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Add information about negative indexes to sequence datamodel doc (#110903) 

Co-authored by Terry Jan Reedy
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Adorilson Bezerra 2024-03-25 22:34:20 +00:00 committed by GitHub
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Doc/reference/datamodel.rst
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@ -299,14 +299,17 @@ Sequences
These represent finite ordered sets indexed by non-negative numbers. The
built-in function :func:`len` returns the number of items of a sequence. When
the length of a sequence is *n*, the index set contains the numbers 0, 1,
..., *n*-1. Item *i* of sequence *a* is selected by ``a[i]``.
..., *n*-1. Item *i* of sequence *a* is selected by ``a[i]``. Some sequences,
including built-in sequences, interpret negative subscripts by adding the
sequence length. For example, ``a[-2]`` equals ``a[n-2]``, the second to last
item of sequence a with length ``n``.
.. index:: single: slicing
Sequences also support slicing: ``a[i:j]`` selects all items with index *k* such
that *i* ``<=`` *k* ``<`` *j*. When used as an expression, a slice is a
sequence of the same type. This implies that the index set is renumbered so
that it starts at 0.
sequence of the same type. The comment above about negative indexes also applies
to negative slice positions.
Some sequences also support "extended slicing" with a third "step" parameter:
``a[i:j:k]`` selects all items of *a* with index *x* where ``x = i + n*k``, *n*