code: def get_objective(self, objective_id=None):
          
          if objective_id is None:
              raise NullArgument()
          url_path = construct_url('objectives', obj_id=objective_id)
          return objects.Objective(self._get_request(url_path))
docs: Gets the Objective specified by its Id.
          In plenary mode, the exact Id is found or a NotFound results.
          Otherwise, the returned Objective may have a different Id than
          requested, such as the case where a duplicate Id was assigned to
          an Objective and retained for compatibility.
          arg:    objectiveId (osid.id.Id): Id of the Objective
          return: (osid.learning.Objective) - the objective
          raise:  NotFound - objectiveId not found
          raise:  NullArgument - objectiveId is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method is must be implemented.