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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 _layout_to_vdev(layout, device_dir=None):
    '''
    Turn the layout data into usable vdevs spedcification

    We need to support 2 ways of passing the layout:

    .. code::
        layout_new:
          - mirror:
            - disk0
            - disk1
          - mirror:
            - disk2
            - disk3

    .. code:
        layout_legacy:
          mirror-0:
            disk0
            disk1
          mirror-1:
            disk2
            disk3

    '''
    vdevs = []

    # NOTE: check device_dir exists
    if device_dir and not os.path.exists(device_dir):
        device_dir = None

    # NOTE: handle list of OrderedDicts (new layout)
    if isinstance(layout, list):
        # NOTE: parse each vdev as a tiny layout and just append
        for vdev in layout:
            if isinstance(vdev, OrderedDict):
                vdevs.extend(_layout_to_vdev(vdev, device_dir))
            else:
                if device_dir and vdev[0] != '/':
                    vdev = os.path.join(device_dir, vdev)
                vdevs.append(vdev)

    # NOTE: handle nested OrderedDict (legacy layout)
    #       this is also used to parse the nested OrderedDicts
    #       from the new layout
    elif isinstance(layout, OrderedDict):
        for vdev in layout:
            # NOTE: extract the vdev type and disks in the vdev
            vdev_type = vdev.split('-')[0]
            vdev_disk = layout[vdev]

            # NOTE: skip appending the dummy type 'disk'
            if vdev_type != 'disk':
                vdevs.append(vdev_type)

            # NOTE: ensure the disks are a list (legacy layout are not)
            if not isinstance(vdev_disk, list):
                vdev_disk = vdev_disk.split(' ')

            # NOTE: also append the actualy disks behind the type
            #       also prepend device_dir to disks if required
            for disk in vdev_disk:
                if device_dir and disk[0] != '/':
                    disk = os.path.join(device_dir, disk)
                vdevs.append(disk)

    # NOTE: we got invalid data for layout
    else:
        vdevs = None

    return vdevs