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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def request(self, request):
 """ Send a request to the Kafka broker. :param bytes request: The bytes of a Kafka `RequestMessage`_ structure. It must have a unique (to this connection) correlation ID. :returns: `Deferred` which will: - Succeed with the bytes of a Kafka `ResponseMessage`_ - Fail when the connection terminates .. _RequestMessage:: https://kafka.apache.org/protocol.html#protocol_messages """

        if self._failed is not None:
            return fail(self._failed)
        correlation_id = request[4:8]
        assert correlation_id not in self._pending
        d = Deferred()
        self.sendString(request)
        self._pending[correlation_id] = d
        return d