models: Make sure del on attributes does not work

mopidy/models.py

@@ -152,6 +152,9 @@ class ImmutableObject(object):
             return super(ImmutableObject, self).__setattr__(name, value)
         raise AttributeError('Object is immutable.')
 
+    def __delattr__(self, name):
+        raise AttributeError('Object is immutable.')
+
     def __repr__(self):
         kwarg_pairs = []
         for (key, value) in sorted(self.__dict__.items()):

tests/models/test_models.py

@@ -74,6 +74,12 @@ class RefTest(unittest.TestCase):
         with self.assertRaises(AttributeError):
             ref.name = None
 
+    # TODO: add these for the more of the models?
+    def test_del_name(self):
+        ref = Ref(name='foo')
+        with self.assertRaises(AttributeError):
+            del ref.name
+
     def test_invalid_kwarg(self):
         with self.assertRaises(TypeError):
             Ref(foo='baz')