code: def check_constraints(self, instance):
          '''
          Return fieldnames which need recalculation.
          '''
          recalc_fields = []
          for constraint in self.constraints:
              try:
                  constraint(self.model, instance)
              except constraints.InvalidConstraint as e:
                  recalc_fields.extend(e.fields)
          return recalc_fieldf check_constraints(self, instance):
          '''
          Return fieldnames which need recalculation.
          '''
          recalc_fields = []
          for constraint in self.constraints:
              try:
                  constraint(self.model, instance)
              except constraints.InvalidConstraint as e:
                  recalc_fields.extend(e.fields)
          return recalc_fields
docs: Return fieldnames which need recalculation.