[issue21667] Clarify status of O(1) indexing semantics of str objects

[Jim Jewett]

Jim Jewett added the comment:

I think the new wording is an improvement, but keeping the changes minimal left it in an awkward in-between state.

Proposal:

A string is a sequence of Unicode code points.  Strings can include any sequence of code points, including some which are semantically meaningless, or explicitly undefined.

Python doesn't have a :c:type:`char` type; a single code point is represented as a string of length ``1``.  The built-in function :func:`chr` translates an integer in the range ``U+0000 - U+10FFFF`` to the corresponding length ``1`` string object, and :func:`ord` does the reverse.

:meth:`str.encode` provides a concrete representation (as :class:`bytes` in the given text encoding) suitable for transport and communication with non-Python utilities.  :meth:`bytes.decode` decodes such byte sequences into text strings.

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nosy: +Jim.Jewett

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<http://bugs.python.org/issue21667>
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