6 PEP 278: Universal Newline Support
The three major operating systems used today are Microsoft Windows, Apple's Macintosh OS, and the various Unix derivatives. A minor irritation of cross-platform work is that these three platforms all use different characters to mark the ends of lines in text files. Unix uses the linefeed (ASCII character 10), MacOS uses the carriage return (ASCII character 13), and Windows uses a two-character sequence of a carriage return plus a newline.
Python's file objects can now support end of line conventions other than the one followed by the platform on which Python is running. Opening a file with the mode 'U' or 'rU' will open a file for reading in universal newline mode. All three line ending conventions will be translated to a "\n" in the strings returned by the various file methods such as read() and readline().
Universal newline support is also used when importing modules and when executing a file with the execfile() function. This means that Python modules can be shared between all three operating systems without needing to convert the line-endings.
This feature can be disabled when compiling Python by specifying the --without-universal-newlines switch when running Python's configure script.
標記文本文檔一行結束的符號：
1、unix LF(ascii 10;0x10)
2、mac CR（ascii 13;0x0C）
3、windows (CRLF 0x0c10)
python讀取文件的時候使用U或者rU，來實現跨平台，它把所有的符號統一轉換為\n
with open(filename,'rU') as f:
f.read()/f.readling()
https://docs.python.org/2.3/whatsnew/node7.html