Bug in slice type

Bryan Olson fakeaddress at nowhere.org
Wed Aug 10 16:54:54 CEST 2005


The Python slice type has one method 'indices', and reportedly:

     This method takes a single integer argument /length/ and
     computes information about the extended slice that the slice
     object would describe if applied to a sequence of length
     items. It returns a tuple of three integers; respectively
     these are the /start/ and /stop/ indices and the /step/ or
     stride length of the slice. Missing or out-of-bounds indices
     are handled in a manner consistent with regular slices.

     http://docs.python.org/ref/types.html


It behaves incorrectly when step is negative and the slice
includes the 0 index.


     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.


-- 
--Bryan



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