class Networkerror(RuntimeError):
 def __init__(self, arg):
 self.args = arg

在你定义以上类后，你可以触发该异常，如下所示：

try:
 raise Networkerror("Bad hostname")
except Networkerror,e:
 print e.args

在下面这个例子中，默认的__init__()异常已被我们重写。

>>> class MyError(Exception):
... def __init__(self, value):
... self.value = value
... def __str__(self):
... return repr(self.value)
...
>>> try:
... raise MyError(2*2)
... except MyError as e:
... print 'My exception occurred, value:', e.value
...
My exception occurred, value: 4
>>> raise MyError, 'oops!'
Traceback (most recent call last):
 File "", line 1, in ?
__main__.MyError: 'oops!'

常见的做法是创建一个由该模块定义的异常基类和子类，创建特定的异常类不同的错误条件。

我们通常定义的异常类，会让它比较简单，允许提取异常处理程序的错误信息，当创建一个异常模块的时候，常见的做法是创建一个由该模块定义的异常基类和子类，根据不同的错误条件，创建特定的异常类：

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