如果一行不存在，如何在不同子目录下的多个文本文件中添加一行？

import os

def add_line_in_files(directory, line):
    for root, dirs, files in os.walk(directory):
        for file in files:
            if file.endswith(".txt"):  # 只处理后缀为.txt的文本文件，可以根据实际情况调整
                file_path = os.path.join(root, file)
                with open(file_path, 'r') as f:
                    lines = f.readlines()

                # 在列表的第一行之前插入新行
                lines.insert(0, line + '\n')

                with open(file_path, 'w') as f:
                    f.writelines(lines)

# 示例用法
directory = "/path/to/parent_directory"  # 替换为实际的父目录路径
line_to_insert = "This is the line to insert"
add_line_in_files(directory, line_to_insert)