code: def add_attachment(self, attachment):
          
          log = logging.getLogger(self.cls_logger + '.add_attachment')
          if not isinstance(attachment, SlackAttachment):
              msg = 'attachment must be of type: SlackAttachment'
              log.error(msg)
              raise ValueError(msg)
          self.attachments.append(attachment.attachment)
          log.debug('Added attachment: {a}'.format(a=attachment))
docs: Adds an attachment to the SlackMessage payload
  
          This public method adds a slack message to the attachment
          list.
  
          :param attachment: SlackAttachment object
          :return: None