docs: def set_objective(self, expression):
          """Set linear objective of problem"""
  
          if isinstance(expression, numbers.Number):
              # Allow expressions with no variables as objective,
              # represented as a number
              expression = Expression()
  
          self._p.set_linear_objective(
              (lp_name, expression.value(var))
              for var, lp_name in iteritems(self._variables))