你可以使用raise语句 引发 异常。你还得指明错误/异常的名称和伴随异常 触发的 异常对象。你
可以引发的错误或异常应该分别是一个Error或Exception类的直接或间接导出类。
#!/usr/bin/python
# Filename: raising.py
class ShortInputException(Exception):
'''A user-defined exception class.'''
def __init__(self, length, atleast):
Exception.__init__(self)
self.length = length
self.atleast = atleast
try:
s = raw_input('Enter something --> ')
if len(s) < 3:
raise ShortInputException(len(s), 3)
# Other work can continue as usual here
except EOFError:
print '\nWhy did you do an EOF on me?'
except ShortInputException, x:
print 'ShortInputException: The input was of length %d, \
was expecting at least %d' % (x.length, x.atleast)
else:
print 'No exception was raised.'
$ python raising.py
Enter something -->
Why did you do an EOF on me?
$ python raising.py
Enter something --> ab
ShortInputException: The input was of length 2, was expecting at least 3
$ python raising.py
Enter something --> abc
No exception was raised.
它如何工作这里，我们创建了我们自己的异常类型，其实我们可以使用任何预定义的异常/错误。这个新
的异常类型是ShortInputException类。它有两个域——length是给定输入的长度，atleast则是程序
期望的最小长度。
在except从句中，我们提供了错误类和用来表示错误/异常对象的变量。这与函数调用中的形参
和实参概念类似。在这个特别的except从句中，我们使用异常对象的length和atleast域来为用户
打印一个恰当的消息。