Make the constant Sentinel() class public

This pattern can be useful so make it publicly available.

Change-Id: I8fab1a5804aecf17a00a908646b6a20869c4a03b

neutron_lib/constants.py

@@ -246,15 +246,18 @@ DEVICE_NAME_MAX_LEN = 15
 ISO8601_TIME_FORMAT = '%Y-%m-%dT%H:%M:%S.%f'
 
 
+class Sentinel(object):
+    """A constant object that does not change even when copied."""
+    def __deepcopy__(self, memo):
+        # Always return the same object because this is essentially a constant.
+        return self
+
+
 #############################
 # Attribute related constants
 #############################
-class _Sentinel(object):
-    def __deepcopy__(self, memo):
-        # always return the same object because this is essentially a constant
-        return self
 
-ATTR_NOT_SPECIFIED = _Sentinel()
+ATTR_NOT_SPECIFIED = Sentinel()
 
 HEX_ELEM = '[0-9A-Fa-f]'
 UUID_PATTERN = '-'.join([HEX_ELEM + '{8}', HEX_ELEM + '{4}',

neutron_lib/tests/unit/test_neutron_lib.py

@@ -12,17 +12,16 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-"""
-test_neutron_lib
-----------------------------------
+import copy
 
-Tests for `neutron_lib` module.
-"""
+from neutron_lib import constants
 
 from neutron_lib.tests import _base as base
 
 
-class TestNeutron_lib(base.BaseTestCase):
+class TestNeutronLib(base.BaseTestCase):
 
-    def test_something(self):
-        pass
+    def test_sentinel_constant(self):
+        foo = constants.Sentinel()
+        bar = copy.deepcopy(foo)
+        self.assertEqual(id(foo), id(bar))