try...finally的用法

finally里面只是执行完成try中的代码后，必须执行的代码，
即使是try中有异常抛出，也是会去执行finally。

>>> try:
...     1/0
... finally:
...     print "I just finally do something ,eg: clear!"
...
I just finally do something ,eg: clear!
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: integer division or modulo by zero
>>>

一般情况下，finally里面执行的都是一些清理工作，比如：关闭文件描述符，释放锁等