def append(self, key, val):
        """
        Append given data to the value of an existing key.

        @param key: The key to modify.
        @type key: L{bytes}

        @param val: The value to append to the current value associated with
            the key.
        @type val: L{bytes}

        @return: A deferred that will fire with C{True} if the operation has
            succeeded, C{False} otherwise.
        @rtype: L{Deferred}
        """
        # Even if flags and expTime values are ignored, we have to pass them
        return self._set(b"append", key, val, 0, 0, b"")

        Append given data to the value of an existing key.

        @param key: The key to modify.
        @type key: L{bytes}

        @param val: The value to append to the current value associated with
            the key.
        @type val: L{bytes}

        @return: A deferred that will fire with C{True} if the operation has
            succeeded, C{False} otherwise.
        @rtype: L{Deferred}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/memcache.py