如何在linux中使用python修改文件的最后一行

Question

我有一个file.txt，其内容如下：

A "Hello, World!" program generally is a computer program that outputs or displays the message "Hello, World!". 
Such a program is very simple in most programming languages, and is often used to illustrate the basic syntax of a programming language. 
It is often the first program written by people learning to code.[1][2] It can also be used as a sanity test to make sure that a computer 
language is correctly installed, and that the operator understands how to use it

Hello world

在文件的最后一行，我有一个句子Hello world，我想评论#，使它变成#Hello world。如何使用 python3 注释/取消注释文件的最后一行。谢谢

Answer 1

你不能像预期的那样简单。一种解决方案是将# 添加到相关行并重写整个文件。

file.txt:

A "Hello, World!" program generally is a computer program that outputs or displays the message "Hello, World!". 
Such a program is very simple in most programming languages, and is often used to illustrate the basic syntax of a programming language. 
It is often the first program written by people learning to code.[1][2] It can also be used as a sanity test to make sure that a computer 
language is correctly installed, and that the operator understands how to use it

Hello world

main.py:

fname = "file.txt"

with open(fname, "r") as f:
  contents = f.readlines()

# Last line is at -1.
contents[-1] = '#' + contents[-1]

with open(fname, "w") as f:
  f.write("".join(contents))

"".join(contents) 将行列表变回字符串。