Python异常处理

Python 用异常对象(exception object)来表示异常情况。

1. 查看内建的异常类:
   >>> import exceptions
   >>> dir(exceptions)
   所有这些异常都可以用在raise语句中:
   >>> raise ArithmeticError

2. 创建自己的异常类:
   就像其他类一样----只要确保从Exception类继承(不管是间接的或者是直接的，也就是
   说继承其他的内建异常类也是可以的)。
   例如:
class SomeCustomException(Exception): pass

3. 捕捉异常:
   例如:
       

	try:
	    x = input('Enter the first number: ')
	    y = input('Enter the second number: ')
	    print x/y
	except ZeroDivisionError:
 	    print "The second number can't be zero!"
	except TypeError:
	    print "That wasn't a number, was it?"
        

   例如:
       

	try:
	    x = input('Enter the first number: ')
	    y = input('Enter the second number: ')
	except (ZeroDivisionError, TypeError, NameError):
	    print 'You numbers were bogus...'
        

4. 捕捉对象:
   例如:
       

	try:
	    x = input('Enter the first number: ')
            y = input('Enter the second number: ')
	    print x/y
	except (ZeroDivisionError, TypeError), e:
	    print e
        

5. 真正的全捕捉:
   例:
       

	try:
	    x = input('Enter the first number: ')
	    y = input('Enter the second number: ')
	    print x/y
	except:
	    print 'Something wrong happened...'
        

6. 万事大吉:
   一些坏事发生时执行一段代码,例:
  

   try:
       print 'A simple task'
   except:
       print 'What? Something went wrong?'
   else:
       print 'Ah... It went as planned.'
   

   例:
  

   while True:
       try:
           x = input('Enter the first number: ')
           y = input('Enter the second number: ')
           value = x/y
           print 'x/y is', value
       except:
           print 'Invalid input. Please try again.'
       else:
           break
   

7. 最后:
   Finally子句，用来在可能的异常后进行清理，它和try子句联合使用:
  

   x = None
   try:
       x = 1/0
   finally:
       print 'Cleaning up...'
       del x