[Python-3000] Removing __getslice__ et al.

[Thomas Wouters]

A long-standing part of Py3K is removing 'old style slices' (meaning the
__get/set/delslice__ methods and the sq_slice/sq_ass_slice sequence-struct
functions, but not slice objects.) I started removing them, only to find out
that it's damned inconvenient to make all slicing go through
tp_as_mapping->mp_subscript, for two reasons:

 - Many classes don't behave as mapping at all, so the mp_subscript function
(and the whole tp_as_mapping struct) would be added just for the benefit of
accepting slices, which is really a sequence thing, not a mapping thing.

 - There's actually a PyMapping_Check that relies on sq_slice: a type is a
mapping type when it has tp_as_mapping->mp_subscript but not
tp_as_sequence->sq_slice. I'm not sure how to express that if there is no
special method for slicing.

So, I imagine we need a special (C level) hook for slicing after all, just
one that always takes a slice object (or tuple of
slice-objects-or-tuples-of-same. Oh, and/or Ellipsis. Did I forget anything?
:) Either that, or a set of conveniency functions (which we already have,
for the most part) and a mapping/sequence-agnostic getitem/subscript. But if
we do that, we need a new way of implementing PyMapping_Check.

(While on the subject, howmuch of the C API dealing with slices should be
adjusted? There's PySequence_GetSlice() that takes Py_ssize_t's, which is
(IMHO) pretty convenient for C code, but it'd have to create a slice object
behind the scenes. And maybe creating a slice object manually isn't that
much effort, after all.)

--
Thomas Wouters <thomas at python.org>

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