Guido van Rossum <guido@python.org> writes:
> (I changed the subject)
>
> > When I was going through the sources of sliceobject.c I found the function
> > PySlice_GetIndicesEx. It performs the magic of trimming a slice into the
> > range of indices of a sequence, including negative indices and intervals
> > with None as start or stop value. A comment in this function says:
> >
> > /* this is harder to get right than you might think */
And it is.
> > Wouldn't it be a good idea to expose this nontrivial functionality to
> > Python code as a method of slice objects?
>
> I dunno. It seems that most code that actually uses slices is written
> in C anyway.
>
> > The method would take an integer argument (length) and return an
> > xrange object.
>
> Why an xrange object? That's not inspectable. *If* we were to do
> this (which I doubt) it should return a tuple of three ints.
Yes.
> > It should make it much
> > easier to implement user types that support extended slicing:
> >
> > def __getitem__(self, index):
> > if isinstance(index, slice):
> > return [get_item_at(i) for i in index.trim(len(self))]
> > else:
> > return get_item_at(index)
> >
> > Suggestions for a better name than trim?
>
> getindices()
When I was debugging this function, I wrote a method called indices().
Actually, I think I'm probably in favour of adding this method, if
only to make writing clearer test cases easier.
[...]
[Tim]
> Just to be helpfully irritating, I'll note that Zope's C
> implementation of slice index normalization for BTreeItems objects
> was off in nearly every way possible, until a few weeks ago. It
> really is difficult to get this right.
No kidding.
Cheers,
M.
--
I think perhaps we should have electoral collages and construct
our representatives entirely of little bits of cloth and papier
mache.
-- Owen Dunn, ucam.chat, from his review of the year
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