def test_casUnknowKey(self):
        """
        When L{MemCacheProtocol.checkAndSet} response is C{EXISTS}, the
        resulting L{Deferred} fires with C{False}.
        """
        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"EXISTS\r\n", False)

        When L{MemCacheProtocol.checkAndSet} response is C{EXISTS}, the
        resulting L{Deferred} fires with C{False}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_memcache.py