<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def open_usb_handle(self, port_num):
 """open usb port Args: port_num: port number on the Cambrionix unit Return: usb handle """

    serial = self.get_usb_serial(port_num)
    return local_usb.LibUsbHandle.open(serial_number=serial)