Hi Anil,
Can you tell me when your on_new_focus is getting called? Are you trying to do On_new_focus on changing of Dropdown? Excuse me if i'm wrong, but I didnt understand exactly when you expect this method to be executed in your case.
If you are trying to execute this along with drop down change, then it is wrong. This method will be called when you change focus from an entity,
Consider a simple scenario, you have 2 Context Node College and Students, Now we have a relation between these 2 entities, and College is the Higher level node.
When a College entity is Filled in College Node the Student Node should automatically be filled with the Student entities related to that college,
on_new_focus is used for this purpose,
So each time you change entity in College Node, on_new_focus will be called and Related Entities will be filled into Student.
Again Excuse me if this was not your question, Please explain your doubt in detail 
Thanks and Regards,
Pratheek