Support dict subtypes

Author: sobolevn

Lib/test/test_capi/test_dict.py

@@ -628,7 +628,14 @@ def test_dict_as_frozendict_and_clear(self):
 
         class DictSubtype(dict): ...
 
-        for wrong_input in (frozendict(), DictSubtype(), [], None):
+        d = DictSubtype({'x': None})
+        f = as_frozendict(d)
+        self.assertIs(type(f), frozendict)
+        self.assertEqual(f, frozendict({'x': None}))
+        self.assertIs(type(d), DictSubtype)
+        self.assertEqual(d, DictSubtype({}))
+
+        for wrong_input in (frozendict(), [], None):
             with self.subTest(wrong_input=wrong_input):
                 with self.assertRaises(SystemError):
                     as_frozendict(wrong_input)

Objects/dictobject.c

@@ -8561,7 +8561,7 @@ transfer_keys_and_values_lock_held(PyObject *res, PyObject *dict)
 PyObject *
 PyDict_AsFrozenDictAndClear(PyObject *dict)
 {
-    if (dict == NULL || !PyDict_CheckExact(dict)) {
+    if (dict == NULL || !PyDict_Check(dict)) {
         PyErr_BadInternalCall();
         return NULL;
     }