def partion(key):
    """
    split the key have the format of donkey

    Arguments
    ---------
    key: string

    Examples
    --------
    >>>partion( 'a__b__c' )
    [ 'a', 'b', 'c' ]
    """
    return key.split('__')
    parts = key.rsplit('__', 1)
    return tuple(parts) if len(parts) > 1 else (parts[0], None)
