An article to help you understand Python exception delivery and custom exceptions

import time


try:
    f = open('test.txt')
    try:
        while True:
            content = f.readline()
            if len(content) == 0:
                break
            time.sleep(2)
    finally:
        f.close()
        print('关闭文件')
except :
   print("没有这个文件")
finally:
    print("最后的finally")


# 这是test.txt文件中读取到信息

   def test1():
        print("----test1-1----")
        print(num)
        print("----test1-2----")




    def test2():
        print("----test2-1----")
        test1()
        print("----test2-2----")




    def test3():
        try:
            print("----test3-1----")
            test1()
            print("----test3-2----")
        except Exception as result:
            print("捕获到了异常，信息是:%s"%result)


        print("----test3-2----")




    test3()
    print("------华丽的分割线-----")
    test2()

class ShortInputException(Exception):
    '''自定义的异常类'''
    def __init__(self, length, atleast):
        #super().__init__()
        self.length = length
        self.atleast = atleast


def main():
    try:
        s = input('请输入 --> ')
        if len(s) < 3:
            # raise引发一个你定义的异常
            raise ShortInputException(len(s), 3) #自定义异常长度为3。
            
    except ShortInputException as result:#x这个变量被绑定到了错误的实例
      
        print(&#39;ShortInputException: 输入的长度是 %d,长度至少应是 %d&#39;% (result.length, result.atleast))
    else:
        print(&#39;没有异常发生.&#39;)


main()

class Test(object):
    def __init__(self, switch):
        self.switch = switch #开关
    def calc(self, a, b):
        try:
            return a/b
        except Exception as result:
            if self.switch:
                print("捕获开启，已经捕获到了异常，信息如下:")
                print(result)
            else:
                #重新抛出这个异常，此时就不会被这个异常处理给捕获到，从而触发默认的异常处理
                raise


a = Test(True)


a.calc(11,0)


print("----------------------华丽的分割线----------------")


a.switch = False
a.calc(11,0)