extended slicing and negative stop value problem

Max maxmoroz at gmail.com
Sun Aug 21 19:27:26 CEST 2011


On Aug 20, 1:40 pm, Steven D'Aprano <steve
+comp.lang.pyt... at pearwood.info> wrote:
> Pardon me for breaking threading, but I don't have Max's original post.

Not sure why; I also can't see it! I'll copy it at the end just in
case.

> On Sat, Aug 20, 2011 at 7:20 PM, Max Moroz <maxmo... at gmail.com> wrote:
> > Would it be a good idea to change Python definition so that a[10, -1, -1]
>
> I presume you mean slice notation a[10:-1:-1].
>
> > referred to the elements starting with position 10, going down to the
> > beginning?
>
> If this was going to be "fixed", the time was probably
> about three years ago, when Python3 was just starting. Now such a change
> will probably need to wait for the hypothetical Python 4000.

Yeah, I was surprised that it didn't bother anyone..

> The usual advice is to do your slicing twice, reversing it the second time:
>
> a[0:11][::-1]
> # Instead of a[10:-1:-1], which looks like it should work, but doesn't.

It works nicely, but it is 1.3 times slower in my code (I am surprised
the interpreter doesn't optimize this).




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