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def search_index_file(search_dir=None,index=None):
    if search_dir is None:
        search_dir = '/'
        print(search_dir)
        result = search_dir

    if index is not None:
        index = int(index)
        dirs = sorted(os.listdir(search_dir))[index]
        path_file = os.path.join(search_dir,dirs)
        if os.path.isdir(path_file):
            result=search_index_file(search_dir=path_file,index=index)
        else:
            result = path_file
    #这里的return很有意义，第二次进入search_index_file的return result是 找到文件的else返回的结果
    #该结果给第一次执行search_index_file的if os.path.isdir(path_file)的result，
    # 这时候第二次执行return result就是整个最终的结果
    return result