bpo-35838: document optionxform must be idempotent (GH-12656)
diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index 185b4a1..04b52dc 100644
--- a/Doc/library/configparser.rst
+++ b/Doc/library/configparser.rst
@@ -721,6 +721,12 @@
>>> 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