def test_checkAndSet(self):
        """
        L{MemCacheProtocol.checkAndSet} passes an additional cas identifier
        that the server handles to check if the data has to be updated.
        """
        return self._test(
            self.proto.checkAndSet(b"foo", b"bar", cas=b"1234"),
            b"cas foo 0 0 3 1234\r\nbar\r\n", b"STORED\r\n", True)

        L{MemCacheProtocol.checkAndSet} passes an additional cas identifier
        that the server handles to check if the data has to be updated.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_memcache.py