Slicing [N::-1]

Fri Mar 5 16:42:01 EST 2010

Mensanator wrote:
> On Mar 5, 12:28 pm, Steven D'Aprano <st... at REMOVE-THIS-
> cybersource.com.au> wrote:
>>> On 2010年3月05日 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
>>>>