On Sat, Aug 2, 2014 at 1:35 PM, David Wilson <dw+python-dev@hmmz.org> wrote:
> Repeated list and str concatenation both have quadratic O(N**2)
> performance, but people frequently build up strings with + 
join() isn't preferable in cases where it damages readability while
simultaneously providing zero or negative performance benefit, such as
when concatenating a few short strings, e.g. while adding a prefix to a

Good point -- I was trying to make the point about .join() vs + for strings in an intro python class last year, and made the mistake of having the students test the performance.

You need to concatenate a LOT of strings to see any difference at all --  I know that O() of algorithms is unavoidable, but between efficient python optimizations and a an apparently good memory allocator, it's really a practical non-issue.
Although it's true that join() is automatically the safer option, and
especially when dealing with user supplied data, the net harm caused by
teaching rote and ceremony seems far less desirable compared to fixing a
trivial slowdown in a script, if that slowdown ever became apparent.

and it rarely would. 

Blocking sum( some_strings) because it _might_ have poor performance seems awfully pedantic.

As a long-time numpy user, I think sum(a_long_list_of_numbers) has pathetically bad performance, but I wouldn't block it!



Christopher Barker, Ph.D.

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception