在程序中可以通过创建新的异常类型来命名自己的异常。异常类通常应该直接或间接的从Exception 类派生,例如:
>>> class MyError(Exception): ... def __init__(self, value): ... self.value = value ... def __str__(self): ... return repr(self.value) ... >>> try: ... raise MyError(2*2) ... except MyError, e: ... print 'My exception occurred, value:', e.value ... My exception occurred, value: 4 >>> raise MyError, 'oops!' Traceback (most recent call last): File "<stdin>", line 1, in ? __main__.MyError: 'oops!'
异常类中可以定义任何其它类中可以定义的东西,但是通常为了保持简单,只在其中加入几个属性信息,以供异常处理句柄提取。如果一个新创建的模块中需要抛出几种不同的错误时,一个通常的作法是为该模块定义一个异常基类,然后针对不同的错误类型派生出对应的异常子类。
class Error(Exception): """Base class for exceptions in this module.""" pass class InputError(Error): """Exception raised for errors in the input. Attributes: expression -- input expression in which the error occurred message -- explanation of the error """ def __init__(self, expression, message): self.expression = expression self.message = message class TransitionError(Error): """Raised when an operation attempts a state transition that's not allowed. Attributes: previous -- state at beginning of transition next -- attempted new state message -- explanation of why the specific transition is not allowed """ def __init__(self, previous, next, message): self.previous = previous self.next = next self.message = message
与标准异常相似,大多数异常的命名都以“Error”结尾。
很多标准模块中都定义了自己的异常,用以报告在他们所定义的函数中可能发生的错误。关于类的进一步信息请参见第 9章,“类”。