Python Exception Handling: Syntax, Usage, and Examples

Python exception handling lets you catch and manage errors, preventing program crashes. Instead of stopping execution when an error occurs, Python gives you tools to handle issues gracefully and keep your code running smoothly.

How to Use Exception Handling in Python

You can use the try-except block to catch exceptions and execute alternative code. The basic syntax looks like this:

try:
    # Code that might raise an exception
except ExceptionType:
    # Code to handle the exception

• try: Runs a block of code that might cause an error.
• except: Catches and handles the specified exception.
• ExceptionType: Defines the type of error to handle, such as ZeroDivisionError or FileNotFoundError.

A finally block ensures that specific code runs no matter what happens.

try:
    file = open("data.txt", "r")
except FileNotFoundError:
    print("File not found.")
finally:
    print("Execution complete.")

When to Use Exception Handling in Python

Preventing Program Crashes

If your program processes user input, files, or network requests, it must handle errors properly. Without exception handling, a single unexpected input can crash your entire application.

try:
    number = int(input("Enter a number: "))
    print(10 / number)
except ValueError:
    print("You must enter a number.")
except ZeroDivisionError:
    print("You cannot divide by zero.")

Handling Multiple Exceptions

You can catch different errors in a single try block using multiple except statements. This keeps your program running even when unexpected issues occur.

try:
    data = open("file.txt", "r").read()
    result = 10 / int(data)
except FileNotFoundError:
    print("The file is missing.")
except ValueError:
    print("The file contains non-numeric data.")
except ZeroDivisionError:
    print("You cannot divide by zero.")

Logging Errors for Debugging

Instead of displaying errors to users, you can log them for debugging. The logging module lets you record exceptions in a structured way.

import logging

logging.basicConfig(filename="errors.log", level=logging.ERROR)

try:
    with open("config.json", "r") as file:
        config = file.read()
except FileNotFoundError as e:
    logging.error(f"Error: {e}")

Examples of Exception Handling in Python

Catching All Exceptions

If you need to handle every possible error, catch Exception. Be cautious with this approach, as it hides specific errors, making debugging harder.

try:
    result = 10 / 0
except Exception as e:
    print(f"An error occurred: {e}")

Using else in Exception Handling

An else block runs only if no exceptions occur. This keeps your code organized and makes it clear what happens when errors don't occur.

try:
    number = int(input("Enter a number: "))
except ValueError:
    print("Invalid input.")
else:
    print(f"You entered: {number}")

Raising Custom Exceptions

You can create your own exceptions using raise. This helps you enforce rules in your program.

def check_age(age):
    if age < 18:
        raise ValueError("You must be 18 or older.")
    print("Access granted.")

try:
    check_age(16)
except ValueError as e:
    print(e)

Learn More About Exception Handling in Python

Handling Multiple Exceptions in One Block

You can catch multiple exception types in a single except statement by using a tuple.

try:
    num = int("abc")
    result = 10 / num
except (ValueError, ZeroDivisionError):
    print("Invalid operation.")

Using finally for Cleanup

A finally block runs whether an exception occurs or not. Use it for cleanup tasks like closing files or releasing resources.

try:
    file = open("data.txt", "r")
except FileNotFoundError:
    print("File not found.")
finally:
    print("Closing resources.")

Handling Nested Exceptions

If you run code inside loops or functions, you may need nested try-except blocks. This approach helps manage errors at different levels.

try:
    for i in range(3):
        try:
            value = int(input(f"Enter a number for iteration {i}: "))
            print(10 / value)
        except ZeroDivisionError:
            print("You cannot divide by zero.")
except KeyboardInterrupt:
    print("Process interrupted.")

Logging Errors to a File

Logging errors instead of printing them keeps track of issues over time.

import logging

logging.basicConfig(filename="app.log", level=logging.ERROR)

try:
    with open("data.csv", "r") as file:
        content = file.read()
except FileNotFoundError as e:
    logging.error(f"File error: {e}")

Python exception handling helps you prevent crashes, improve debugging, and write reliable applications. Exception handling ensures your program runs smoothly by handling user input errors, file access issues, or unexpected failures.