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Tue Feb 12 22:46:34 CET 2013 · https://mail.python.org/pipermail/python-dev/2013-February/123987.html

On Tue, 12 Feb 2013 16:40:38 -0500, Ned Batchelder <ned at nedbatchelder.com> wrote:
> On 2/12/2013 4:16 PM, Brett Cannon wrote:
> > On Tue, Feb 12, 2013 at 4:06 PM, Antoine Pitrou <solipsis at pitrou.net 
> > <mailto:solipsis at pitrou.net>> wrote:
> >     On Tue, 12 Feb 2013 23:03:04 +0200
> >     Maciej Fijalkowski <fijall at gmail.com <mailto:fijall at gmail.com>> wrote:
> >     >
> >     > We recently encountered a performance issue in stdlib for pypy. It
> >     > turned out that someone commited a performance "fix" that uses
> >     += for
> >     > strings instead of "".join() that was there before.
> >     >
> >     > Now this hurts pypy (we can mitigate it to some degree though) and
> >     > possible Jython and IronPython too.
> >     >
> >     > How people feel about generally not having += on long strings in
> >     > stdlib (since the refcount = 1 thing is a hack)?
> >
> >     I agree that += should not be used as an optimization (on strings) in
> >     the stdlib code. The optimization is there so that uncareful code does
> >     not degenerate, but deliberately relying on it is a bit devilish.
> >     (optimisare diabolicum :-))
> >
> > Ditto from me. If you're going so far as to want to optimize Python 
> > code then you probably are going to care enough to accelerate it in C, 
> > in which case you can leave the Python code idiomatic.
> 
> But the only reason "".join() is a Python idiom in the first place is 
> because it was "the fast way" to do what everyone initially coded as "s 
> += ...".   Just because we all learned a long time ago that joining was 
> the fast way to build a string doesn't mean that "".join() is the clean 
> idiomatic way to do it.

If 'idiomatic' (a terrible term) means "the standard way in this
language", which is how it is employed in the programming community,
then yes, "".join() is the idiomatic way to write that *in Python*,
and thus is cleaner code *in Python*.

--David

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[Python-Dev] Usage of += on strings in loops in stdlib