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] [] STeVe -- http://mail.python.org/mailman/listinfo/python-list
