code: def role(self, role_name):
          
          field_name = self.name
          field_roles = {field_name: MLField.translate_role_name(role_name)}
          if field_roles:
              return _change_singleton_roles(self, field_roles, True)
          else:
              return self
docs: Set role of current column
  
          :param role_name: name of the role to be selected.
          :return: