def deal_with_input(box):
    """
    This function will check the format of input rows.
    If user input non-alphabet or not single letters or no space between letters,
    it will print 'Illegal input'.
    If the input format is correct, the input string will be transform into list

    for example:
    an input of "a b c d" will be save into a list ['a', 'b', 'c', 'd']
    :return: list_of_box (list) when legal format: a list contain 4 alphabets, when illegal format: empty list
    """
    list_of_box = []
    if len(box) != 7:
        print('Illegal input')
    else:
        if box[1] is not ' ' or box[3] is not ' ' or box[5] is not ' ':
            print('Illegal input')
        elif box[0].isalpha() and box[2].isalpha() and box[4].isalpha() and box[6].isalpha():
            list_of_box.append(box[0])
            list_of_box.append(box[2])
            list_of_box.append(box[4])
            list_of_box.append(box[6])
        else:
            print('Illegal input')
    return list_of_box