[Python-ideas] Where did we go wrong with negative stride?

[MRAB]

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.html
> 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?
>