[Python-checkins] bpo-35838: document optionxform must be idempotent (GH-12656)

[Miss Islington (bot)]

https://github.com/python/cpython/commit/9a838c593f6ada69a37025d7ded8ac822816a74c
commit: 9a838c593f6ada69a37025d7ded8ac822816a74c
branch: 3.7
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2019-04-02T02:29:16-07:00
summary:

bpo-35838: document optionxform must be idempotent (GH-12656)

(cherry picked from commit 04694a306b8f4ab54ef5fc4ba673c26fa53b0ac1)

Co-authored-by: Inada Naoki <songofacandy at gmail.com>

files:
M Doc/library/configparser.rst

diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index 95cc352010e0..68b663ff7f65 100644
--- a/Doc/library/configparser.rst
+++ b/Doc/library/configparser.rst
@@ -723,6 +723,12 @@ be overridden by subclasses or by attribute assignment.
      >>> list(custom['Section2'].keys())
      ['AnotherKey']
 
+  .. note::
+     The optionxform function transforms option names to a canonical form.
+     This should be an idempotent function: if the name is already in
+     canonical form, it should be returned unchanged.
+
+
 .. attribute:: ConfigParser.SECTCRE
 
   A compiled regular expression used to parse section headers.  The default