Custom error pages (404, 500) in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a user visits a non-existent route?

Given this Flask app with a custom 404 error handler, what will the user see if they visit /unknown?

Flask

from flask import Flask, render_template_string
app = Flask(__name__)

@app.errorhandler(404)
def page_not_found(e):
    return render_template_string('<h1>Page Not Found</h1><p>Sorry, no page here.</p>'), 404

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

if __name__ == '__main__':
    app.run()

A The browser shows 'Welcome Home!' message.

B The browser shows a default Flask error page with a traceback.

C The browser shows a 404 page with 'Page Not Found' and 'Sorry, no page here.' message.

D The browser shows a 500 Internal Server Error page.

💡 Hint

Think about what Flask does when a route is not found and a custom 404 handler is defined.

When a user visits a route that does not exist, Flask triggers the 404 error handler. Since a custom handler is defined, Flask returns the custom HTML with status 404.

📝 Syntax

intermediate

Identify the syntax error in this custom 500 error handler

Which option correctly fixes the syntax error in this Flask 500 error handler?

Flask

from flask import Flask, render_template_string
app = Flask(__name__)

@app.errorhandler(500)
def internal_error(e):
    return render_template_string('<h1>Server Error</h1><p>Something went wrong.</p>') 500

if __name__ == '__main__':
    app.run()

A Remove 500 entirely: return render_template_string(...)

B Add a comma before 500: return render_template_string(...) , 500

C Replace 500 with status=500: return render_template_string(..., status=500)

D Put 500 inside parentheses: return (render_template_string(...)) 500

💡 Hint

Remember how to return a tuple with response and status code in Flask.

Flask expects a tuple (response, status_code). The status code must be separated by a comma, not just placed after the return value.

❓ state_output

advanced

What is the HTTP status code returned by this custom error handler?

Consider this Flask app snippet. What HTTP status code will the client receive when a 404 error occurs?

Flask

from flask import Flask, render_template_string
app = Flask(__name__)

@app.errorhandler(404)
def not_found(e):
    return render_template_string('<h1>Oops!</h1>'), 200

if __name__ == '__main__':
    app.run()

A 200

B 500

C 404

D None (default Flask 404 page)

💡 Hint

Check the status code explicitly returned by the handler.

The handler returns a 200 status code explicitly, so the client receives 200 OK even though the route was not found.

🔧 Debug

advanced

Why does this custom 404 handler not work as expected?

Review this Flask app code. Why does the custom 404 page never show?

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/hello')
def hello():
    return 'Hello!'

@app.errorhandler(404)
def page_not_found():
    return '<h1>Not Found</h1>', 404

if __name__ == '__main__':
    app.run()

A The error handler function is missing the required error parameter.

B The route '/hello' conflicts with the 404 handler.

C The return statement in the error handler is missing render_template.

D Flask does not support custom 404 handlers without blueprints.

💡 Hint

Check the function signature of error handlers in Flask.

Flask error handlers must accept the error as a parameter. Missing it causes the handler not to be called properly.

🧠 Conceptual

expert

Which statement about Flask custom error pages is true?

Choose the correct statement about how Flask handles custom error pages for 404 and 500 errors.

A Flask automatically uses templates named 404.html and 500.html without defining error handlers.

B Flask requires restarting the server to activate custom error handlers after defining them.

C Custom error handlers can only be defined inside blueprints, not the main app.

D Custom error handlers must return a response and the correct HTTP status code to override default error pages.

💡 Hint

Think about what Flask expects from error handlers to replace default pages.

To override default error pages, Flask custom handlers must return a response and the matching HTTP status code. Otherwise, default pages show.