Bug in slice type

[John Machin]

Steven Bethard wrote:
> Bryan Olson wrote:
> 
>>
>>     class BuggerAll:
>>
>>         def __init__(self, somelist):
>>             self.sequence = somelist[:]
>>
>>         def __getitem__(self, key):
>>             if isinstance(key, slice):
>>                 start, stop, step = key.indices(len(self.sequence))
>>                 # print 'Slice says start, stop, step are:', start, 
>> stop, step
>>                 return self.sequence[start : stop : step]
>>
>>
>>     print           range(10) [None : None : -2]
>>     print BuggerAll(range(10))[None : None : -2]
>>
>> The above prints:
>>
>>     [9, 7, 5, 3, 1]
>>     []
>>
>> Un-commenting the print statement in __getitem__ shows:
>>
>>     Slice says start, stop, step are: 9 -1 -2
>>
>> The slice object seems to think that -1 is a valid exclusive
>> bound, but when using it to actually slice, Python interprets
>> negative numbers as an offset from the high end of the sequence.
>>
>> Good start-stop-step values are (9, None, -2), or (9, -11, -2),
>> or (-1, -11, -2). The later two have the advantage of being
>> consistend with the documented behavior of returning three
>> integers.
> 
> 
> I suspect there's a reason that it's done this way, but I agree with you 
> that this seems strange.  Have you filed a bug report on Sourceforge?
> 
> BTW, a simpler example of the same phenomenon is:
> 
> py> range(10)[slice(None, None, -2)]
> [9, 7, 5, 3, 1]
> py> slice(None, None, -2).indices(10)
> (9, -1, -2)
> py> range(10)[9:-1:-2]
> []
> 

 >>> rt = range(10)
 >>> rt[slice(None, None, -2)]
[9, 7, 5, 3, 1]
 >>> rt[::-2]
[9, 7, 5, 3, 1]
 >>> slice(None, None, -2).indices(10)
(9, -1, -2)
 >>> [rt[x] for x in range(9, -1, -2)]
[9, 7, 5, 3, 1]
 >>>

Looks good to me. indices has returned a usable (start, stop, step).
Maybe the docs need expanding.