code: def __load_pst(self):
          
          if self.pst_arg is None:
              return None
          if isinstance(self.pst_arg, Pst):
              self.__pst = self.pst_arg
              return self.pst
          else:
              try:
                  self.log("loading pst: " + str(self.pst_arg))
                  self.__pst = Pst(self.pst_arg)
                  self.log("loading pst: " + str(self.pst_arg))
                  return self.pst
              except Exception as e:
                  raise Exception("linear_analysis.__load_pst(): error loading"+\
                                  " pest control from argument: " +
                                  str(self.pst_arg) + '\n->' + str(e))
docs: private method set the pst attribute