code: def rollback(awsclient, function_name, alias_name=ALIAS_NAME, version=None):
      
      if version:
          log.info('rolling back to version {}'.format(version))
      else:
          log.info('rolling back to previous version')
          version = _get_previous_version(awsclient, function_name, alias_name)
          if version == '0':
              log.error('unable to find previous version of lambda function')
              return 1
  
          log.info('new version is %s' % str(version))
  
      _update_alias(awsclient, function_name, version, alias_name)
      return 0
docs: Rollback a lambda function to a given version.
  
      :param awsclient:
      :param function_name:
      :param alias_name:
      :param version:
      :return: exit_code