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

[Bruce Leban]

On Sun, Oct 27, 2013 at 4:45 PM, Greg Ewing <greg.ewing at canterbury.ac.nz>wrote:

> I think the only way to really fix this cleanly is to have
> a different *syntax* for counting from the end, rather than
> trying to guess from the value of the argument.
>

I was thinking the exact same thing today. Suppose the slice syntax was
changed to:

[start:stop:stride:reverse]

where 0 or None or False for reverse leaves the slice in order while any
True value reverses it. This would replace

'abcde'[2:5] == 'bcd'
'abcde'[2:5::True] == 'dcb'
'abcde'[::-2] == 'abcde'[::2:True] == 'eca'
'abcdef'[::-2] == 'fdb'
'abcdef'[::2:True] == 'eca'

As the last three examples, illustrate, sometimes the reverse is equivalent
to a negative stride and sometimes it's not.

--- Bruce
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