Slicing [N::-1]

[Robert Kern]

On 2010-03-05 12:28 PM, Steven D'Aprano wrote:
> 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):

Rather, they have 0 and len(seq), respectively, when the step is positive, and 
len(seq)-1 and -1 when the step is negative.

>>>> 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.

True, I don't think it is.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco