Master Python Exception Handling: Real-World Examples and Best Practices

Python Exception Handling Overview

When Python encounters an error, the interpreter cannot continue and raises an exception, providing an error message.

2. Example Analysis

Attempting to open a non‑existent file 123.txt raises an IOError ("No such file or directory").

print('-----test--1---')
open('123.txt','r')
print('-----test--2---')

Result:

<1> Catching Exceptions with try...except...

Example:

try:
    print('-----test--1---')
    open('123.txt','r')
    print('-----test--2---')
except IOError:
    pass

Result:

Explanation:

The program shows no error because the IOError is caught. pass does nothing; replacing it with print would display a message.

Summary:

Place code that may raise an error inside try.

Handle the exception inside except.

<2> Catching Multiple Exceptions

Example:

try:
    print(num)
except IOError:
    print('产生错误了')

Result:

Why does the program still show an error message even though an except block is present?

Answer: The except catches IOError, but the raised exception is NameError, so it is not handled.

Corrected code:

try:
    print(num)
except NameError:
    print('产生错误了')

Result:

In practice, multiple exceptions can be caught with a tuple:

#coding=utf-8
try:
    print('-----test--1---')
    open('123.txt','r')  # IOError if file missing
    print('-----test--2---')
    print(num)  # NameError if undefined
except (IOError, NameError):
    # handle both exceptions
    pass

Note: When catching multiple exceptions, list their names in a tuple after except.

<3> Accessing Exception Information

try:
    open("a.txt")
except (NameError, IOError) as result:
    print("捕抓到异常")
    print("信息展示：", result)

Result:

<4> Catching All Exceptions

try:
    open("a.txt")
except Exception as result:
    print("捕抓到异常")
    print("信息展示：", result)

Result:

<5> Using else with try...except

The else block runs when no exception is raised.

try:
    num = 100
    print(num)
except NameError as errorMsg:
    print('产生错误了:%s' % errorMsg)
else:
    print('没有捕获到异常，真高兴')

Result:

<6> try...finally...

The finally clause executes regardless of whether an exception occurs, useful for cleanup such as closing files.

import time
try:
    f = open('test.txt')
    try:
        while True:
            content = f.readline()
            if len(content) == 0:
                break
            time.sleep(2)
            print(content)
    except:
        # handle read errors, e.g., Ctrl+C
        pass
    finally:
        f.close()
        print('关闭文件')
except:
    print("没有这个文件")

Result:

Explanation: The script reads each line of test.txt, pauses 2 seconds before printing, and demonstrates that a KeyboardInterrupt triggers, but the finally block still runs to close the file.

Conclusion

This article explained Python exception fundamentals, covering common exception handling techniques with rich examples to help readers understand basic operations.