Issue #24254: Preserve class attribute definition order.

2 files changed, 19 insertions, 1 deletions

diff --git a/Doc/reference/compound_stmts.rst b/Doc/reference/compound_stmts.rst
index 1c5bbdf..2eab29a 100644
--- a/Doc/reference/compound_stmts.rst
+++ b/Doc/reference/compound_stmts.rst
@@ -632,6 +632,17 @@ list for the base classes and the saved local namespace for the attribute
 dictionary.  The class name is bound to this class object in the original local
 namespace.
 
+The order in which attributes are defined in the class body is preserved
+in the ``__definition_order__`` attribute on the new class.  If that order
+is not known then the attribute is set to :const:`None`.  The class body
+may include a ``__definition_order__`` attribute.  In that case it is used
+directly.  The value must be a tuple of identifiers or ``None``, otherwise
+:exc:`TypeError` will be raised when the class statement is executed.
+
+.. versionchanged:: 3.6
+
+   Add ``__definition_order__`` to classes.
+
 Class creation can be customized heavily using :ref:`metaclasses <metaclasses>`.
 
 Classes can also be decorated: just like when decorating functions, ::
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index e7328ab..00785ed 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1750,7 +1750,14 @@ as ``namespace = metaclass.__prepare__(name, bases, **kwds)`` (where the
 additional keyword arguments, if any, come from the class definition).
 
 If the metaclass has no ``__prepare__`` attribute, then the class namespace
-is initialised as an empty :func:`dict` instance.
+is initialised as an empty ordered mapping.
+
+.. impl-detail::
+
+   In CPython the default is :class:`collections.OrderedDict`.
+
+.. versionchanged:: 3.6
+   Defaults to :class:`collections.OrderedDict` instead of :func:`dict`.
 
 .. seealso::