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）