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