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:

But the same thing applies to positive strides:

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:

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.