自定义异常

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。

实例：

>>> 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 "<stdin>", line 1, in ?
__main__.MyError: 'oops!'

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