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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def __load_pst(self):
 """private method set the pst attribute """

        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))