code: def set_icon(self, icon_url):
          
          log = logging.getLogger(self.cls_logger + '.set_icon')
          if not isinstance(icon_url, basestring):
              msg = 'icon_url arg must be a string'
              log.error(msg)
              raise ValueError(msg)
          self.payload['icon_url'] = icon_url
          log.debug('Set Icon URL to: {u}'.format(u=icon_url))
docs: Sets the icon_url for the message
  
          :param icon_url: (str) Icon URL
          :return: None