bpo-33462: Add __reversed__ to dict and dict views (GH-6827)

Doc/library/stdtypes.rst

@@ -4285,6 +4285,11 @@ pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
          LIFO order is now guaranteed. In prior versions, :meth:`popitem` would
          return an arbitrary key/value pair.
 
+   .. describe:: reversed(d)
+
+      Return a reversed iterator over the keys of the dictionary. This is a
+      shortcut for ``reversed(d.keys())``.
+
    .. method:: setdefault(key[, default])
 
       If *key* is in the dictionary, return its value.  If not, insert *key*
@@ -4332,6 +4337,22 @@ pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
       Dictionary order is guaranteed to be insertion order.  This behavior was
       implementation detail of CPython from 3.6.
 
+   Dictionaries and dictionary views are reversible. ::
+
+      >>> d = {"one": 1, "two": 2, "three": 3, "four": 4}
+      >>> d
+      {'one': 1, 'two': 2, 'three': 3, 'four': 4}
+      >>> list(reversed(d))
+      ['four', 'three', 'two', 'one']
+      >>> list(reversed(d.values()))
+      [4, 3, 2, 1]
+      >>> list(reversed(d.items()))
+      [('four', 4), ('three', 3), ('two', 2), ('one', 1)]
+
+   .. versionchanged:: 3.8
+      Dictionaries are now reversible.
+
+
 .. seealso::
    :class:`types.MappingProxyType` can be used to create a read-only view
    of a :class:`dict`.
@@ -4375,6 +4396,14 @@ support membership tests:
    Return ``True`` if *x* is in the underlying dictionary's keys, values or
    items (in the latter case, *x* should be a ``(key, value)`` tuple).
 
+.. describe:: reversed(dictview)
+
+   Return an reversed iterator over the keys, values or items of the dictionnary.
+   The view will be iterated in reverse order of the insertion.
+
+   .. versionchanged:: 3.8
+      Dictionary views are now reversible.
+
 
 Keys views are set-like since their entries are unique and hashable.  If all
 values are hashable, so that ``(key, value)`` pairs are unique and hashable,

Doc/whatsnew/3.8.rst

@@ -98,6 +98,9 @@ Other Language Changes
 * Added support of ``\N{name}`` escapes in :mod:`regular expressions <re>`.
   (Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.)
 
+* Dict and dictviews are now iterable in reversed insertion order using
+  :func:`reversed`. (Contributed by Rémi Lapeyre in :issue:`33462`.)
+
 * The syntax allowed for keyword names in function calls was further
   restricted. In particular, ``f((keyword)=arg)`` is no longer allowed. It was
   never intended to permit more than a bare name on the left-hand side of a

Include/dictobject.h

@@ -51,6 +51,9 @@ PyAPI_DATA(PyTypeObject) PyDict_Type;
 PyAPI_DATA(PyTypeObject) PyDictIterKey_Type;
 PyAPI_DATA(PyTypeObject) PyDictIterValue_Type;
 PyAPI_DATA(PyTypeObject) PyDictIterItem_Type;
+PyAPI_DATA(PyTypeObject) PyDictRevIterKey_Type;
+PyAPI_DATA(PyTypeObject) PyDictRevIterItem_Type;
+PyAPI_DATA(PyTypeObject) PyDictRevIterValue_Type;
 PyAPI_DATA(PyTypeObject) PyDictKeys_Type;
 PyAPI_DATA(PyTypeObject) PyDictItems_Type;
 PyAPI_DATA(PyTypeObject) PyDictValues_Type;

Lib/test/test_collections.py

@@ -796,22 +796,21 @@ class TestOneTrickPonyABCs(ABCTestCase):
 
