commit 9a838c593f6ada69a37025d7ded8ac822816a74c
author Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Tue Apr 02 02:29:16 2019 -0700
committer GitHub <noreply@github.com> Tue Apr 02 02:29:16 2019 -0700
tree 8ce6b0220506cc0de207f77e657235a6cb499aa0
parent 5ca4fe04784fa278c319a3764c9a755f49cc0944

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

(cherry picked from commit 04694a306b8f4ab54ef5fc4ba673c26fa53b0ac1)

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

Doc/library/configparser.rst

diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index 95cc352..68b663f 100644
--- a/Doc/library/configparser.rst
+++ b/Doc/library/configparser.rst
@@ -723,6 +723,12 @@
      >>> 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