How to open the serial device exclusively in pyserial?

I want to open the serial device exclusively by pyserial in ubuntu 20.04. I tried the following two approaches.

1. To use the exclusive flag in pyserial.

ser = Serial(port=serialdevice, baudrate=115200, bytesize=8, timeout=2, exclusive=True)

2. To use fcntl as below.

        try:
            ser = Serial(port=serialdevice, baudrate=115200, bytesize=8, timeout=2)
            if ser.isOpen():
                try:
                    fcntl.flock(ser.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
                except Exception as e:
                    print(e)

        except Exception a e:
                    print(e)

Both approaches do not work. There is no any lock file generated under /var/lock.

Finally, I can only created the lock file manually under /var/lock as below.

   try:
        device_name = serialdevice.split("/")[2]

        lock_file_name = "/var/lock/LCK.."+device_name
        ser = Serial(port=serialdevice, baudrate=115200, bytesize=8, timeout=2)
        if ser.isOpen():
            open(lock_file_name, 'w').write(str(os.getpid()))
        else:
            print("port not open")

    except Exception as e:
        print("Failed to open serial port exclusively. Pls check if the serial port is already used by other tools")
        print(e)
        return

May I ask what could be wrong in the first two approaches? Is the manual method used generally in opening the serial port exclusively? Do I need to delete the lock file manually when the python script quit?

Thanks!