Steven Bethard wrote: > Bryan Olson wrote: > >> Steven Bethard wrote: >> > Well, I couldn't find where the general semantics of a negative stride >> > index are defined, but for sequences at least[1]: >> > >> > "The slice of s from i to j with step k is defined as the sequence of >> > items with index x = i + n*k such that 0 <= n < (j-i)/k." >> > >> > This seems to contradict list behavior though. [...] >> >> The conclusion is inescapable: Python's handling of negative >> subscripts is a wart. > > > I'm not sure I'd go that far. Note that my confusion above was the > order of combination of points (3) and (5) on the page quoted above[1]. > I think the problem is not the subscript handling so much as the > documentation thereof.
Any bug can be pseudo-fixed by changing the documentation to conform to the behavior. Here, the doc clearly went wrong by expecting Python's behavior to follow from a few consistent rules. The special-case handling of negative indexes looks handy, but raises more difficulties than people realized. I believe my PPEP avoids the proliferation of special cases. The one additional issue I've discovered is that user-defined types that are to support __getitem__ and/or __setitem__ *must* also implement __len__. Sensible sequence types already do, so I don't think it's much of an issue. > This is already valid syntax, and is used > heavily by the numarray/numeric folks. Yeah, I thought that variant might break some code. I didn't know it would be that much. Forget that variant. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
