args, an attribute of exception

When an exception occurs, it may have associated values, also known as the exception’s arguments.

The except clause may specify a variable after the exception name. The variable is bound to the exception instance which typically has an args attribute that stores the arguments. For convenience, builtin exception types define __str__() to print all the arguments without explicitly accessing .args

也就是说，当我们打印一个异常的时候，实际发生了如下调用

print -> exception.__str__() -> exception.args

>>>
try:
    raise Exception('spam', 'eggs')
except Exception as inst:
    print(type(inst))    # the exception type
    print(inst.args)     # arguments stored in .args
    print(inst)          # __str__ allows args to be printed directly,
                         # but may be overridden in exception subclasses

    x, y = inst.args     # unpack args
    print('x =', x)
    print('y =', y)

<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs

The exception’s __str__() output is printed as the last part (‘detail’) of the message for unhandled exceptions.

how to define customed exception

两个关键方法：__init__() __str__()

关于 自定义异常 的__init__()

如果你想定义的新异常重写了 __init__() 方法， 确保你使用所有参数调用 Exception.__init__() ，例如：

class CustomError(Exception):
    def __init__(self, message, status):
        super().__init__(message, status)
        self.message = message
        self.status = status

Exception的默认行为是接受所有传递的参数并将它们以元组形式存储在 .args 属性中. 很多其他函数库和部分Python库默认所有异常都必须有 .args 属性， 因此如果你忽略了这一步，你会发现有些时候你定义的新异常不会按照期望运行

只有显式地调用super().__init__()给自定义异常实例的.args 属性赋值，后续打印异常时__str__()方法才有内容可用

当然，如果你想改变异常打印的内容样式，重写__str__()即可