
[Guido]
I wouldn't take out negative strides completely, but I might consider deprecating lower and upper bounds other than None (== missing). So a[::-1] would still work, and a[None:None:-1] would be a verbose way of spelling the same,
Happy idea.
but a[-1:-6:-1] would be deprecated.
Not sure I've _ever_ seen that in real life. Where it comes up is on places like stackoverflow, when somebody (mistakely) suggests using seq[:-1:-1] to do a reverse slice. Then it's pointed out that this doesn't work like range(len(seq), -1, -1). Then some wiseass with too much obscure knowledge of implementation details ;-) points out that seq[:-len(seq)-1:-1] does work (well, in CPython - I don't know whether all implementations follow this quirk - although the docs imply that they should).
Then we could triumphantly (re-)introduce upper and lower bounds in Python 4, with the meaning a[i:j:-1] == a[i:j][::-1].
+1.