用户自定义异常
你可以通过创建一个新的异常类来拥有自己的异常。异常类继承自 Exception 类，可以直接继承，或者间接继承，例如:
>>>class MyError(Exception):
def__init__(self, value):
self.value= value
def__str__(self):
returnrepr(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!'
在这个例子中，类 Exception 默认的 __init__() 被覆盖。
当创建一个模块有可能抛出多种不同的异常时，一种通常的做法是为这个包建立一个基础异常类，然后基于这个基础类为不同的错误情况创建不同的子类:
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
大多数的异常的名字都以"Error"结尾，就跟标准的异常命名一样。
