diff --git a/gitlab/base.py b/gitlab/base.py
index 5eb111851..53bf45237 100644
--- a/gitlab/base.py
+++ b/gitlab/base.py
@@ -132,7 +132,7 @@ def __ne__(self, other: object) -> bool:
         return super(RESTObject, self) != other
 
     def __dir__(self):
-        return super(RESTObject, self).__dir__() + list(self.attributes)
+        return super(RESTObject, self).__dir__() | self.attributes.keys()
 
     def __hash__(self) -> int:
         if not self.get_id():