python 文本模式读写文件时不应使用 os.linesep 简介

旧城等待， 2021-09-02 05:52 124阅读 0赞

[os.linesep][]官方文档

> The string used to separate (or, rather, terminate) lines on the current platform. This may be a single character, such as '\\n' for POSIX, or multiple characters, for example, '\\r\\n' for Windows. Do not use os.linesep as a line terminator when writing files opened in text mode (the default); use a single '\\n' instead, on all platforms.

如上，os.linesep是用来分割文件的每一行（即文件结束符），由于在不同操作系统下文件结束符不一定相同，所以os.linesep是跨平台的文件描述符，比如在Windows平台上是'\\r\\n'，在Linux平台上则是'\\n'

Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
    Type "copyright", "credits" or "license()" for more information.
    >>> import os
    >>> os.linesep
    '\r\n'

Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
    [GCC 5.4.0 20160609] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import os
    >>> os.linesep
    '\n'

但是以`open`默认的文本模式读写时，'\\n'会被自动转换成'\\r\\n'。在Windows平台实验如下

>>> with open(r'D:\test.txt', 'w') as f:
              f.write(os.linesep)
    
              
    2
    >>> with open(r'D:\test.txt', 'rb') as f:
              f.read()
    
              
    b'\r\r\n'

本来是要写入结束符'\\r\\n'，结果由于python自动把'\\n'替换成'\\r\\n'导致写入的是'\\r\\n\\n'。因此按照官方的建议，此时使用'\\n'代替os.linesep即可。  
不过在二进制模式下，为文本文件添加换行符的操作用os.linesep来实现跨平台更好。

>>> with open(r'D:\test.txt', 'wb') as f:
              f.write(os.linesep.encode())
    
              
    2
    >>> with open(r'D:\test.txt', 'rb') as f:
              f.read()
    
              
    b'\r\n'

参考资料  
[https://stackoverflow.com/questions/21636213/why-you-shouldnt-use-os-linesep-when-editing-on-text-mode][https_stackoverflow.com_questions_21636213_why-you-shouldnt-use-os-linesep-when-editing-on-text-mode]

[os.linesep]: https://link.jianshu.com?t=https%3A%2F%2Fdocs.python.org%2F3%2Flibrary%2Fos.html%3Fhighlight%3Dlinesep%23os.linesep
[https_stackoverflow.com_questions_21636213_why-you-shouldnt-use-os-linesep-when-editing-on-text-mode]: https://link.jianshu.com?t=https%3A%2F%2Fstackoverflow.com%2Fquestions%2F21636213%2Fwhy-you-shouldnt-use-os-linesep-when-editing-on-text-mode