Make sure ConfigType is an abstract class

Use the abstract class to ensure that all types provide the
method _formatter.

Change-Id: Ifa2e9ca7b91d71887d2bd4ce2e6bb21c2f616722

oslo_config/tests/test_types.py

@@ -18,6 +18,25 @@ import unittest
 from oslo_config import types
 
 
+class ConfigTypeTests(unittest.TestCase):
+    def test_none_concrete_class(self):
+        class MyString(types.ConfigType):
+            def __init__(self, type_name='mystring value'):
+                super(MyString, self).__init__(type_name=type_name)
+
+        self.assertRaises(TypeError, MyString)
+
+    def test_concrete_class(self):
+        class MyString(types.ConfigType):
+            def __init__(self, type_name='mystring value'):
+                super(MyString, self).__init__(type_name=type_name)
+
+            def _formatter(self, value):
+                return value
+
+        MyString()
+
+
 class TypeTestHelper(object):
     def setUp(self):
         super(TypeTestHelper, self).setUp()

oslo_config/types.py

@@ -28,6 +28,7 @@ import netaddr
 import six
 
 
+@six.add_metaclass(abc.ABCMeta)
 class ConfigType(object):
     def __init__(self, type_name='unknown type'):
         self.type_name = type_name