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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 _send_mbus(self, frame):
 """Send modbus frame :param frame: modbus frame to send (with MBAP for TCP/CRC for RTU) :type frame: str (Python2) or class bytes (Python3) :returns: number of bytes send or None if error :rtype: int or None """

        # for auto_open mode, check TCP and open if need
        if self.__auto_open and not self.is_open():
            self.open()
        # send request
        bytes_send = self._send(frame)
        if bytes_send:
            if self.__debug:
                self._pretty_dump('Tx', frame)
            return bytes_send
        else:
            return None