Error handling in production in Flask - Mini Project: Build & Apply

Error handling in production

📖 Scenario: You are building a simple Flask web application that shows a welcome message. You want to make sure that if a user visits a page that does not exist, the app shows a friendly error page instead of a default error message.

🎯 Goal: Create a Flask app with a route for the home page. Then add error handling for 404 errors to show a custom message. This helps users understand when they visit a wrong page.

📋 What You'll Learn

Create a Flask app instance named app

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

Create 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 applications need to handle errors gracefully to improve user experience and avoid showing confusing default error pages.

💼 Career

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

1

Set up the Flask app and home route

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

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    # Your code here

Need a hint?

Remember to import Flask and create the app instance before adding routes.

2

Add error handler for 404 errors

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

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):
    # Your code here

Need a hint?

Use the @app.errorhandler(404) decorator to catch 404 errors.

3

Return custom message and status code in 404 handler

In the page_not_found function, return the text '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):
    # Your code here

Need a hint?

Return a tuple with the message and the status code 404.

4

Add the app run block for production

Add the block if __name__ == '__main__': and inside it call app.run() to start the Flask app.

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

# Your code here

Need a hint?

This block makes sure the app runs when you start the script directly.