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'
