# 引发异常 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.' ``` 源文件（[code/raising.py](code/raising.py)） ## 输出 ``` $ 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。