On Sun, Oct 27, 2013 at 10:04 AM, Guido van Rossum <guido@python.org> wrote:
"abcde"[::-1] == "edcba"

as you'd expect, but there is no number you can put as the second bound to get the same result:

"abcde"[:1:-1] == "edc"
"abcde"[:0:-1] == "edcb"

This isn't really a negative stride issue. [x:y] is a half-open range == [x, y) in mathematical notation and therefore you need a value for y that is one more. As others have pointed out there is a number you can put in the second bound but it's not a valid index:

'abcde'[:-6:-1] == 'abcde'

But the same thing applies to positive strides:

'abcde'[::1] == 'abcde'[:5:1] == 'abcde'

And the only values you can replace 5 with that work are out of bounds as well or the special value None. None represents both the left edge and the right edge and if we deem that confusing slices could be modified to accept -inf as representing the left edge and inf as representing the right edge. Thus we'd have:

'abcde'[-inf:inf] == 'abcde'
'abcde'[inf:-inf] == ''

On Sun, Oct 27, 2013 at 12:23 PM, MRAB <python@mrabarnett.plus.com> wrote:
The difference might be because the left end is at offset 0 but the
right end is at offset -1.

If the left end was offset 1 and the right end was offset -1 then some of the asymmetry goes away. 

On Sun, Oct 27, 2013 at 1:02 PM, Ron Adam <ron3200@gmail.com> wrote:
And I've never liked the property where when counting down, and you pass 0, it wraps around.   (And the other case of counting up when passing 0.)

And then when you count down and it passes 0, you'd get an index error.

I'm *not* proposing we change how strings are indexed. I think that might break a few programs. I'm just pointing out that you can't count from zero in both directions and that introduces some weirdness.

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