Add ObjectListBase concat methods

This adds support for using + to concatenate two objects derived from
ObjectListBase. The original objects are not modified, a new list object
is returned.

The requirements for this are that the two objects are
of the same class, and there is only an 'objects' field.

Change-Id: I4610db6b52f3f576a6d0c2e64af39927077586cd

oslo_versionedobjects/base.py

@@ -880,6 +880,28 @@ def obj_what_changed(self):
                 changes.add('objects')
         return changes
 
+    def __add__(self, other):
+        # Handling arbitrary fields may not make sense if those fields are not
+        # all concatenatable. Only concatenate if the base 'objects' field is
+        # the only one and the classes match.
+        if (self.__class__ == other.__class__ and
+                list(self.__class__.fields.keys()) == ['objects']):
+            return self.__class__(objects=self.objects + other.objects)
+        else:
+            raise TypeError("List Objects should be of the same type and only "
+                            "have an 'objects' field")
+
+    def __radd__(self, other):
+        if (self.__class__ == other.__class__ and
+                list(self.__class__.fields.keys()) == ['objects']):
+            # This should never be run in practice. If the above condition is
+            # met then __add__ would have been run.
+            raise NotImplementedError('__radd__ is not implemented for '
+                                      'objects of the same type')
+        else:
+            raise TypeError("List Objects should be of the same type and only "
+                            "have an 'objects' field")
+
 
 class VersionedObjectSerializer(messaging.NoOpSerializer):
     """A VersionedObject-aware Serializer.

oslo_versionedobjects/tests/test_objects.py

@@ -2251,3 +2251,92 @@ class TestObject(base.VersionedObject):
                           'TestChild': '2.34',
                           'TestChildTwo': '4.56'},
                          tree)
+
+
+class TestListObjectConcat(test.TestCase):
+    def test_list_object_concat(self):
+        @base.VersionedObjectRegistry.register_if(False)
+        class MyList(base.ObjectListBase, base.VersionedObject):
+            fields = {'objects': fields.ListOfObjectsField('MyOwnedObject')}
+
+        values = [1, 2, 42]
+
+        list1 = MyList(objects=[MyOwnedObject(baz=values[0]),
+                                MyOwnedObject(baz=values[1])])
+        list2 = MyList(objects=[MyOwnedObject(baz=values[2])])
+
+        concat_list = list1 + list2
+        for idx, obj in enumerate(concat_list):
+            self.assertEqual(values[idx], obj.baz)
+
+        # Assert that the original lists are unmodified
+        self.assertEqual(2, len(list1.objects))
+        self.assertEqual(1, list1.objects[0].baz)
+        self.assertEqual(2, list1.objects[1].baz)
+        self.assertEqual(1, len(list2.objects))
+        self.assertEqual(42, list2.objects[0].baz)
+
+    def test_list_object_concat_fails_different_objects(self):
+        @base.VersionedObjectRegistry.register_if(False)
+        class MyList(base.ObjectListBase, base.VersionedObject):
+            fields = {'objects': fields.ListOfObjectsField('MyOwnedObject')}
+
+        @base.VersionedObjectRegistry.register_if(False)
+        class MyList2(base.ObjectListBase, base.VersionedObject):
+            fields = {'objects': fields.ListOfObjectsField('MyOwnedObject')}
+
+        list1 = MyList(objects=[MyOwnedObject(baz=1)])
+        list2 = MyList2(objects=[MyOwnedObject(baz=2)])
+
+        def add(x, y):
+            return x + y
+
+        self.assertRaises(TypeError, add, list1, list2)
+        # Assert that the original lists are unmodified
+        self.assertEqual(1, len(list1.objects))
+        self.assertEqual(1, len(list2.objects))
+        self.assertEqual(1, list1.objects[0].baz)
+        self.assertEqual(2, list2.objects[0].baz)
+
+    def test_list_object_concat_fails_extra_fields(self):
+        @base.VersionedObjectRegistry.register_if(False)
+        class MyList(base.ObjectListBase, base.VersionedObject):
+            fields = {'objects': fields.ListOfObjectsField('MyOwnedObject'),
+                      'foo': fields.IntegerField(nullable=True)}
+
+        list1 = MyList(objects=[MyOwnedObject(baz=1)])
+        list2 = MyList(objects=[MyOwnedObject(baz=2)])
+
+        def add(x, y):
+            return x + y
+
+        self.assertRaises(TypeError, add, list1, list2)
+        # Assert that the original lists are unmodified
+        self.assertEqual(1, len(list1.objects))
+        self.assertEqual(1, len(list2.objects))
+        self.assertEqual(1, list1.objects[0].baz)
+        self.assertEqual(2, list2.objects[0].baz)
+
+    def test_builtin_list_add_fails(self):
+        @base.VersionedObjectRegistry.register_if(False)
+        class MyList(base.ObjectListBase, base.VersionedObject):
+            fields = {'objects': fields.ListOfObjectsField('MyOwnedObject')}
+
+        list1 = MyList(objects=[MyOwnedObject(baz=1)])
+
+        def add(obj):
+            return obj + []
+
+        self.assertRaises(TypeError, add, list1)
+
+    def test_builtin_list_radd_fails(self):
+        @base.VersionedObjectRegistry.register_if(False)
+        class MyList(base.ObjectListBase, base.VersionedObject):
+            fields = {'objects': fields.ListOfObjectsField('MyOwnedObject')}
+
+        list1 = MyList(objects=[MyOwnedObject(baz=1)])
+
+        def add(obj):
+            return [] + obj
+
+        self.assertRaises(TypeError, add, list1)