如何用字符串变量替换文本文件中的一行？

# 读取文本文件
with open('file.txt', 'r') as file:
    lines = file.readlines()

# 定位目标行并替换
target_line_index = -1
for i, line in enumerate(lines):
    if '目标行的关键词' in line:
        target_line_index = i
        break

if target_line_index != -1:
    # 构建替换字符串
    new_line = '替换后的内容'

    # 替换目标行
    lines[target_line_index] = new_line

    # 保存修改后的文本文件
    with open('file.txt', 'w') as file:
        file.writelines(lines)