Prevent 2 calls to queue_position

website/events/api/serializers.py

@@ -179,8 +179,8 @@ class RegistrationSerializer(serializers.ModelSerializer):
         return instance.is_late_cancellation()
 
     def _queue_position(self, instance):
-        return (instance.queue_position()
-                if instance.queue_position() > 0 else None)
+        pos = instance.queue_position()
+        return pos if pos > 0 else None
 
     def _is_cancelled(self, instance):
         return instance.date_cancelled is not None