with 用法.打开一个上下文管理器.实现enter和exit方法即可

#!/usr/bin/env python
class Test(obj):
    def __enter__(self):
        print "In __enter__()"
              return "test_with"
    def __exit__(self, type, value, trace):
        print "In __exit__()"
def get_example():
    return Test()
with get_example() as example:
    print "example:", example

> 平时都是 with open() ,自己如果实现了__enter__和__exit__ 方法,也可以用with语句打开