code: def _check_required(self, value):
          '''
          Internal method to check if assigned value is None while it is required.
          Exception is thrown if ``True``
          '''
          if value is None and self._required:
              err_msg = self._errors['required'].format(self.__class__.__name__, self.name)
              if self.container_model:
                  err_msg += self._errors['required_extra'].format(self.container_model.__name__)
              raise ValueError(err_msgf _check_required(self, value):
          '''
          Internal method to check if assigned value is None while it is required.
          Exception is thrown if ``True``
          '''
          if value is None and self._required:
              err_msg = self._errors['required'].format(self.__class__.__name__, self.name)
              if self.container_model:
                  err_msg += self._errors['required_extra'].format(self.container_model.__name__)
              raise ValueError(err_msg)
docs: Internal method to check if assigned value is None while it is required.
          Exception is thrown if ``True``