Help with Python UnboundLocalError: local variable referenced before assignment

Question

I have post the similar question before,however,I think I may have misinterpreted my question,so may I just post my origin code here,and looking for someone can help me,I am

Answer 1

where do you call force in this code?

In any event, the problem is in update_r. You reference vs in the first line of update_r even though vs is not defined in this function. Python is not looking at the vs defined above. Try adding

global vs

as the first line of update_r or adding vs to the parameter list for update_r