Returns the initial reply. This is emitted before any subscription
        data is emitted. This function raises an exception if the subscription
        attempt failed.

def reply(self, timeout=None):
        """
        Returns the initial reply. This is emitted before any subscription
        data is emitted. This function raises an exception if the subscription
        attempt failed.
        """
        self._wait_on_signal(self._response_received)
        if self._response_exception is not None:
            msg = self._response_exception.message
            raise YamcsError(msg)
        return self._response_reply