如何用python替换sed-like文本？

Question

我想启用此文件中的所有apt存储库

cat /etc/apt/sources.list
## Note, this file is written by cloud-init on first boot of an instance                                                                                                            
## modifications made here will not survive a re-bundle.                                                                                                                            
## if you wish to make changes you can:                                                                                                                                             
## a.) add 'apt_preserve_sources_list: true' to /etc/cloud/cloud.cfg                                                                                                                
##     or do the same in user-data
## b.) add sources in /etc/apt/sources.list.d                                                                                                                                       
#                                                                                                                                                                                   

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to                                                                                                           
# newer versions of the distribution.                                                                                                                                               
deb http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick main                                                                                                                   
deb-src http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick main                                                                                                               

## Major bug fix updates produced after the final release of the                                                                                                                    
## distribution.                                                                                                                                                                    
deb http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-updates main                                                                                                           
deb-src http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-updates main                                                                                                       

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu                                                                                                         
## team. Also, please note that software in universe WILL NOT receive any                                                                                                           
## review or updates from the Ubuntu security team.                                                                                                                                 
deb http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick universe                                                                                                               
deb-src http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick universe                                                                                                           
deb http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-updates universe
deb-src http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
# deb http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick multiverse
# deb-src http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick multiverse
# deb http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-updates multiverse
# deb-src http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-updates multiverse

## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
# deb http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-backports main restricted universe multiverse
# deb-src http://us-east-1.ec2.archive.ubuntu.com/ubuntu/ maverick-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu maverick partner
# deb-src http://archive.canonical.com/ubuntu maverick partner

deb http://security.ubuntu.com/ubuntu maverick-security main
deb-src http://security.ubuntu.com/ubuntu maverick-security main
deb http://security.ubuntu.com/ubuntu maverick-security universe
deb-src http://security.ubuntu.com/ubuntu maverick-security universe
# deb http://security.ubuntu.com/ubuntu maverick-security multiverse
# deb-src http://security.ubuntu.com/ubuntu maverick-security multiverse

有了sed，这是一个简单的sed -i 's/^# deb/deb/' /etc/apt/sources.list最优雅的（“pythonic”）方法是什么？

Answer 1

在纯Python中使用no外部命令或附加依赖项创建一个自制的sed替换是一项崇高的任务，充满了崇高的地雷。谁会想到呢？

尽管如此，这是可行的。这也是可取的。我们都去过那里，人们：“我需要咀嚼一些纯文本文件，但我只有Python，两个塑料鞋带，和一罐发霉的地堡级马拉希诺樱桃。帮助。”

在这个答案中，我们提供了一个最佳的解决方案，将先前答案的令人不快的地方拼凑在一起，而不是令人不快的地方。正如plundra所指出的，David Miller的otherwise top-notch answer以非原子的方式写入所需的文件，因此会引发竞争条件（例如，来自其他试图同时读取该文件的线程和/或进程）。真糟糕。Plundra的otherwise excellent answer解决了这个问题，同时引入了更多的错误，包括许多致命的编码错误、一个关键的安全漏洞（无法保留原始文件的权限和其他元数据）以及过早的优化，用低级字符索引替换正则表达式。那也不好。

太棒了，团结起来！

import re, shutil, tempfile

def sed_inplace(filename, pattern, repl):
    '''
    Perform the pure-Python equivalent of in-place `sed` substitution: e.g.,
    `sed -i -e 's/'${pattern}'/'${repl}' "${filename}"`.
    '''
    # For efficiency, precompile the passed regular expression.
    pattern_compiled = re.compile(pattern)

    # For portability, NamedTemporaryFile() defaults to mode "w+b" (i.e., binary
    # writing with updating). This is usually a good thing. In this case,
    # however, binary writing imposes non-trivial encoding constraints trivially
    # resolved by switching to text writing. Let's do that.
    with tempfile.NamedTemporaryFile(mode='w', delete=False) as tmp_file:
        with open(filename) as src_file:
            for line in src_file:
                tmp_file.write(pattern_compiled.sub(repl, line))

    # Overwrite the original file with the munged temporary file in a
    # manner preserving file attributes (e.g., permissions).
    shutil.copystat(filename, tmp_file.name)
    shutil.move(tmp_file.name, filename)

# Do it for Johnny.
sed_inplace('/etc/apt/sources.list', r'^\# deb', 'deb')

Answer 2

massedit.py（http://github.com/elmotec/massedit）为您搭建脚手架，只留下要编写的regex。它仍在测试阶段，但我们正在寻找反馈。

python -m massedit -e "re.sub(r'^# deb', 'deb', line)" /etc/apt/sources.list

将以差异格式显示差异（之前/之后）。

添加-w选项以将更改写入原始文件：

python -m massedit -e "re.sub(r'^# deb', 'deb', line)" -w /etc/apt/sources.list

或者，现在可以使用api：

>>> import massedit
>>> filenames = ['/etc/apt/sources.list']
>>> massedit.edit_files(filenames, ["re.sub(r'^# deb', 'deb', line)"], dry_run=True)

Answer 3

你可以这样做：

with open("/etc/apt/sources.list", "r") as sources:
    lines = sources.readlines()
with open("/etc/apt/sources.list", "w") as sources:
    for line in lines:
        sources.write(re.sub(r'^# deb', 'deb', line))

with语句确保文件正确关闭，在"w"模式下重新打开文件会在写入文件之前清空该文件。sub（pattern，replace，string）与sed/perl中的s/pattern/replace/等价。

编辑：示例中的固定语法