diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
index 0ad83ed..671ed02 100644
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -21,6 +21,8 @@
     msg.lineno = lineno
 
 class CatchWarningTest(unittest.TestCase):
+    # base class used for catching warnings issued by the
+    # warning framework (this is reused by test_structmembers.py)
 
     def setUp(self):
         global msg