Issue #26146: marshal.loads() now uses the empty frozenset singleton

Lib/test/test_marshal.py

@@ -135,6 +135,13 @@ class ContainerTestCase(unittest.TestCase, HelperMixin):
         for constructor in (set, frozenset):
             self.helper(constructor(self.d.keys()))
 
+    @support.cpython_only
+    def test_empty_frozenset_singleton(self):
+        # marshal.loads() must reuse the empty frozenset singleton
+        obj = frozenset()
+        obj2 = marshal.loads(marshal.dumps(obj))
+        self.assertIs(obj2, obj)
+
 
 class BufferTestCase(unittest.TestCase, HelperMixin):
 

Python/marshal.c

@@ -1266,6 +1266,16 @@ r_object(RFILE *p)
             PyErr_SetString(PyExc_ValueError, "bad marshal data (set size out of range)");
             break;
         }
+
+        if (n == 0 && type == TYPE_FROZENSET) {
+            /* call frozenset() to get the empty frozenset singleton */
+            v = PyObject_CallFunction((PyObject*)&PyFrozenSet_Type, NULL);
+            if (v == NULL)
+                break;
+            R_REF(v);
+            retval = v;
+        }
+        else {
             v = (type == TYPE_SET) ? PySet_New(NULL) : PyFrozenSet_New(NULL);
             if (type == TYPE_SET) {
                 R_REF(v);
@@ -1301,6 +1311,7 @@ r_object(RFILE *p)
             if (type != TYPE_SET)
                 v = r_ref_insert(v, idx, flag, p);
             retval = v;
+        }
         break;
 
     case TYPE_CODE: