From 77bf6da7258b4a312e224860ea50ac010aa17c1e Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Fri, 15 Dec 2017 06:23:24 -0800
Subject: [PATCH] bpo-28393: Update encoding lookup docs wrt bpo-27938
 (GH-4871) (#4881)

(cherry picked from commit 297fd876aad8ef443d8992618de22c46dbda258b)
---
 Doc/library/codecs.rst | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index 6e249ecf2b1af4..74b24e10ede647 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -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