# Slicing [N::-1]

Gary Herron gherron at islandtraining.com
Fri Mar 5 22:42:01 CET 2010

```Mensanator wrote:
> On Mar 5, 12:28 pm, Steven D'Aprano <st... at REMOVE-THIS-
> cybersource.com.au> 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):
>>
>
> The only way to get a 0 from a reverse range() is to have a bound of
> -1.
>

Not quite.  An empty second bound goes all the way to the zero index:

>>> range(9)[2::-1]
[2, 1, 0]

Gary Herron

>
>>>>> 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.
>>
>
> Well, it's implied. If the stopping bound in a reverse range()
> is greater than the starting bound, you get an empty return.
>
>
>> --
>> Steven
>>
>
>

```