Expose object context thru a public property

This is useful for consuming projects to cary on the context in base
class without duplicating it in their own base classes.

Change-Id: I28f49740b82a3063709c3c239b5199e901ad898e

oslo_versionedobjects/base.py

@@ -664,6 +664,10 @@ class VersionedObject(object):
     def obj_fields(self):
         return list(self.fields.keys()) + self.obj_extra_fields
 
+    @property
+    def obj_context(self):
+        return self._context
+
 
 class ComparableVersionedObject(object):
     """Mix-in to provide comparison methods

oslo_versionedobjects/tests/test_objects.py

@@ -1083,6 +1083,21 @@ class _TestObject(object):
         obj = TestObj()
         self.assertEqual(['foo', 'bar'], obj.obj_fields)
 
+    def test_obj_context(self):
+        class TestObj(base.VersionedObject):
+            pass
+
+        # context is available through the public property
+        context = mock.Mock()
+        obj = TestObj(context)
+        self.assertEqual(context, obj.obj_context)
+
+        # ..but it's not available for update
+        new_context = mock.Mock()
+        self.assertRaises(
+            AttributeError,
+            setattr, obj, 'obj_context', new_context)
+
     def test_obj_constructor(self):
         obj = MyObj(context=self.context, foo=123, bar='abc')
         self.assertEqual(123, obj.foo)