     def test_Reversible(self):
         # Check some non-reversibles
-        non_samples = [None, 42, 3.14, 1j, dict(), set(), frozenset()]
+        non_samples = [None, 42, 3.14, 1j, set(), frozenset()]
         for x in non_samples:
             self.assertNotIsInstance(x, Reversible)
             self.assertFalse(issubclass(type(x), Reversible), repr(type(x)))
         # Check some non-reversible iterables
-        non_reversibles = [dict().keys(), dict().items(), dict().values(),
-                           Counter(), Counter().keys(), Counter().items(),
-                           Counter().values(), _test_gen(),
-                           (x for x in []), iter([]), reversed([])]
+        non_reversibles = [_test_gen(), (x for x in []), iter([]), reversed([])]
         for x in non_reversibles:
             self.assertNotIsInstance(x, Reversible)
             self.assertFalse(issubclass(type(x), Reversible), repr(type(x)))
         # Check some reversible iterables
         samples = [bytes(), str(), tuple(), list(), OrderedDict(),
                    OrderedDict().keys(), OrderedDict().items(),
-                   OrderedDict().values()]
+                   OrderedDict().values(), Counter(), Counter().keys(),
+                   Counter().items(), Counter().values(), dict(),
+                   dict().keys(), dict().items(), dict().values()]
         for x in samples:
             self.assertIsInstance(x, Reversible)
             self.assertTrue(issubclass(type(x), Reversible), repr(type(x)))
@@ -1612,7 +1611,7 @@ class TestCollectionABCs(ABCTestCase):
         self.assertIsInstance(z, set)
         list(z)
         mymap['blue'] = 7               # Shouldn't affect 'z'
-        self.assertEqual(sorted(z), [('orange', 3), ('red', 5)])
+        self.assertEqual(z, {('orange', 3), ('red', 5)})
 
     def test_Sequence(self):
         for sample in [tuple, list, bytes, str]:
@@ -1767,10 +1766,10 @@ class TestCounter(unittest.TestCase):
         self.assertTrue(issubclass(Counter, Mapping))
         self.assertEqual(len(c), 3)
         self.assertEqual(sum(c.values()), 6)
-        self.assertEqual(sorted(c.values()), [1, 2, 3])
-        self.assertEqual(sorted(c.keys()), ['a', 'b', 'c'])
-        self.assertEqual(sorted(c), ['a', 'b', 'c'])
-        self.assertEqual(sorted(c.items()),
+        self.assertEqual(list(c.values()), [3, 2, 1])
+        self.assertEqual(list(c.keys()), ['a', 'b', 'c'])
+        self.assertEqual(list(c), ['a', 'b', 'c'])
+        self.assertEqual(list(c.items()),
                          [('a', 3), ('b', 2), ('c', 1)])
         self.assertEqual(c['b'], 2)
         self.assertEqual(c['z'], 0)
@@ -1784,7 +1783,7 @@ class TestCounter(unittest.TestCase):
         for i in range(5):
             self.assertEqual(c.most_common(i),
                              [('a', 3), ('b', 2), ('c', 1)][:i])
-        self.assertEqual(''.join(sorted(c.elements())), 'aaabbc')
+        self.assertEqual(''.join(c.elements()), 'aaabbc')
         c['a'] += 1         # increment an existing value
         c['b'] -= 2         # sub existing value to zero
         del c['c']          # remove an entry
@@ -1793,7 +1792,7 @@ class TestCounter(unittest.TestCase):
         c['e'] = -5         # directly assign a missing value
         c['f'] += 4         # add to a missing value
         self.assertEqual(c, dict(a=4, b=0, d=-2, e=-5, f=4))
-        self.assertEqual(''.join(sorted(c.elements())), 'aaaaffff')
+        self.assertEqual(''.join(c.elements()), 'aaaaffff')
         self.assertEqual(c.pop('f'), 4)
         self.assertNotIn('f', c)
         for i in range(3):

Lib/test/test_dict.py

@@ -1021,7 +1021,7 @@ class DictTest(unittest.TestCase):
             it = iter(data)
             d = pickle.dumps(it, proto)
             it = pickle.loads(d)
-            self.assertEqual(sorted(it), sorted(data))
+            self.assertEqual(list(it), list(data))
 
             it = pickle.loads(d)
             try:
@@ -1031,7 +1031,7 @@ class DictTest(unittest.TestCase):
             d = pickle.dumps(it, proto)
             it = pickle.loads(d)
             del data[drop]
-            self.assertEqual(sorted(it), sorted(data))
+            self.assertEqual(list(it), list(data))
 
     def test_itemiterator_pickling(self):
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
@@ -1062,7 +1062,7 @@ class DictTest(unittest.TestCase):
             it = iter(data.values())
             d = pickle.dumps(it, proto)
             it = pickle.loads(d)
-            self.assertEqual(sorted(list(it)), sorted(list(data.values())))
+            self.assertEqual(list(it), list(data.values()))
 
             it = pickle.loads(d)
             drop = next(it)
@@ -1071,6 +1071,62 @@ class DictTest(unittest.TestCase):
             values = list(it) + [drop]
             self.assertEqual(sorted(values), sorted(list(data.values())))
 
