Exception handling in routes in Flask - Mini Project: Build & Apply

Exception handling in routes

📖 Scenario: You are building a simple Flask web app that returns user information based on user ID. Sometimes the user ID might not exist, so you need to handle errors gracefully.

🎯 Goal: Create a Flask app with a route /user/<int:user_id> that returns user data. Add exception handling to return a friendly error message if the user ID is not found.

📋 What You'll Learn

Create a dictionary called users with exact entries: 1: 'Alice', 2: 'Bob', 3: 'Charlie'

Create a Flask app instance called app

Create a route /user/<int:user_id> that returns the user name for the given user_id

Add exception handling using try and except KeyError to catch missing user IDs

Return a JSON response with {'error': 'User not found'} and status code 404 when user ID is missing

💡 Why This Matters

🌍 Real World

Web apps often need to handle errors gracefully when users request data that might not exist. This project shows how to do that in Flask.

💼 Career

Knowing how to handle exceptions in web routes is essential for backend developers to build reliable and user-friendly APIs.

1

Create user data dictionary

Create a dictionary called users with these exact entries: 1: 'Alice', 2: 'Bob', and 3: 'Charlie'.

Flask

# Create the users dictionary with IDs and names
# Your code here

Need a hint?

Use curly braces {} to create the dictionary with keys as integers and values as strings.

2

Create Flask app instance

Import Flask from flask and create a Flask app instance called app.

Flask

users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

# Import Flask and create app instance
# Your code here

Need a hint?

Use Flask(__name__) to create the app instance.

3

Create user route with exception handling

Create a route /user/<int:user_id> using @app.route. Inside the route function get_user, use try and except KeyError to return the user name from users[user_id]. If the user ID is missing, return a JSON response {'error': 'User not found'} with status code 404. Import jsonify from flask to create JSON responses.

Flask

from flask import Flask

users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

app = Flask(__name__)

# Import jsonify
# Create route /user/
# Use try-except to handle missing user_id
# Your code here

Need a hint?

Use @app.route decorator and a function named get_user with parameter user_id. Use jsonify to return JSON responses.

4

Add app run block

Add the standard Flask app run block: if __name__ == '__main__': and inside it call app.run(debug=True) to start the server in debug mode.

Flask

from flask import Flask, jsonify

users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

app = Flask(__name__)

@app.route('/user/')
def get_user(user_id):
    try:
        return jsonify({'user': users[user_id]})
    except KeyError:
        return jsonify({'error': 'User not found'}), 404

# Add app run block below
# Your code here

Need a hint?

This block runs the Flask app only if the script is executed directly.