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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 modify_snapshot(snapshot_id=None,
                    description=None,
                    userdata=None,
                    cleanup=None,
                    config="root"):
    '''
    Modify attributes of an existing snapshot.

    config
        Configuration name. (Default: root)

    snapshot_id
        ID of the snapshot to be modified.

    cleanup
        Change the cleanup method of the snapshot. (str)

    description
        Change the description of the snapshot. (str)

    userdata
        Change the userdata dictionary of the snapshot. (dict)

    CLI example:

    .. code-block:: bash

        salt '*' snapper.modify_snapshot 54 description="my snapshot description"
        salt '*' snapper.modify_snapshot 54 description="my snapshot description"
        salt '*' snapper.modify_snapshot 54 userdata='{"foo": "bar"}'
        salt '*' snapper.modify_snapshot snapshot_id=54 cleanup="number"
    '''
    if not snapshot_id:
        raise CommandExecutionError('Error: No snapshot ID has been provided')

    snapshot = get_snapshot(config=config, number=snapshot_id)
    try:
        # Updating only the explicitly provided attributes by the user
        updated_opts = {
            'description': description if description is not None else snapshot['description'],
            'cleanup': cleanup if cleanup is not None else snapshot['cleanup'],
            'userdata': userdata if userdata is not None else snapshot['userdata'],
        }
        snapper.SetSnapshot(config,
                            snapshot_id,
                            updated_opts['description'],
                            updated_opts['cleanup'],
                            updated_opts['userdata'])
        return get_snapshot(config=config, number=snapshot_id)
    except dbus.DBusException as exc:
        raise CommandExecutionError(_dbus_exception_to_reason(exc, locals()))