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
--
http://mail.python.org/mailman/listinfo/python-list
