code: def touch(self, conn, key, exptime):
          
          assert self._validate_key(key)
  
          _cmd = b' '.join([b'touch', key, str(exptime).encode('utf-8')])
          cmd = _cmd + b'\r\n'
          resp = yield from self._execute_simple_command(conn, cmd)
          if resp not in (const.TOUCHED, const.NOT_FOUND):
              raise ClientException('Memcached touch failed', resp)
          return resp == const.TOUCHED
docs: The command is used to update the expiration time of
          an existing item without fetching it.
  
          :param key: ``bytes``, is the key to update expiration time
          :param exptime: ``int``, is expiration time. This replaces the existing
          expiration time.
          :return: ``bool``, True in case of success.