<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_username(uid,**kwargs):
 """ Return the username of a given user_id """

    rs = db.DBSession.query(User.username).filter(User.id==uid).one()

    if rs is None:
        raise ResourceNotFoundError("User with ID %s not found"%uid)

    return rs.username