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

Chris Barker chris.barker at noaa.gov
Mon Aug 4 18:25:12 CEST 2014

On Sat, Aug 2, 2014 at 1:35 PM, David Wilson <dw+python-dev at 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
> filename.

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.

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Chris.Barker at noaa.gov
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