+    def test_reverseiterator_pickling(self):
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            data = {1:"a", 2:"b", 3:"c"}
+            it = reversed(data)
+            d = pickle.dumps(it, proto)
+            it = pickle.loads(d)
+            self.assertEqual(list(it), list(reversed(data)))
+
+            it = pickle.loads(d)
+            try:
+                drop = next(it)
+            except StopIteration:
+                continue
+            d = pickle.dumps(it, proto)
+            it = pickle.loads(d)
+            del data[drop]
+            self.assertEqual(list(it), list(reversed(data)))
+
+    def test_reverseitemiterator_pickling(self):
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            data = {1:"a", 2:"b", 3:"c"}
+            # dictviews aren't picklable, only their iterators
+            itorg = reversed(data.items())
+            d = pickle.dumps(itorg, proto)
+            it = pickle.loads(d)
+            # note that the type of the unpickled iterator
+            # is not necessarily the same as the original.  It is
+            # merely an object supporting the iterator protocol, yielding
+            # the same objects as the original one.
+            # self.assertEqual(type(itorg), type(it))
+            self.assertIsInstance(it, collections.abc.Iterator)
+            self.assertEqual(dict(it), data)
+
+            it = pickle.loads(d)
+            drop = next(it)
+            d = pickle.dumps(it, proto)
+            it = pickle.loads(d)
+            del data[drop[0]]
+            self.assertEqual(dict(it), data)
+
+    def test_reversevaluesiterator_pickling(self):
+        for proto in range(pickle.HIGHEST_PROTOCOL):
+            data = {1:"a", 2:"b", 3:"c"}
+            # data.values() isn't picklable, only its iterator
+            it = reversed(data.values())
+            d = pickle.dumps(it, proto)
+            it = pickle.loads(d)
+            self.assertEqual(list(it), list(reversed(data.values())))
+
+            it = pickle.loads(d)
+            drop = next(it)
+            d = pickle.dumps(it, proto)
+            it = pickle.loads(d)
+            values = list(it) + [drop]
+            self.assertEqual(sorted(values), sorted(data.values()))
+
     def test_instance_dict_getattr_str_subclass(self):
         class Foo:
             def __init__(self, msg):
@@ -1222,6 +1278,13 @@ class DictTest(unittest.TestCase):
 
         self.assertRaises(RuntimeError, iter_and_mutate)
 
+    def test_reversed(self):
+        d = {"a": 1, "b": 2, "foo": 0, "c": 3, "d": 4}
+        del d["foo"]
+        r = reversed(d)
+        self.assertEqual(list(r), list('dcba'))
+        self.assertRaises(StopIteration, next, r)
+
     def test_dict_copy_order(self):
         # bpo-34320
         od = collections.OrderedDict([('a', 1), ('b', 2)])

Lib/test/test_enumerate.py

@@ -160,9 +160,9 @@ class TestReversed(unittest.TestCase, PickleTest):
                 raise StopIteration
             def __len__(self):
                 return 5
-        for data in 'abc', range(5), tuple(enumerate('abc')), A(), range(1,17,5):
+        for data in ('abc', range(5), tuple(enumerate('abc')), A(),
+                    range(1,17,5), dict.fromkeys('abcde')):
             self.assertEqual(list(data)[::-1], list(reversed(data)))
-        self.assertRaises(TypeError, reversed, {})
         # don't allow keyword arguments
         self.assertRaises(TypeError, reversed, [], a=1)
 

Misc/NEWS.d/next/Core and Builtins/2018-05-23-17-18-02.bpo-33462.gurbpbrhe.rst

@@ -0,0 +1 @@
+Make dict and dict views reversible. Patch by Rémi Lapeyre.

