async def exists(self, key, namespace=None, _conn=None):
        """
        Check key exists in the cache.

        :param key: str key to check
        :param namespace: str alternative namespace to use
        :param timeout: int or float in seconds specifying maximum timeout
            for the operations to last
        :returns: True if key exists otherwise False
        :raises: :class:`asyncio.TimeoutError` if it lasts more than self.timeout
        """
        start = time.monotonic()
        ns_key = self.build_key(key, namespace=namespace)
        ret = await self._exists(ns_key, _conn=_conn)
        logger.debug("EXISTS %s %d (%.4f)s", ns_key, ret, time.monotonic() - start)
        return ret
Check key exists in the cache.

        :param key: str key to check
        :param namespace: str alternative namespace to use
        :param timeout: int or float in seconds specifying maximum timeout
            for the operations to last
        :returns: True if key exists otherwise False
        :raises: :class:`asyncio.TimeoutError` if it lasts more than self.timeout