Why error handling matters in Flask - See It in Action

Why error handling matters

📖 Scenario: You are building a simple web app using Flask. You want to make sure your app handles errors gracefully so users see friendly messages instead of confusing crash pages.

🎯 Goal: Create a Flask app that defines a route and adds error handling for 404 Not Found errors with a custom message.

📋 What You'll Learn

Create a Flask app instance named app

Define a route / that returns the text 'Welcome to the homepage!'

Add an error handler for 404 errors using @app.errorhandler(404)

The 404 error handler should return the text 'Sorry, page not found.' and the status code 404

💡 Why This Matters

🌍 Real World

Web apps often face user requests for pages that do not exist. Handling these errors gracefully improves user trust and experience.

💼 Career

Knowing how to add error handling in Flask is essential for backend web developers to build robust and user-friendly applications.

1

Create the Flask app and homepage route

Import Flask from flask. Create a Flask app instance called app. Define a route for '/' that returns the string 'Welcome to the homepage!'.

Flask

from flask import Flask

# Create the Flask app instance
# Define the homepage route
# Your code here

Need a hint?

Remember to import Flask and create the app with Flask(__name__). Use @app.route('/') to define the homepage.

2

Add an error handler for 404 errors

Add an error handler for 404 errors using the decorator @app.errorhandler(404). Define a function called page_not_found that takes a parameter error.

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Welcome to the homepage!'

# Add the 404 error handler decorator and function
# Your code here

Need a hint?

Use @app.errorhandler(404) above a function named page_not_found that accepts error as a parameter.

3

Return a friendly message and status code in the error handler

Inside the page_not_found function, return the string 'Sorry, page not found.' and the status code 404 as a tuple.

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Welcome to the homepage!'

@app.errorhandler(404)
def page_not_found(error):
    # Return friendly message and 404 status code
    # Your code here

Need a hint?

Return a tuple with the message string and the number 404 to set the HTTP status code.

4

Add the app run command for local testing

Add the code to run the Flask app locally by checking if __name__ == '__main__' and calling app.run() inside that block.

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Welcome to the homepage!'

@app.errorhandler(404)
def page_not_found(error):
    return 'Sorry, page not found.', 404

# Add the app run command below
# Your code here

Need a hint?

Use the standard Python check if __name__ == '__main__' to run app.run() for local testing.