Apply suggestions from code review

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: eendebakpt

Lib/test/test_set.py

@@ -2159,7 +2159,7 @@ class TestPySet_Add(unittest.TestCase):
     def test_set(self):
         # Test the PySet_Add c-api for set objects
         s = set()
-        assert _testcapi.pyset_add(s, 1) == {1}
+        self.assertEqual(_testcapi.pyset_add(s, 1), {1})
         self.assertRaises(TypeError, _testcapi.pyset_add, s, [])
 
     def test_frozenset(self):
@@ -2175,9 +2175,9 @@ class TrackedHashableClass():
 
         a = TrackedHashableClass()
         result_set = _testcapi.pyset_add(frozenset(), 1)
-        assert not gc.is_tracked(result_set)
+        self.assertFalse(gc.is_tracked(result_set))
         result_set = _testcapi.pyset_add(frozenset(), a)
-        assert gc.is_tracked(result_set)
+        self.assertTrue(gc.is_tracked(result_set))
 
 
 #==============================================================================

Modules/_testcapimodule.c

@@ -2452,7 +2452,6 @@ pyset_add(PyObject* self, PyObject* const* args, Py_ssize_t nargsf)
     if (return_value < 0) {
         return NULL;
     }
-
     return Py_NewRef(set);
 }
 

Objects/setobject.c

@@ -1180,7 +1180,7 @@ _PyFrozenSet_MaybeUntrack(PyObject *op)
     if (op == NULL || !PyFrozenSet_CheckExact(op)) {
         return;
     }
-    // if no elements of a frozenset are tracked, we untrack the object
+    // if no elements of a frozenset are tracked by the GC, we untrack the object
     Py_ssize_t pos = 0;
     setentry *entry;
     while (set_next((PySetObject *)op, &pos, &entry)) {
@@ -1203,7 +1203,9 @@ make_new_frozenset(PyTypeObject *type, PyObject *iterable)
         return Py_NewRef(iterable);
     }
     PyObject *obj = make_new_set(type, iterable);
-    _PyFrozenSet_MaybeUntrack(obj);
+    if (obj != NULL) {
+        _PyFrozenSet_MaybeUntrack(obj);
+    }
     return obj;
 }