Re: Feature request: sorting a list slice

Am Sonntag 21 Mai 2006 18:55 schrieb Raymond Hettinger:
If the perf gain is small and the use cases are infrequent, the
addition is likely unwarranted. There is an entire class of feature
requests that are more appropriate as recipes than for inclusion in the
language.

The thing is: having an explicit start/stop argument to reverse() and sort()
doesn't slow down method call much (it's just one if() whose body is skipped
when the parameters aren't passed, I'd say that the time that's lost here is
pretty insignificant, in the order of 10^-6 seconds, on _any_ modern
machine), and on the other hand warrants huge memory gains (if not
performance gains by saving a memcpy) when you do need to sort or reverse
slices of large lists.

I've had use cases of the latter (reversing large parts of even larger lists
in memory) in several data archiving and evaluation programs I wrote, but I
can also understand the use case that was made by George for having these
arguments for sort(), so that groupBy() can be extended easily to work on
subgroups without requiring slicing.

Anyway: having these "extensions" as a recipe won't work here: the main idea
is saving the slicing by having sort() and reverse() do the "slicing"
internally (rather, they just add a constant to the lower and upper bound,
and the user specifies these constants, the internal functions they call
already work on slices, and the current listobject.c gives them the whole
list as the slice by default).

The user can't patch this as an extension on the fly, because it requires
changes to the underlying listobject.c source. That's why George is asking
for inclusion of this patch.

I just wrote the patch because I had the time to do so, and I won't battle for
it's inclusion, but again, I see the use cases clearly, at the very least for
slice support in list.reverse() and array.reverse() (which the patch also
implements).

--- Heiko.
.

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