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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 list_available_devices():
 """ List all available devices for the respective backend returns: devices: a list of dictionaries with the keys 'identifier' and 'instance': \ [ {'identifier': 'usb://0x04f9:0x2015/C5Z315686', 'instance': pyusb.core.Device()}, ] The 'identifier' is of the format idVendor:idProduct_iSerialNumber. """


    class find_class(object):
        def __init__(self, class_):
            self._class = class_
        def __call__(self, device):
            # first, let's check the device
            if device.bDeviceClass == self._class:
                return True
            # ok, transverse all devices to find an interface that matches our class
            for cfg in device:
                # find_descriptor: what's it?
                intf = usb.util.find_descriptor(cfg, bInterfaceClass=self._class)
                if intf is not None:
                    return True
            return False

    # only Brother printers
    printers = usb.core.find(find_all=1, custom_match=find_class(7), idVendor=0x04f9)

    def identifier(dev):
        try:
            serial = usb.util.get_string(dev, 256, dev.iSerialNumber)
            return 'usb://0x{:04x}:0x{:04x}_{}'.format(dev.idVendor, dev.idProduct, serial)
        except:
            return 'usb://0x{:04x}:0x{:04x}'.format(dev.idVendor, dev.idProduct)

    return [{'identifier': identifier(printer), 'instance': printer} for printer in printers]