On 27/10/2013 17:04, Guido van Rossum wrote:

In the comments of http://python-history.blogspot.com/2013/10/why-python-uses-0-based-indexing.... there were some complaints about the interpretation of the bounds for negative strides, and I have to admin it feels wrong. Where did we go wrong? For example,

"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"

but

"abcde":-1:-1] == ""

I'm guessing it all comes from the semantics I assigned to negative stride for range() long ago, unthinkingly combined with the rules for negative indices.

For a positive stride, omitting the second bound is equivalent to length + 1:

"abcde"[:6:1] 'abcde'

For a negative stride, omitting the second bound is equivalent to -(length + 1):

"abcde"[:-6:-1] 'edcba'

Are we stuck with this forever? If we want to fix this in Python 4 we'd have to start deprecating negative stride with non-empty lower/upper bounds now. And we'd have to start deprecating negative step for range() altogether, recommending reversed(range(lower, upper)) instead.

Thoughts? Is NumPy also affected?