python删除文件中的行

Question

python怎么删除文件中的行，比如下面这个文件demo.txt，内容如下：

hello world!
hello beijing!
hello shanghai!
hello hangzhou!

//delete
hello NewYork!
hello London!

我想把从注释//delete开始后面的行都删掉，应该怎么做呢？
需要给删除的每行添加一个标记吗？看到一个示例是这样写的：

def remove_lines():
    with open("demo.txt", "r") as f:
        lines = f.readlines()

    with open("demo.txt", "w") as f_w:
        for line in lines:
            if "mark" in line:  #要删除的每行有个mark标记字符串
                continue
            f_w.write(line)

有其他更简洁一点的写法吗？

Answer 1

with open('demo.txt', 'r') as f:
    lines = f.readlines()

index = lines.index('//delete\n')
lines = lines[:index]
    
with open('demo.txt', 'w') as f:
    for line in lines:
        f.write(line)

Answer 2

import re
with open("hello.txt", "r") as f:
    lines = f.read()

ll = re.sub('//delete[\s\S]*','',lines,re.S)
f = file('hello.txt','w')
f.write(ll)
f.close()

结合正则表达式