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

Lib/test/test_dict.py

@@ -1601,6 +1601,21 @@ def __hash__(self):
         with self.assertRaises(KeyError):
             d.get(key2)
 
+    def test_clear_at_lookup(self):
+        d = {}
+
+        class X(object):
+            def __hash__(self):
+                return 1
+            def __eq__(self, other):
+                nonlocal d
+                d.clear()
+
+        for _ in range(10):
+            d[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 ``dict`` if ``clear`` is called at the lookup stage. Patch by
+Mikhail Efimov.

Objects/dictobject.c

@@ -1899,6 +1899,14 @@ insertdict(PyInterpreterState *interp, PyDictObject *mp,
     if (ix == DKIX_ERROR)
         goto Fail;
 
+    // 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)
+            goto Fail;
+        assert(mp->ma_keys->dk_kind == DICT_KEYS_GENERAL);
+    }
+
     if (ix == DKIX_EMPTY) {
         assert(!_PyDict_HasSplitTable(mp));
         /* Insert into new slot. */