python异常捕获try except

python的异常处理机制设计的比较传统，在日常的开发中，基本满足我的需要

下面就python的异常，几点小小的讨论

1. try...except

tommy@lab3:~$ python
Python 2.5.2 (r252:60911, Jan  4 2009, 17:40:26)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>>
>>> try:
...     1/0
... except:
...     print "do something..."
...
do something...
>>>

2. try...finally

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
>>>

所以，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

>>> try:
...     fd=open("have-exists-file", "r")
...     print "do some thing ,read file ,write file..."
... finally:
...     fd.close()
...
do some thing ,read file ,write file...
>>>

多线程中，对锁的使用

tommy@lab3:~$ python
Python 2.5.2 (r252:60911, Jan  4 2009, 17:40:26)
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import threading
>>> l_lock=threading.RLock()
>>> try:
...     l_lock.acquire()
...     print "do some thing."
... finally:
...     l_lock.release()
...
True
do some thing.
>>>

注意，finally中，如果出现异常，外部如果没有相应的捕获机制，该异常会层层抛出，直到最顶端，然后解释器停止
一般我们会这样做

>>> try:
...     try:
...         fd=open("no-exists-file", "r")
...         print "do some thing ,read file ,write file..."
...     finally:
...         fd.close()
...except:
...    print "catch finally exception."
do some thing ,read file ,write file...
>>>

3. 封装了一个全局的捕获异常的函数（偶比较懒，就用了全部的捕获异常方法）

import traceback
import sys
def trace_back():
    try:
        return traceback.print_exc()
    except:
        return ''

具体使用

try:
    do_something()
except:
    print trace_back()

写的比较散，主要是想说说，python异常的简单使用，已经封装的一个简单的获得异常发生时，全局保持的异常信息（可能不准确，但基本够用）