Add factory helper function which returns a singleton

Not that ServiceTypes is super expensive or anything, but it's also
an encapsultation of static data, so there's really no need to construct
more than one of them in a program.

Change-Id: I2515fd9be27421006ed22a3ab01bef8cb48196f8

os_service_types/__init__.py

@@ -18,3 +18,24 @@ import pbr.version
 from os_service_types.service_types import ServiceTypes  # flake8: noqa
 
 __version__ = pbr.version.VersionInfo('os-service-types').version_string()
+_service_type_manager = None
+
+
+def get_service_types(*args, **kwargs):
+    """Return singleton instance of the ServiceTypes object.
+
+    Parameters are all passed through to the
+    :class:`~os_service_types.service_types.ServiceTypes` constructor.
+
+    .. note::
+
+      Only one singleton is kept, so if instances with different parameter
+      values are desired, directly calling the constructor is necessary.
+
+    :returns: Singleton instance of
+        :class:`~os_service_types.service_types.ServiceTypes`
+    """
+    global _service_type_manager
+    if not _service_type_manager:
+        _service_type_manager = ServiceTypes(*args, **kwargs)
+    return _service_type_manager

os_service_types/tests/test_singleton.py

@@ -0,0 +1,38 @@
+# -*- coding: utf-8 -*-
+
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+"""
+test_singleton
+------------
+
+Tests for `get_service_types` singleton factory function.
+"""
+import os_service_types
+from os_service_types.tests import base
+
+
+class TestSingleton(base.TestCase):
+
+    def setUp(self):
+        super(TestSingleton, self).setUp()
+        # Make an object with no network access
+        self.service_types = os_service_types.get_service_types()
+
+    def test_singleton_same(self):
+        service_types = os_service_types.get_service_types()
+        self.assertTrue(service_types is self.service_types)
+
+    def test_singleton_different(self):
+        service_types = os_service_types.ServiceTypes()
+        self.assertFalse(service_types is self.service_types)

releasenotes/notes/get-service-types-f5e56799a55a6df4.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Factory function ``os_service_types.get_service_types`` added. Returns
+    a singleton instance of ServiceTypes. ServiceTypes is a very low cost
+    object in the first place, but it does read a data file from disk and
+    then the data is pretty static.