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[3.6] bpo-28393: Update encoding lookup docs wrt bpo-27938 (GH-4871) #4881

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Dec 15, 2017
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[3.6] bpo-28393: Update encoding lookup docs wrt bpo-27938 (GH-4871) #4881

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12 changes: 8 additions & 4 deletions Doc/library/codecs.rst
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Expand Up @@ -977,10 +977,14 @@ e.g. ``'utf-8'`` is a valid alias for the ``'utf_8'`` codec.

Some common encodings can bypass the codecs lookup machinery to
improve performance. These optimization opportunities are only
recognized by CPython for a limited set of aliases: utf-8, utf8,
latin-1, latin1, iso-8859-1, mbcs (Windows only), ascii, utf-16,
and utf-32. Using alternative spellings for these encodings may
result in slower execution.
recognized by CPython for a limited set of (case insensitive)
aliases: utf-8, utf8, latin-1, latin1, iso-8859-1, iso8859-1, mbcs
(Windows only), ascii, us-ascii, utf-16, utf16, utf-32, utf32, and
the same using underscores instead of dashes. Using alternative
aliases for these encodings may result in slower execution.

.. versionchanged:: 3.6
Optimization opportunity recognized for us-ascii.

Many of the character sets support the same languages. They vary in individual
characters (e.g. whether the EURO SIGN is supported or not), and in the
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