Custom error pages (404, 500) in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a custom 404 error page in a Flask app?

A custom 404 error page shows a friendly message when a user tries to visit a page that does not exist. It improves user experience by guiding them back to valid pages instead of showing a generic error.

beginner

How do you define a custom error handler for a 500 Internal Server Error in Flask?

Use the @app.errorhandler(500) decorator on a function that returns a response or template. This function runs when a server error happens, letting you show a custom message or page.

beginner

Which Flask function is used to render a custom error page template?

The render_template() function is used to load and show an HTML template for the error page, allowing you to design a nice layout and message.

intermediate

Why is it important to return the correct HTTP status code with a custom error page?

Returning the correct status code (like 404 or 500) helps browsers and search engines understand the error type. It also helps tools and users know the page is an error, not a normal page.

beginner

What happens if you don't define a custom error handler for 404 in Flask?

Flask shows a default plain text 404 error page. It works but looks basic and may confuse users. Custom pages improve clarity and keep your site style consistent.

Which decorator is used to create a custom 404 error page in Flask?

A @app.errorhandler(404)

B @app.route('/404')

C @app.handle_error(404)

D @app.custom_error(404)

The @app.errorhandler(404) decorator tells Flask to use the decorated function when a 404 error occurs.

What should a custom error handler function return?

A A rendered template and the error status code

B Only a string message

C Nothing, just print an error

D Redirect to homepage

The function should return a rendered template (HTML) and the correct HTTP status code to properly show the error page.

What HTTP status code is used for Internal Server Error?

A 404

B 500

C 403

D 200

500 is the standard HTTP status code for Internal Server Error.

If you want to show a custom page for any error, which decorator can you use?

A @app.route('/error')

B @app.errorhandler(404)

C @app.errorhandler(Exception)

D @app.catch_all()

Using @app.errorhandler(Exception) catches all exceptions and can be used to show a generic error page.

Why should custom error pages be user-friendly?

A To confuse users

B To hide errors completely

C To slow down the website

D To help users understand what went wrong and guide them

User-friendly error pages help users know what happened and how to continue, improving their experience.

Explain how to create a custom 404 error page in Flask and why it is useful.

Think about how Flask handles errors and how you can show a nicer page.

Describe the difference between default Flask error pages and custom error pages.

Consider what users see and how it affects their experience.

Practice

1. What is the main purpose of creating custom error pages like 404 or 500 in a Flask app?

easy

A. To show friendly messages instead of default error text

B. To speed up the server response time

C. To disable error logging

D. To automatically fix bugs in the code

Solution

1. Step 1: Understand default error behavior

   By default, Flask shows plain error messages like '404 Not Found' which are not user-friendly.

2. Step 2: Purpose of custom error pages

   Custom error pages replace these default messages with nicer, clearer pages that help users understand the problem.

3. Final Answer:

   To show friendly messages instead of default error text -> Option A

4. Quick Check:

   Custom error pages = Friendly messages [OK]

Hint: Custom error pages improve user experience with friendly messages [OK]

Common Mistakes:

• Thinking custom pages fix server speed
• Believing they disable error logs
• Assuming they auto-correct bugs

2. Which of the following is the correct way to define a custom 404 error handler in Flask?

easy

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

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

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

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

Solution

1. Step 1: Identify correct decorator for error handling

   Flask uses @app.errorhandler(error_code) to register error handlers.

2. Step 2: Check function signature and return

   The handler function takes an error argument and returns a response with the error code.

3. Final Answer:

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

4. Quick Check:

   @app.errorhandler(404) = Correct syntax [OK]

Hint: Use @app.errorhandler(code) to define error handlers [OK]

Common Mistakes:

• Using @app.route instead of @app.errorhandler
• Wrong decorator names like @app.handle_error
• Missing error parameter in handler function

3. Given this Flask code snippet, what will be the output when a user visits a non-existing page?

@app.errorhandler(404)
def page_not_found(e):
    return '
Oops! Page missing.
', 404

@app.route('/')
def home():
    return 'Welcome Home'

medium

A. The browser shows 'Welcome Home'

B. The server crashes with an error

C. The browser shows default Flask 404 page

D. The browser shows 'Oops! Page missing.' with 404 status

Solution

1. Step 1: Understand route and error handler

   The '/' route returns 'Welcome Home'. The 404 error handler returns a custom HTML message with status 404.

2. Step 2: Accessing a non-existing page triggers 404

   Visiting a page not defined triggers the 404 handler, so the custom message is returned with status 404.

3. Final Answer:

   The browser shows '<h1>Oops! Page missing.</h1>' with 404 status -> Option D

4. Quick Check:

   404 error triggers custom handler = The browser shows '<h1>Oops! Page missing.</h1>' with 404 status [OK]

Hint: Non-existing pages trigger 404 handler, not normal routes [OK]

Common Mistakes:

• Thinking '/' route handles all pages
• Expecting default Flask 404 page instead of custom
• Assuming server crashes on 404

4. Identify the error in this Flask code for a custom 500 error page:

@app.errorhandler(500)
def internal_error():
    return 'Server error occurred', 500

medium

A. The handler function must accept an error parameter

B. The decorator should be @app.handle_error(500)

C. The return statement must not include the status code

D. The function name must be 'handle_500_error'

Solution

1. Step 1: Check function signature for error handlers

   Flask error handlers must accept one argument for the error object.

2. Step 2: Verify the given function signature

   The function internal_error() has no parameters, which is incorrect for error handlers.

3. Final Answer:

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

4. Quick Check:

   Error handler functions need error argument [OK]

Hint: Error handlers always take one error argument [OK]

Common Mistakes:

• Omitting the error parameter in handler function
• Using wrong decorator names
• Thinking function name matters for error handlers

5. You want to create a custom 404 error page that renders an HTML template named '404.html' and returns status 404. Which Flask code snippet correctly does this?

hard

A. @app.route('/404')
def not_found():
return render_template('404.html'), 404

B. @app.errorhandler(404)
def not_found(error):
return render_template('404.html'), 404

C. @app.errorhandler(404)
def not_found():
return render_template('404.html')

D. @app.errorhandler(404)
def not_found(error):
return render_template('404.html', status=404)

Solution

1. Step 1: Use correct decorator and function signature

   Use @app.errorhandler(404) and define function with error parameter.

2. Step 2: Return rendered template with status code

   Return render_template('404.html') and the status code 404 as a tuple.

3. Final Answer:

   @app.errorhandler(404)\ndef not_found(error):\n return render_template('404.html'), 404 -> Option B

4. Quick Check:

   Custom 404 with template and status code = @app.errorhandler(404) def not_found(error): return render_template('404.html'), 404 [OK]

Hint: Return (render_template, status_code) tuple in error handler [OK]

Common Mistakes:

• Using @app.route instead of @app.errorhandler
• Missing error parameter in function
• Not returning status code with template