Objects/clinic/dictobject.c.h

@@ -103,4 +103,22 @@ dict_setdefault(PyDictObject *self, PyObject *const *args, Py_ssize_t nargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=d7508c5091609a23 input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(dict___reversed____doc__,
+"__reversed__($self, /)\n"
+"--\n"
+"\n"
+"Return a reverse iterator over the dict keys.");
+
+#define DICT___REVERSED___METHODDEF    \
+    {"__reversed__", (PyCFunction)dict___reversed__, METH_NOARGS, dict___reversed____doc__},
+
+static PyObject *
+dict___reversed___impl(PyDictObject *self);
+
+static PyObject *
+dict___reversed__(PyDictObject *self, PyObject *Py_UNUSED(ignored))
+{
+    return dict___reversed___impl(self);
+}
+/*[clinic end generated code: output=b9923851cbd9213a input=a9049054013a1b77]*/

Objects/dictobject.c

@@ -3100,6 +3100,7 @@ static PyMethodDef mapp_methods[] = {
      clear__doc__},
     {"copy",            (PyCFunction)dict_copy,         METH_NOARGS,
      copy__doc__},
+    DICT___REVERSED___METHODDEF
     {NULL,              NULL}   /* sentinel */
 };
 
@@ -3335,22 +3336,32 @@ dictiter_new(PyDictObject *dict, PyTypeObject *itertype)
 {
     dictiterobject *di;
     di = PyObject_GC_New(dictiterobject, itertype);
-    if (di == NULL)
+    if (di == NULL) {
         return NULL;
+    }
     Py_INCREF(dict);
     di->di_dict = dict;
     di->di_used = dict->ma_used;
-    di->di_pos = 0;
     di->len = dict->ma_used;
-    if (itertype == &PyDictIterItem_Type) {
+    if ((itertype == &PyDictRevIterKey_Type ||
+         itertype == &PyDictRevIterItem_Type ||
+         itertype == &PyDictRevIterValue_Type) && dict->ma_used) {
+            di->di_pos = dict->ma_keys->dk_nentries - 1;
+    }
+    else {
+        di->di_pos = 0;
+    }
+    if (itertype == &PyDictIterItem_Type ||
+        itertype == &PyDictRevIterItem_Type) {
         di->di_result = PyTuple_Pack(2, Py_None, Py_None);
         if (di->di_result == NULL) {
             Py_DECREF(di);
             return NULL;
         }
     }
-    else
+    else {
         di->di_result = NULL;
+    }
     _PyObject_GC_TRACK(di);
     return (PyObject *)di;
 }
@@ -3664,6 +3675,120 @@ PyTypeObject PyDictIterItem_Type = {
 };
 
