gh-140551: Fix dict crash if clear is called at lookup stage

Lib/test/test_dict.py

@@ -1601,6 +1601,26 @@ def __hash__(self):
         with self.assertRaises(KeyError):
             d.get(key2)
 
+    def test_clear_at_lookup(self):
+        class X(object):
+            def __hash__(self):
+                return 1
+            def __eq__(self, other):
+                nonlocal d
+                d.clear()
+
+        d = {}
+        for _ in range(10):
+            d[X()] = None
+
+        self.assertEqual(len(d), 1)
+
+        d = {}
+        for _ in range(10):
+            d.setdefault(X(), None)
+
+        self.assertEqual(len(d), 1)
+
 
 class CAPITest(unittest.TestCase):
 

Misc/NEWS.d/next/Core_and_Builtins/2025-10-24-20-42-33.gh-issue-140551.-9swrl.rst

@@ -0,0 +1,2 @@
+Fixed crash in :class:`dict` if :meth:`dict.clear` is called at the lookup
+stage. Patch by Mikhail Efimov.

Objects/dictobject.c

@@ -1775,6 +1775,14 @@ static inline int
 insert_combined_dict(PyInterpreterState *interp, PyDictObject *mp,
                      Py_hash_t hash, PyObject *key, PyObject *value)
 {
+    // gh-140551: If dict was cleaned in _Py_dict_lookup,
+    // we have to resize one more time to force general key kind.
+    if (DK_IS_UNICODE(mp->ma_keys) && !PyUnicode_CheckExact(key)) {
+        if (insertion_resize(mp, 0) < 0)
+            return -1;
+        assert(mp->ma_keys->dk_kind == DICT_KEYS_GENERAL);
+    }
+
     if (mp->ma_keys->dk_usable <= 0) {
         /* Need to resize. */
         if (insertion_resize(mp, 1) < 0) {