaka at gmail.com> wrote: > I prefer "x = '%s%s%s%s' % (a, b, c, d)" when string's number is more > than 3 > and some of them are literal strings. This has the benefit of being slow both on CPython and PyPy. Although using .format() is even slower. :-) Did you really try it?

Yes.

PyPy is really fast with str.mod, when the format string is a constant. Yes, it's jitted.

Simple concatenation: s1 = s1 + s2 PyPy-1.9 time for 100 concats of 10000 length strings = 7.133 CPython time for 100 concats of 10000 length strings = 0.005

Making a list of strings and joining after the loop: s1 = ''.join(l) PyPy-1.9 time for 100 concats of 10000 length strings = 0.005 CPython time for 100 concats of 10000 length strings = 0.003

Old formatting: s1 = '%s%s' % (s1, s2) PyPy-1.9 time for 100 concats of 10000 length strings = 20.924 CPython time for 100 concats of 10000 length strings = 3.787

New formatting: s1 = '{0}{1}'.format(s1, s2) PyPy-1.9 time for 100 concats of 10000 length strings = 13.249 CPython time for 100 concats of 10000 length strings = 3.751

I have, by the way, yet to find a usecase where the fastest method in CPython is not also the fastest in PyPy.

//Lennart

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