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

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

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

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

Hint

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

Practice

1. What is the main purpose of using @app.errorhandler in a Flask route?

easy

A. To connect to a database automatically

B. To define a new route in the Flask app

C. To catch specific errors and return custom responses

D. To start the Flask application server

Solution

1. Step 1: Understand the role of @app.errorhandler

   This decorator is used to catch exceptions or errors that happen during request handling.

2. Step 2: Purpose in routes

   It allows you to return a custom message or page instead of a default error, improving user experience.

3. Final Answer:

   To catch specific errors and return custom responses -> Option C

4. Quick Check:

   Error handling = catch and respond [OK]

Hint: Errorhandler decorates functions to catch errors in routes [OK]

Common Mistakes:

• Confusing errorhandler with route decorator
• Thinking it starts the server
• Assuming it connects to databases

2. Which of the following is the correct syntax to handle a 404 error in Flask?

easy

A. @app.errorhandler(404)
def not_found(error):
return 'Page not found', 404

B. @app.route(404)
def not_found():
return 'Page not found'

C. @app.errorhandler('404')
def not_found(error):
return 'Page not found', 404

D. @app.errorhandler(404)
def not_found():
return 'Page not found'

Solution

1. Step 1: Check the decorator usage

   The correct decorator is @app.errorhandler(404) with the status code as an integer.

2. Step 2: Verify function signature and return

   The handler function should accept an error argument and return a response with the status code.

3. Final Answer:

   @app.errorhandler(404)\ndef not_found(error):\n return 'Page not found', 404 -> Option A

4. Quick Check:

   Correct decorator and function signature = A [OK]

Hint: Use integer status code and error parameter in handler [OK]

Common Mistakes:

• Using @app.route instead of @app.errorhandler
• Passing status code as string '404'
• Missing error parameter in handler function

3. Given this Flask code snippet, what will be the output when accessing /divide/10/0?

from flask import Flask
app = Flask(__name__)

@app.route('/divide/<int:a>/<int:b>')
def divide(a, b):
    try:
        result = a / b
        return str(result)
    except ZeroDivisionError:
        return 'Cannot divide by zero', 400

medium

A. ZeroDivisionError exception traceback

B. 0

C. 10

D. Cannot divide by zero with HTTP status 400

Solution

1. Step 1: Analyze the route and input

   The route divides a by b. Input is 10 and 0, so division by zero occurs.

2. Step 2: Check exception handling

   The ZeroDivisionError is caught, returning 'Cannot divide by zero' with status 400.

3. Final Answer:

   Cannot divide by zero with HTTP status 400 -> Option D

4. Quick Check:

   ZeroDivisionError caught returns message and 400 [OK]

Hint: Try-except catches zero division, returns error message [OK]

Common Mistakes:

• Expecting a crash or traceback
• Ignoring the exception handling block
• Assuming division returns 0

4. Identify the error in this Flask route error handling code:

@app.errorhandler(500)
def internal_error():
    return 'Internal Server Error', 500

medium

A. The handler function must accept an error parameter

B. The status code 500 is invalid for errorhandler

C. The return statement should not include status code

D. The decorator should be @app.route, not @app.errorhandler

Solution

1. Step 1: Check function signature for errorhandler

   Functions decorated with @app.errorhandler must accept one parameter for the error.

2. Step 2: Verify other parts

   Status code 500 is valid, return with status code is correct, and decorator usage is correct.

3. Final Answer:

   The handler function must accept an error parameter -> Option A

4. Quick Check:

   Errorhandler function needs error argument [OK]

Hint: Errorhandler functions always take an error argument [OK]

Common Mistakes:

• Omitting the error parameter in handler function
• Using wrong decorator for error handling
• Returning without status code

5. You want to handle both 404 and 500 errors in your Flask app with custom JSON responses. Which approach correctly implements this?

hard

A. Use one function with @app.errorhandler([404, 500]) to handle both errors

B. Define two separate functions with @app.errorhandler(404) and @app.errorhandler(500), returning JSON and status codes

C. Handle errors inside each route function with try-except blocks only

D. Set app.config['ERROR_404_JSON'] = True and Flask will auto-handle JSON errors

Solution

1. Step 1: Understand Flask errorhandler usage

   Flask requires separate functions for different error codes with their own decorators.

2. Step 2: Confirm JSON response and status code

   Each handler should return a JSON response and the correct HTTP status code for clarity.

3. Final Answer:

   Define two separate functions with @app.errorhandler(404) and @app.errorhandler(500), returning JSON and status codes -> Option B

4. Quick Check:

   Separate handlers per error code = A [OK]

Hint: Use separate errorhandler functions for each error code [OK]

Common Mistakes:

• Trying to handle multiple errors in one function decorator
• Relying only on try-except inside routes
• Assuming Flask auto-converts errors to JSON