[3.15] gh-158254: Detect keys changing during reverse dict iteration (GH-158255)

Lib/test/test_dict.py

@@ -1424,6 +1424,31 @@ def test_reversed_dict_after_clear_and_restore(self):
         for it in iterators:
             self.assertEqual(list(it), [])
 
+    def test_reversed_dict_keys_changed_during_iteration(self):
+        d = dict.fromkeys(range(10))
+        for i in range(7):
+            del d[i]
+
+        iterators = (
+            reversed(d),
+            reversed(d.keys()),
+            reversed(d.values()),
+            reversed(d.items()),
+        )
+        for it in iterators:
+            next(it)
+
+        # Same size as before, but with different keys below
+        # the iterators' current position.
+        d.clear()
+        d.update(dict.fromkeys(range(10)))
+        for i in range(3, 10):
+            del d[i]
+
+        for it in iterators:
+            with self.assertRaisesRegex(RuntimeError, 'keys changed'):
+                list(it)
+
     def test_dict_copy_order(self):
         # bpo-34320
         od = collections.OrderedDict([('a', 1), ('b', 2)])

Misc/NEWS.d/next/Core_and_Builtins/2026-09-26-15-39-32.gh-issue-158254.qT7vRk.rst

@@ -0,0 +1,3 @@
+Reverse iterators over a :class:`dict` and its views now raise
+:exc:`RuntimeError` if the dictionary's keys change during iteration, like
+forward iterators, instead of yielding entries for the new keys.

Objects/dictobject.c

@@ -6254,6 +6254,12 @@ dictreviter_iter_lock_held(PyDictObject *d, PyObject *self)
             value = entry_ptr->me_value;
         }
     }
+    // We found an element, but did not expect it
+    if (di->len == 0) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "dictionary keys changed during iteration");
+        goto fail;
+    }
     di->di_pos = i-1;
     di->len--;