Index: Objects/frameobject.c
===================================================================
--- Objects/frameobject.c	(revision 64228)
+++ Objects/frameobject.c	(working copy)
@@ -881,9 +881,11 @@
 	if (ncells || nfreevars) {
 		dict_to_map(co->co_cellvars, ncells,
 			    locals, fast + co->co_nlocals, 1, clear);
-		dict_to_map(co->co_freevars, nfreevars,
-			    locals, fast + co->co_nlocals + ncells, 1, 
-			    clear);
+		if (co->co_flags & CO_OPTIMIZED) {
+			dict_to_map(co->co_freevars, nfreevars,
+			        locals, fast + co->co_nlocals + ncells, 1, 
+			        clear);
+		}
 	}
 	PyErr_Restore(error_type, error_value, error_traceback);
 }
Index: Lib/test/test_scope.py
===================================================================
--- Lib/test/test_scope.py	(revision 64228)
+++ Lib/test/test_scope.py	(working copy)
@@ -519,6 +519,24 @@
         self.assert_("x" not in varnames)
         self.assert_("y" in varnames)
 
+    def testLocalsClass_WithTrace(self):
+        # Issue23728: after the trace trace function returns, the
+        # locals() dictionary is used to update all variables, this
+        # used to include free variables. But in class statements,
+        # free variables are not inserted...
+        import sys
+        sys.settrace(lambda a,b,c:None)
+        try:
+            x = 12
+
+            class C:
+                def f(self):
+                    return x
+
+            assert x == 12 # Used to raise UnboundLocalError
+        finally:
+            sys.settrace(None)
+
     def testBoundAndFree(self):
         # var is bound and free in class