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/yexcept 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/yexcept (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/yexcept:    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