code: def execute(self, sources, target):
          
          fields = self._get_fields_list_from_eps(sources)
          ret_fields = fields[0]
          if self.clear_feature:
              ret_fields = list(self._remove_field_roles(ret_fields, set(six.iterkeys(self.field_mapping)),
                                                         FieldRole.FEATURE))
          target._ml_fields = list(self._set_singleton_role(ret_fields, self.field_mapping))
docs: :type sources: list[DFAdapter]
          :type target: DFAdapter