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

[Andrew Barnert]

On Oct 29, 2013, at 15:07, Nick Coghlan <ncoghlan at gmail.com> wrote:

> Isn't all that is needed to prevent the default wraparound behaviour clamping negative numbers to zero on input?
> 
> As in:
> 
> def clampleft(start, stop, step):
>     if start is not None and start < 0:
>         start = 0
>     if stop is not None and stop < 0:
>         stop = 0
>     return slice(start, stop, step)
> 
Except many of the wraparound cases people complain about are the other way around, negative stop wrapping around to 0. 

You could fix that almost as easily:

def clampright(start, stop, step):
    if start >= 0:
        start = ???
    if stop >= 0:
        stop = None
    return slice(start, stop, step)

Except... What do you set start to if you want to make sure it's past-end? You could force an empty slice (which is the main thing you want) with, e.g., stop=start=0; is that close enough?
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