<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def touch(self, conn, key, exptime):
 """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. """

        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