diff --git a/ryu/tests/unit/lib/test_stringify.py b/ryu/tests/unit/lib/test_stringify.py
index 4b47fd22..da784149 100644
--- a/ryu/tests/unit/lib/test_stringify.py
+++ b/ryu/tests/unit/lib/test_stringify.py
@@ -52,13 +52,12 @@ class Test_stringify(unittest.TestCase):
         eq_(j, c.to_jsondict())
 
     def test_jsondict2(self):
-        import string
 
         def my_encode(x):
-            return string.lower(x)
+            return x.lower()
 
         def my_decode(x):
-            return string.upper(x)
+            return x.upper()
 
         j = {'C1': {'a': 'aaa', 'c': 'ccc'}}
         eq_(j['C1']['a'], my_encode('AAA'))