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

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

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

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

Hint

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

Practice

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

easy

A. To catch specific errors and provide custom responses

B. To speed up the application startup

C. To automatically restart the server on errors

D. To log user activities

Solution

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

   This decorator is used to catch specific HTTP or application errors in Flask.

2. Step 2: Recognize its use for custom responses

   It allows developers to return user-friendly error pages or messages instead of default error traces.

3. Final Answer:

   To catch specific errors and provide custom responses -> Option A

4. Quick Check:

   Error handling = custom responses [OK]

Hint: Errorhandler decorates functions to catch and respond to errors [OK]

Common Mistakes:

• Thinking it restarts the server automatically
• Confusing errorhandler with logging functions
• Assuming it speeds up app startup

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

easy

A. @app.error(404)
def not_found(error):
return 'Not found', 404

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

C. @app.handle_error(404)
def not_found():
return 'Page missing', 404

D. @app.catch_error(404)
def not_found(error):
return '404 error', 404

Solution

1. Step 1: Identify the correct decorator name

   The correct Flask decorator for error handling is @app.errorhandler.

2. Step 2: Confirm function signature and return

   The function should accept an error parameter and return a response with the error code.

3. Final Answer:

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

4. Quick Check:

   Correct decorator and function signature = @app.errorhandler(404) def not_found(error): return 'Page not found', 404 [OK]

Hint: Use @app.errorhandler with error code and error param [OK]

Common Mistakes:

• Using wrong decorator names like @app.handle_error
• Omitting the error parameter in the function
• Returning without the status code

3. Given this Flask code snippet, what will the user see if a 500 error occurs?

@app.errorhandler(500)
def internal_error(error):
    return 'Something went wrong, please try later.', 500

medium

A. The message: 'Something went wrong, please try later.' with status 500

B. A detailed Python traceback error page

C. A blank page with status code 500

D. The default Flask 404 error page

Solution

1. Step 1: Understand the errorhandler function

   The function returns a custom message and status code 500 when a 500 error occurs.

2. Step 2: Identify the user-visible output

   The user will see the returned string message with HTTP status 500, not the default traceback.

3. Final Answer:

   The message: 'Something went wrong, please try later.' with status 500 -> Option A

4. Quick Check:

   Custom error message shown = The message: 'Something went wrong, please try later.' with status 500 [OK]

Hint: Custom errorhandler returns your message, not default trace [OK]

Common Mistakes:

• Expecting Flask's default error page instead of custom message
• Thinking a blank page will appear
• Confusing 500 error with 404 error page

4. What is wrong with this Flask error handler code?

@app.errorhandler(403)
def forbidden():
    return 'Access denied', 403

medium

A. The return statement must include a JSON response

B. The decorator should be @app.handle_error

C. The function must accept an error parameter

D. The status code 403 is invalid

Solution

1. Step 1: Check function signature for errorhandler

   Flask error handler functions must accept the error argument to receive error details.

2. Step 2: Identify missing error parameter

   The function forbidden lacks the required error parameter.

3. Final Answer:

   The function must accept an error parameter -> Option C

4. Quick Check:

   Error handler function needs error param = The function must accept an error parameter [OK]

Hint: Error handler functions always take an error argument [OK]

Common Mistakes:

• Omitting the error parameter in handler functions
• Using wrong decorator names
• Assuming JSON response is mandatory

5. You want to show a friendly error page for all unhandled exceptions in production without exposing debug info. Which Flask setup is best?

hard

A. Use @app.route('/error') to manually show errors

B. Set app.debug = True to see detailed errors

C. Remove all error handlers to let Flask show default error pages

D. Use @app.errorhandler(Exception) to catch all errors and return a simple message

Solution

1. Step 1: Understand catching all exceptions

   Using @app.errorhandler(Exception) catches all unhandled errors globally.

2. Step 2: Provide user-friendly message without debug info

   Returning a simple message hides technical details, improving user experience and security.

3. Final Answer:

   Use @app.errorhandler(Exception) to catch all errors and return a simple message -> Option D

4. Quick Check:

   Global error handler with simple message = Use @app.errorhandler(Exception) to catch all errors and return a simple message [OK]

Hint: Catch Exception globally to hide debug info in production [OK]

Common Mistakes:

• Enabling debug mode in production
• Relying on default Flask error pages
• Using routes instead of error handlers for errors