How to use the pyftdi.usbtools.UsbTools.find_backend function in pyftdi

loglevel = min(ERROR, loglevel)
        if debug:
            formatter = Formatter('%(asctime)s.%(msecs)03d %(name)-20s '
                                  '%(message)s', '%H:%M:%S')
        else:
            formatter = Formatter('%(message)s')
        FtdiLogger.set_formatter(formatter)
        FtdiLogger.set_level(loglevel)
        FtdiLogger.log.addHandler(StreamHandler(stderr))

        if args.virtual:
            from pyftdi.usbtools import UsbTools
            # Force PyUSB to use PyFtdi test framework for USB backends
            UsbTools.BACKENDS = ('pyftdi.tests.backend.usbvirt', )
            # Ensure the virtual backend can be found and is loaded
            backend = UsbTools.find_backend()
            loader = backend.create_loader()()
            loader.load(args.virtual)

        try:
            add_custom_devices(Ftdi, args.vidpid)
        except ValueError as exc:
            argparser.error(str(exc))

        Ftdi.show_devices()

    except (ImportError, IOError, NotImplementedError, ValueError) as exc:
        print('\nError: %s' % exc, file=stderr)
        if debug:
            print(format_exc(chain=False), file=stderr)
        exit(1)
    except KeyboardInterrupt:

loglevel = min(ERROR, loglevel)
        if debug:
            formatter = Formatter('%(asctime)s.%(msecs)03d %(name)-20s '
                                  '%(message)s', '%H:%M:%S')
        else:
            formatter = Formatter('%(message)s')
        FtdiLogger.set_formatter(formatter)
        FtdiLogger.set_level(loglevel)
        FtdiLogger.log.addHandler(StreamHandler(stderr))

        if args.virtual:
            from pyftdi.usbtools import UsbTools
            # Force PyUSB to use PyFtdi test framework for USB backends
            UsbTools.BACKENDS = ('pyftdi.tests.backend.usbvirt', )
            # Ensure the virtual backend can be found and is loaded
            backend = UsbTools.find_backend()
            loader = backend.create_loader()()
            loader.load(args.virtual)

        try:
            add_custom_devices(Ftdi, args.vidpid)
        except ValueError as exc:
            argparser.error(str(exc))

        I2cBusScanner.scan(args.device)

    except (ImportError, IOError, NotImplementedError, ValueError) as exc:
        print('\nError: %s' % exc, file=stderr)
        if debug:
            print(format_exc(chain=False), file=stderr)
        exit(1)
    except KeyboardInterrupt:

loglevel = min(ERROR, loglevel)
        if debug:
            formatter = Formatter('%(asctime)s.%(msecs)03d %(name)-20s '
                                  '%(message)s', '%H:%M:%S')
        else:
            formatter = Formatter('%(message)s')
        FtdiLogger.set_formatter(formatter)
        FtdiLogger.set_level(loglevel)
        FtdiLogger.log.addHandler(StreamHandler(stderr))

        if args.virtual:
            from pyftdi.usbtools import UsbTools
            # Force PyUSB to use PyFtdi test framework for USB backends
            UsbTools.BACKENDS = ('pyftdi.tests.backend.usbvirt', )
            # Ensure the virtual backend can be found and is loaded
            backend = UsbTools.find_backend()
            loader = backend.create_loader()()
            loader.load(args.virtual)

        try:
            add_custom_devices(Ftdi, args.vidpid)
        except ValueError as exc:
            argparser.error(str(exc))

        eeprom = FtdiEeprom()
        eeprom.open(args.device)
        if args.erase:
            eeprom.erase()
        if args.serial_number:
            eeprom.set_serial_number(args.serial_number)
        if args.manufacturer:
            eeprom.set_manufacturer_name(args.manufacturer)

loglevel = min(ERROR, loglevel)
        if debug:
            formatter = Formatter('%(asctime)s.%(msecs)03d %(name)-20s '
                                  '%(message)s', '%H:%M:%S')
        else:
            formatter = Formatter('%(message)s')
        FtdiLogger.set_formatter(formatter)
        FtdiLogger.set_level(loglevel)
        FtdiLogger.log.addHandler(StreamHandler(stderr))

        if args.virtual:
            from pyftdi.usbtools import UsbTools
            # Force PyUSB to use PyFtdi test framework for USB backends
            UsbTools.BACKENDS = ('pyftdi.tests.backend.usbvirt', )
            # Ensure the virtual backend can be found and is loaded
            backend = UsbTools.find_backend()
            loader = backend.create_loader()()
            loader.load(args.virtual)

        try:
            add_custom_devices(Ftdi, args.vidpid)
        except ValueError as exc:
            argparser.error(str(exc))

        full_mode = args.fullmode if platform != 'win32' else False
        init_term(full_mode)
        miniterm = MiniTerm(device=args.device,
                            baudrate=to_bps(args.baudrate),
                            parity='N',
                            rtscts=args.hwflow,
                            debug=args.debug)
        miniterm.run(full_mode, args.loopback, args.silent, args.localecho,