The constructor for the built-in open() function is:
open(file,mode="r",buffering=None,encoding=None,errors=None,newline=None,closefd=True)
The possible modes are:
* r: Reading
* w: Open for writing
* a: Open for appending
* b: Binary mode
* t: Text mode
* +: Open a disk file for updating
* U: Universal newline mode
The default mode is rt, or open for reading text mode.
（原文地址：http://www.ibm.com/developerworks/linux/library/l-python3-1/）
python 3.0 也有介绍 http://docs.python.org/3.0/library/io.html
