Proposed PEP: New style indexing, was Re: Bug in slice type
fakeaddress at nowhere.org
Wed Aug 24 17:21:22 CEST 2005
Steven Bethard wrote:
> Bryan Olson wrote:
>> Steven Bethard wrote:
>> > Well, I couldn't find where the general semantics of a negative
>> > index are defined, but for sequences at least:
>> > "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.
> 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.
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