bpo-15243: Document __prepare__ as classmethod (GH-17124)
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 1442fbe..9520f82 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1945,7 +1945,8 @@
Once the appropriate metaclass has been identified, then the class namespace
is prepared. If the metaclass has a ``__prepare__`` attribute, it is called
as ``namespace = metaclass.__prepare__(name, bases, **kwds)`` (where the
-additional keyword arguments, if any, come from the class definition).
+additional keyword arguments, if any, come from the class definition). The
+``__prepare__`` method should be implemented as a :func:`classmethod`.
If the metaclass has no ``__prepare__`` attribute, then the class namespace
is initialised as an empty ordered mapping.