# HG changeset patch
# User Eric Snow <ericsnowcurrently@gmail.com>
# Date 1463064725 21600
#      Thu May 12 08:52:05 2016 -0600
# Node ID b734ecfbcf2e11bb9b824a27237584e004ca636d
# Parent fceaa0dc715e0eb833f62a12223fd83df41e5fb5
Issue #24254: Default to OrderedDict for the class definition namespace.

diff --git a/Doc/library/types.rst b/Doc/library/types.rst
--- a/Doc/library/types.rst
+++ b/Doc/library/types.rst
@@ -55,6 +55,11 @@
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.6
+      When the metaclass does not have a ``__prepare__`` attribute,
+      ``prepare_class`` defaults to returning a new
+      :class:`collections.OrderedDict` instance instead of :func:`dict`.
+
 .. seealso::
 
    :ref:`metaclasses`
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1692,7 +1692,10 @@
 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 :class:`collections.OrderedDict` instance.
+
+.. versionchanged:: 3.6
+   Defaults to :class:`collections.OrderedDict` instead of :func:`dict`.
 
 .. seealso::
 
diff --git a/Lib/test/test_metaclass.py b/Lib/test/test_metaclass.py
--- a/Lib/test/test_metaclass.py
+++ b/Lib/test/test_metaclass.py
@@ -180,7 +180,7 @@
     meta: C ()
     ns: [('__module__', 'test.test_metaclass'), ('__qualname__', 'C'), ('a', 42), ('b', 24)]
     kw: []
-    >>> type(C) is dict
+    >>> type(C) is OrderedDict
     True
     >>> print(sorted(C.items()))
     [('__module__', 'test.test_metaclass'), ('__qualname__', 'C'), ('a', 42), ('b', 24)]
@@ -212,7 +212,7 @@
 The default metaclass must define a __prepare__() method.
 
     >>> type.__prepare__()
-    {}
+    OrderedDict()
     >>>
 
 Make sure it works with subclassing.
@@ -248,6 +248,7 @@
 
 """
 
+from collections import OrderedDict
 import sys
 
 # Trace function introduces __locals__ which causes various tests to fail.
diff --git a/Lib/types.py b/Lib/types.py
--- a/Lib/types.py
+++ b/Lib/types.py
@@ -85,7 +85,13 @@
     if hasattr(meta, '__prepare__'):
         ns = meta.__prepare__(name, bases, **kwds)
     else:
-        ns = {}
+        # We follow the pattern in this module of extracting types from
+        # builtins, rather than using collections.OrderedDict directly.
+        nstype = None
+        class _:
+            nonlocal nstype
+            nstype = type(locals())
+        ns = nstype()
     return meta, ns, kwds
 
 def _calculate_meta(meta, bases):
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -61,6 +61,9 @@
 
 - Issue #21099: Switch applicable importlib tests to use PEP 451 API.
 
+- Issue #24254: Default to using OrderedDict in the class *definition*
+  namespace.
+
 - Issue #26563: Debug hooks on Python memory allocators now raise a fatal
   error if functions of the :c:func:`PyMem_Malloc` family are called without
   holding the GIL.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2340,6 +2340,7 @@
         goto error;
     }
 
+    /* Copy the definition namespace into a new dict. */
     dict = PyDict_Copy(orig_dict);
     if (dict == NULL)
         goto error;
@@ -3082,7 +3083,7 @@
 static PyObject *
 type_prepare(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    return PyDict_New();
+    return PyODict_New();
 }
 
 /*
diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -145,7 +145,7 @@
     if (prep == NULL) {
         if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
             PyErr_Clear();
-            ns = PyDict_New();
+            ns = PyODict_New();
         }
         else {
             Py_DECREF(meta);