 
+/* dictreviter */
+
+static PyObject *
+dictreviter_iternext(dictiterobject *di)
+{
+    PyDictObject *d = di->di_dict;
+
+    if (d == NULL) {
+        return NULL;
+    }
+    assert (PyDict_Check(d));
+
+    if (di->di_used != d->ma_used) {
+        PyErr_SetString(PyExc_RuntimeError,
+                         "dictionary changed size during iteration");
+        di->di_used = -1; /* Make this state sticky */
+        return NULL;
+    }
+
+    Py_ssize_t i = di->di_pos;
+    PyDictKeysObject *k = d->ma_keys;
+    PyObject *key, *value, *result;
+
+    if (d->ma_values) {
+        if (i < 0) {
+            goto fail;
+        }
+        key = DK_ENTRIES(k)[i].me_key;
+        value = d->ma_values[i];
+        assert (value != NULL);
+    }
+    else {
+        PyDictKeyEntry *entry_ptr = &DK_ENTRIES(k)[i];
+        while (i >= 0 && entry_ptr->me_value == NULL) {
+            entry_ptr--;
+            i--;
+        }
+        if (i < 0) {
+            goto fail;
+        }
+        key = entry_ptr->me_key;
+        value = entry_ptr->me_value;
+    }
+    di->di_pos = i-1;
+    di->len--;
+
+    if (Py_TYPE(di) == &PyDictRevIterKey_Type) {
+        Py_INCREF(key);
+        return key;
+    }
+    else if (Py_TYPE(di) == &PyDictRevIterValue_Type) {
+        Py_INCREF(value);
+        return value;
+    }
+    else if (Py_TYPE(di) == &PyDictRevIterItem_Type) {
+        Py_INCREF(key);
+        Py_INCREF(value);
+        result = di->di_result;
+        if (Py_REFCNT(result) == 1) {
+            PyObject *oldkey = PyTuple_GET_ITEM(result, 0);
+            PyObject *oldvalue = PyTuple_GET_ITEM(result, 1);
+            PyTuple_SET_ITEM(result, 0, key);  /* steals reference */
+            PyTuple_SET_ITEM(result, 1, value);  /* steals reference */
+            Py_INCREF(result);
+            Py_DECREF(oldkey);
+            Py_DECREF(oldvalue);
+        }
+        else {
+            result = PyTuple_New(2);
+            if (result == NULL) {
+                return NULL;
+            }
+            PyTuple_SET_ITEM(result, 0, key); /* steals reference */
+            PyTuple_SET_ITEM(result, 1, value); /* steals reference */
+        }
+        return result;
+    }
+    else {
+        Py_UNREACHABLE();
+    }
+
+fail:
+    di->di_dict = NULL;
+    Py_DECREF(d);
+    return NULL;
+}
+
+PyTypeObject PyDictRevIterKey_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "dict_reversekeyiterator",
+    sizeof(dictiterobject),
+    .tp_dealloc = (destructor)dictiter_dealloc,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)dictiter_traverse,
+    .tp_iter = PyObject_SelfIter,
+    .tp_iternext = (iternextfunc)dictreviter_iternext,
+    .tp_methods = dictiter_methods
+};
+
+
+/*[clinic input]
+dict.__reversed__
+
+Return a reverse iterator over the dict keys.
+[clinic start generated code]*/
+
+static PyObject *
+dict___reversed___impl(PyDictObject *self)
+/*[clinic end generated code: output=e674483336d1ed51 input=23210ef3477d8c4d]*/
+{
+    assert (PyDict_Check(self));
+    return dictiter_new(self, &PyDictRevIterKey_Type);
+}
+
 static PyObject *
 dictiter_reduce(dictiterobject *di, PyObject *Py_UNUSED(ignored))
 {
@@ -3671,7 +3796,6 @@ dictiter_reduce(dictiterobject *di, PyObject *Py_UNUSED(ignored))
     dictiterobject tmp = *di;
     Py_XINCREF(tmp.di_dict);
 
-    /* iterate the temporary into a list */
     PyObject *list = PySequence_List((PyObject*)&tmp);
     Py_XDECREF(tmp.di_dict);
     if (list == NULL) {
@@ -3680,6 +3804,30 @@ dictiter_reduce(dictiterobject *di, PyObject *Py_UNUSED(ignored))
     return Py_BuildValue("N(N)", _PyObject_GetBuiltin("iter"), list);
 }
 
+PyTypeObject PyDictRevIterItem_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "dict_reverseitemiterator",
+    sizeof(dictiterobject),
+    .tp_dealloc = (destructor)dictiter_dealloc,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)dictiter_traverse,
+    .tp_iter = PyObject_SelfIter,
+    .tp_iternext = (iternextfunc)dictreviter_iternext,
+    .tp_methods = dictiter_methods
+};
+
+PyTypeObject PyDictRevIterValue_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "dict_reversevalueiterator",
+    sizeof(dictiterobject),
+    .tp_dealloc = (destructor)dictiter_dealloc,
+    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    .tp_traverse = (traverseproc)dictiter_traverse,
+    .tp_iter = PyObject_SelfIter,
+    .tp_iternext = (iternextfunc)dictreviter_iternext,
+    .tp_methods = dictiter_methods
+};
+
 /***********************************************/
 /* View objects for keys(), items(), values(). */
 /***********************************************/
@@ -4035,9 +4183,16 @@ dictviews_isdisjoint(PyObject *self, PyObject *other)
 PyDoc_STRVAR(isdisjoint_doc,
 "Return True if the view and the given iterable have a null intersection.");
 
