# 引發異常 你可以使用`raise`語句 引發 異常。你還得指明錯誤/異常的名稱和伴隨異常 觸發的 異常對象。你可以引發的錯誤或異常應該分別是一個`Error`或`Exception`類的直接或間接導出類。 ``` #!/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)） ## 輸出 ``` $ 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 這里，我們創建了我們自己的異常類型，其實我們可以使用任何預定義的異常/錯誤。這個新的異常類型是`ShortInputException`類。它有兩個域——`length`是給定輸入的長度，`atleast`則是程序期望的最小長度。 在`except`從句中，我們提供了錯誤類和用來表示錯誤/異常對象的變量。這與函數調用中的形參和實參概念類似。在這個特別的`except`從句中，我們使用異常對象的`length`和`atleast`域來為用戶打印一個恰當的消息。