On Thu, Apr 02, 2020 at 03:37:34PM +1100, Chris Angelico wrote:
> On Thu, Apr 2, 2020 at 3:27 PM Steven D'Aprano <[email protected]> wrote:
> > Or maybe that's just an argument that no solution is going to solve
> > *every* problem. What do we do about people who write this:
> >
> > buf = f'{buf}{substring}'
> >
> > inside a loop? We can't fix everyone's code with one change.
> >
>
> I don't know whether your point was that this is bad code and can't be
> optimized, or that it's good code but still can't be optimized by this
> proposal.
Neither. It's that anyone building a string like that isn't the target
of this proposal. I could have given various examples, I just happened
to pick f-string:
buf = '%s%s' % (buf, substring)
buf = '{}{}'.format(buf, substring)
buf = ''.join(itertools.chain(buf, substring))
Put any of them into a loop, and they are likely to be exceedingly slow
for large N. But fixing them is not part of Paul's proposal.
> But if the former, then I put it to you that this isn't
> actually bad code.
Repeated string concatenation doesn't suddenly become efficient just
because you wave a magic f-string at it. That appends substring to the
buffer each time through the loop, giving quadratic performance.
On my computer, appending a single character 'a' to the buffer each
time, I get:
10_000 loops, 0.4 seconds (actual time)
50_000 loops, 9.8 seconds # expect 5*0.4 = 2 seconds
100_000 loops 39 seconds # expect 10*0.4 = 4 seconds
200_000 loops 160 seconds # expect 20*0.4 = 8 seconds
The expected times are assuming that the time is proportional to the
number of elements, i.e. O(N). If we assume O(N**2) then the expected
times would be 10, 40 and 160 seconds, so this is a textbook example of
quadratic slowdown.
(At least on my computer.)
--
Steven
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