Slicing [N::-1]

[Steven D'Aprano]

On Fri, 05 Mar 2010 18:12:05 +0000, Arnaud Delobelle wrote:

>>>> l = range(10)
>>>> l
> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>> l[7::-1]
> [7, 6, 5, 4, 3, 2, 1, 0]
>>>> [l[i] for i in range(7, -1, -1)]
> [7, 6, 5, 4, 3, 2, 1, 0]

Where does the first -1 come from? Slices are supposed to have default 
values of 0 and len(seq):

>>> l[7::1]
[7, 8, 9]
>>> [l[i] for i in range(7, len(l), 1)]
[7, 8, 9]
>>> [l[i] for i in range(7, len(l), -1)]
[]


I don't believe the actual behaviour is documented anywhere.


-- 
Steven