Master Python Exception Handling: Propagation, Custom Errors, and Re‑throwing

Python Exception Propagation and Custom Exceptions

This article introduces Python exception handling, covering how exceptions propagate through nested try blocks and function calls, how to create and raise custom exceptions, and how to re‑throw exceptions inside handlers.

1. Exception Propagation

1.1 Try Nesting

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

Result:

1.2 Function Nesting

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()

Result:

Key points:

If a nested try block does not catch an exception, the exception propagates to the outer try block, which can then handle it; if still uncaught, it continues propagating.

When an exception occurs deep in a call chain (e.g., A→B→C), it bubbles up until a function catches it; if no function catches it, the default exception handling is invoked.

2. Raising Custom Exceptions

You can use the raise statement to trigger an exception. Custom exception classes should inherit from Exception or Error.

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

def main():
    try:
        s = input('请输入 --> ')
        if len(s) < 3:
            raise ShortInputException(len(s), 3)
    except ShortInputException as result:
        print('ShortInputException: 输入的长度是 %d,长度至少应是 %d' % (result.length, result.atleast))
    else:
        print('没有异常发生.')

main()

Result:

Note: Calling super().__init__() in the subclass constructor is recommended to ensure proper initialization.

3. Raising Exceptions Inside Handlers

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:
                raise

a = Test(True)
a.calc(11, 0)
print("----------------------华丽的分割线----------------")
a.switch = False
a.calc(11, 0)

Result:

Conclusion

This article provides a detailed walkthrough of Python exception handling, including propagation mechanisms, custom exception creation, and re‑throwing strategies, supported by practical code snippets and output screenshots to help learners master exception management.