Implement a DictRegistry.

Change-Id: I67b177169285f30c7a980a51a81265c621c5f3ff

cloudinit/registry.py

@@ -0,0 +1,23 @@
+import copy
+
+
+class DictRegistry(object):
+    """A simple registry for a mapping of objects."""
+
+    def __init__(self):
+        self._items = {}
+
+    def register_item(self, key, item):
+        """Add item to the registry."""
+        if key in self._items:
+            raise ValueError(
+                'Item already registered with key {0}'.format(key))
+        self._items[key] = item
+
+    @property
+    def registered_items(self):
+        """All the items that have been registered.
+
+        This cannot be used to modify the contents of the registry.
+        """
+        return copy.copy(self._items)

cloudinit/tests/test_registry.py

@@ -0,0 +1,28 @@
+from cloudinit.registry import DictRegistry
+from cloudinit.tests import TestCase
+from cloudinit.tests.util import mock
+
+
+class TestDictRegistry(TestCase):
+
+    def test_added_item_included_in_output(self):
+        registry = DictRegistry()
+        item_key, item_to_register = 'test_key', mock.Mock()
+        registry.register_item(item_key, item_to_register)
+        self.assertEqual({item_key: item_to_register},
+                         registry.registered_items)
+
+    def test_registry_starts_out_empty(self):
+        self.assertEqual({}, DictRegistry().registered_items)
+
+    def test_modifying_registered_items_isnt_exposed_to_other_callers(self):
+        registry = DictRegistry()
+        registry.registered_items['test_item'] = mock.Mock()
+        self.assertEqual({}, registry.registered_items)
+
+    def test_keys_cannot_be_replaced(self):
+        registry = DictRegistry()
+        item_key = 'test_key'
+        registry.register_item(item_key, mock.Mock())
+        self.assertRaises(ValueError,
+                          registry.register_item, item_key, mock.Mock())