读取文件里的文件，按修改时间从小到大（时间戳）排序，合并到一个Excel

def merge_excel_data(folder_path, result_file_path, file_name_list):
    """

    :param folder_path: 文件夹路径
    :param result_file_path: 汇总文件绝对路径
    :param file_name_list: 文件名list
    :return: 汇总文件DataFrame
    """
    # 默认sort排序，是按从小到大来的
    file_name_list = sorted(file_name_list, key=lambda x: os.path.getmtime(os.path.join(folder_path, x)))
    target_list = []
    for file in file_name_list:
        child = os.path.join(folder_path, file)
        target_list.append(child)
    # 读取汇总表
    if os.path.exists(result_file_path):
        merge_df = pd.read_excel(result_file_path, keep_default_na=False, dtype=str)
    else:
        merge_df = DataFrame(dtype=str)
    today_df = DataFrame(dtype=str)
    for file_path in target_list:
        temporary_df = pd.read_excel(file_path, keep_default_na=False, dtype=str)
        today_df = today_df.append(temporary_df)
    merge_df = merge_df.append(today_df)
    merge_df.to_excel(result_file_path, index=False)

    return today_df