+static PyObject* dictkeys_reversed(_PyDictViewObject *dv);
+
+PyDoc_STRVAR(reversed_keys_doc,
+"Return a reverse iterator over the dict keys.");
+
 static PyMethodDef dictkeys_methods[] = {
     {"isdisjoint",      (PyCFunction)dictviews_isdisjoint,  METH_O,
      isdisjoint_doc},
+    {"__reversed__",    (PyCFunction)dictkeys_reversed,    METH_NOARGS,
+     reversed_keys_doc},
     {NULL,              NULL}           /* sentinel */
 };
 
@@ -4080,6 +4235,15 @@ dictkeys_new(PyObject *dict, PyObject *Py_UNUSED(ignored))
     return _PyDictView_New(dict, &PyDictKeys_Type);
 }
 
+static PyObject *
+dictkeys_reversed(_PyDictViewObject *dv)
+{
+    if (dv->dv_dict == NULL) {
+        Py_RETURN_NONE;
+    }
+    return dictiter_new(dv->dv_dict, &PyDictRevIterKey_Type);
+}
+
 /*** dict_items ***/
 
 static PyObject *
@@ -4125,9 +4289,16 @@ static PySequenceMethods dictitems_as_sequence = {
     (objobjproc)dictitems_contains,     /* sq_contains */
 };
 
+static PyObject* dictitems_reversed(_PyDictViewObject *dv);
+
+PyDoc_STRVAR(reversed_items_doc,
+"Return a reverse iterator over the dict items.");
+
 static PyMethodDef dictitems_methods[] = {
     {"isdisjoint",      (PyCFunction)dictviews_isdisjoint,  METH_O,
      isdisjoint_doc},
+    {"__reversed__",    (PyCFunction)dictitems_reversed,    METH_NOARGS,
+     reversed_items_doc},
     {NULL,              NULL}           /* sentinel */
 };
 
@@ -4170,6 +4341,15 @@ dictitems_new(PyObject *dict, PyObject *Py_UNUSED(ignored))
     return _PyDictView_New(dict, &PyDictItems_Type);
 }
 
+static PyObject *
+dictitems_reversed(_PyDictViewObject *dv)
+{
+    if (dv->dv_dict == NULL) {
+        Py_RETURN_NONE;
+    }
+    return dictiter_new(dv->dv_dict, &PyDictRevIterItem_Type);
+}
+
 /*** dict_values ***/
 
 static PyObject *
@@ -4192,7 +4372,14 @@ static PySequenceMethods dictvalues_as_sequence = {
     (objobjproc)0,                      /* sq_contains */
 };
 
+static PyObject* dictvalues_reversed(_PyDictViewObject *dv);
+
+PyDoc_STRVAR(reversed_values_doc,
+"Return a reverse iterator over the dict values.");
+
 static PyMethodDef dictvalues_methods[] = {
+    {"__reversed__",    (PyCFunction)dictvalues_reversed,    METH_NOARGS,
+     reversed_values_doc},
     {NULL,              NULL}           /* sentinel */
 };
 
@@ -4235,6 +4422,16 @@ dictvalues_new(PyObject *dict, PyObject *Py_UNUSED(ignored))
     return _PyDictView_New(dict, &PyDictValues_Type);
 }
 
+static PyObject *
+dictvalues_reversed(_PyDictViewObject *dv)
+{
+    if (dv->dv_dict == NULL) {
+        Py_RETURN_NONE;
+    }
+    return dictiter_new(dv->dv_dict, &PyDictRevIterValue_Type);
+}
+
+
 /* Returns NULL if cannot allocate a new PyDictKeysObject,
    but does not set an error */
 PyDictKeysObject *

Objects/object.c

@@ -1790,6 +1790,15 @@ _Py_ReadyTypes(void)
     if (PyType_Ready(&PyDictItems_Type) < 0)
         Py_FatalError("Can't initialize dict items type");
 
+    if (PyType_Ready(&PyDictRevIterKey_Type) < 0)
+        Py_FatalError("Can't initialize reversed dict keys type");
+
+    if (PyType_Ready(&PyDictRevIterValue_Type) < 0)
+        Py_FatalError("Can't initialize reversed dict values type");
+
+    if (PyType_Ready(&PyDictRevIterItem_Type) < 0)
+        Py_FatalError("Can't initialize reversed dict items type");
+
     if (PyType_Ready(&PyODict_Type) < 0)
         Py_FatalError("Can't initialize OrderedDict type");