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

(cherry picked from commit 04694a306b8f4ab54ef5fc4ba673c26fa53b0ac1)

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

diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index 95cc352010e0e0517382f31d8bbbe945356a38dc..68b663ff7f65e54b1d06b9fc96c689af3dcf8d1d 100644 (file)
--- 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