Seek support for new slice syntax PEP.

[Nobody]

On Mon, 14 Dec 2009 14:18:49 -0500, Terry Reedy wrote:

> Many more people uses range objects (xrange in 2.x). A range object has 
> the same info as a slice object *plus* it is iterable.

This isn't quite true, as a range cannot have a stop value of None, i.e.
you can't represent [n:] or [:] etc as a range. Similarly for using
negative stop values for indices relative to the end of the sequence being
sliced.

Also, aside from the semantics of slice objects themselves, slice notation
isn't limited to a single slice object; it can also return a tuple of
slices and values, e.g.:

	> numpy.s_[1::2,...,3,4:5:6]
	(slice(1, None, 2), Ellipsis, 3, slice(4, 5, 6))

For a single slice, enumerating over a slice with an unspecified stop
value would be equivalent to itertools.count(). Negative stop values won't
work.

For a multi-dimensional slice, with everything specified, you would
probably want to iterate over the cartesian product (i.e. N nested loops
for an N-dimensional slice). But this won't work if anything other than
the outermost loop has an unspecified stop value, or if you use an
ellipsis within a slice.

Oh, and being able to slice a slice could be quite useful, i.e.:

	[10:90:10][2::2] == [30:90:20]

cf:
	> numpy.arange(100)[10:90:10][2::2]
	array([30, 50, 70])
	> numpy.arange(100)[30:90:20]
	array([30, 50, 70])