code: def cas(self, key, value, cas, expire=0, noreply=False):
          
          return self._store_cmd(b'cas', {key: value}, expire, noreply, cas)[key]
docs: The memcached "cas" command.
  
          Args:
            key: str, see class docs for details.
            value: str, see class docs for details.
            cas: int or str that only contains the characters '0'-'9'.
            expire: optional int, number of seconds until the item is expired
                    from the cache, or zero for no expiry (the default).
            noreply: optional bool, False to wait for the reply (the default).
  
          Returns:
            If noreply is True, always returns True. Otherwise returns None if
            the key didn't exist, False if it existed but had a different cas
            value and True if it existed and was changed.