restore historical behavior of defaultdict

Partial revert of GH-142668 (a043407).

Author: Changaco

Lib/test/test_defaultdict.py

@@ -51,7 +51,11 @@ def test_missing(self):
         d1 = defaultdict()
         self.assertRaises(KeyError, d1.__missing__, 42)
         d1.default_factory = list
-        self.assertEqual(d1.__missing__(42), [])
+        v1 = d1.__missing__(42)
+        self.assertEqual(v1, [])
+        v2 = d1.__missing__(42)
+        self.assertEqual(v2, [])
+        self.assertIsNot(v2, v1)
 
     def test_repr(self):
         d1 = defaultdict()
@@ -186,7 +190,7 @@ def test_union(self):
         with self.assertRaises(TypeError):
             i |= None
 
-    def test_factory_conflict_with_set_value(self):
+    def test_reentering_getitem_method(self):
         key = "conflict_test"
         count = 0
 
@@ -201,7 +205,7 @@ def default_factory():
         test_dict = defaultdict(default_factory)
 
         self.assertEqual(count, 0)
-        self.assertEqual(test_dict[key], 2)
+        self.assertEqual(test_dict[key], 1)
         self.assertEqual(count, 2)
 
     def test_repr_recursive_factory(self):

Modules/_collectionsmodule.c

@@ -2247,11 +2247,11 @@ defdict_missing(PyObject *op, PyObject *key)
     value = _PyObject_CallNoArgs(factory);
     if (value == NULL)
         return value;
-    PyObject *result = NULL;
-    (void)PyDict_SetDefaultRef(op, key, value, &result);
-    // 'result' is NULL, or a strong reference to 'value' or 'op[key]'
-    Py_DECREF(value);
-    return result;
+    if (PyObject_SetItem(op, key, value) < 0) {
+        Py_DECREF(value);
+        return NULL;
+    }
+    return value;
 }
 
 static inline PyObject*