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

[Oscar Benjamin]

On 1 November 2013 13:26, אלעזר <elazarg at gmail.com> wrote:
> 2013/11/1 Oscar Benjamin <oscar.j.benjamin at gmail.com>:
>> On 1 November 2013 10:17, Steven D'Aprano <steve at pearwood.info> wrote:
>>> To break the ambiguity, we'd need a rule that End objects can only occur
>>> inside slices with at least one colon.
>>
>> This would defeat much of the point of having a new notation. The
>> problem with negative wraparound applies just as much to ordinary
>> indexing as to slicing.
>>
> Not exactly as much. taking
>
>     x[:-0] # intention: x[:len(x)]
>
> is a reasonable, which happens to fail in Python. While
>
>     x[-0]  # intention: x[len(x)]
>
> is an error in the first place, which happens not to raise an
> Exception in Python, but rather gives you a wrong result.

I'm not really sure what you mean by this so I'll clarify what I mean:

If I write x[-n] then my intention is that n should always be
positive. If n happens to be zero or negative then I really want an
IndexError but I won't get one because it coincidentally has an
alternate meaning. I would rather be able to spell that as x[end-n]
and have any negative index be an error. While I can write x[len(x)-n]
right now it still does the wrong thing when n>len(x).


Oscar