Merge "Build Resource from either existing object or id"

openstack/resource.py

@@ -230,6 +230,22 @@ class Resource(collections.MutableMapping):
         """
         return cls(kwargs, loaded=True)
 
+    @classmethod
+    def from_id(cls, value):
+        """Create an instance from an ID or return an existing instance.
+
+        Instance creation is done via cls.new.
+        """
+        # This method is useful in the higher level, in cases where operations
+        # need to depend on having Resource objects, but the API is flexible
+        # in taking text values which represent those objects.
+        if isinstance(value, cls):
+            return value
+        elif isinstance(value, six.string_types):
+            return cls.new(**{cls.id_attribute: value})
+        else:
+            raise ValueError("value must be %s instance or id" % cls.__name__)
+
     ##
     # MUTABLE MAPPING IMPLEMENTATION
     ##

openstack/tests/test_resource.py

@@ -370,6 +370,26 @@ class ResourceTests(base.TestTransportBase):
         del t.id
         self.assertTrue(Test.id_attribute not in t._attrs)
 
+    def test_from_id_with_name(self):
+        name = "Sandy Koufax"
+
+        obj = FakeResource.from_id(name)
+        self.assertEqual(obj.id, name)
+
+    def test_from_id_with_object(self):
+        name = "Mickey Mantle"
+        obj = FakeResource.new(name=name)
+
+        new_obj = FakeResource.from_id(obj)
+        self.assertIs(new_obj, obj)
+        self.assertEqual(new_obj.name, obj.name)
+
+    def test_from_id_with_bad_value(self):
+        def should_raise():
+            FakeResource.from_id(3.14)
+
+        self.assertThat(should_raise, matchers.raises(ValueError))
+
 
 class FakeResponse:
     def __init__(self, response):