def pipe_data_received(self, fd, data):
        """Used to signal `asyncio.SubprocessProtocol` of incoming data."""
        if fd == 2:  # stderr fd number
            self._on_stderr(data)
        elif self._on_data:
            self._on_data(data)
        else:
            self._queued_data.append(data)
Used to signal `asyncio.SubprocessProtocol` of incoming data.