Error handler decorators in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a 404 error occurs?

Given this Flask app snippet, what response will the client receive when accessing a non-existent route?

Flask

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

@app.errorhandler(404)
def not_found(error):
    return jsonify({'error': 'Not found'}), 404

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

# Client requests /missing

A A plain text response 'Not found' with status code 200

B A JSON response with {'error': 'Not found'} and status code 404

C A redirect to '/' route

D A server error 500 response

💡 Hint

Look at the @app.errorhandler decorator and what it returns.

The @app.errorhandler(404) decorator catches 404 errors and returns a JSON response with the error message and status code 404.

📝 Syntax

intermediate

Which option correctly defines a 500 error handler?

Choose the correct Flask error handler syntax for HTTP 500 errors.

A

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

B

@app.errorhandler(500):
def server_error(e):
    return 'Server error', 500

C

@app.errorhandler(500)
def server_error(e):
    return 'Server error', 500

D

@app.errorhandler(500)
server_error = lambda e: ('Server error', 500)

💡 Hint

Check the decorator syntax and function parameters.

The correct syntax uses @app.errorhandler(500) without colon and the handler function must accept the error argument.

🔧 Debug

advanced

Why does this 403 error handler not work as expected?

Identify the issue in this Flask error handler code that prevents it from catching 403 errors.

Flask

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

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

@app.route('/secret')
def secret():
    abort(403)

A The route '/secret' must be decorated with @app.errorhandler(403) instead.

B The abort(403) call is missing import and causes a NameError.

C The errorhandler decorator should be @app.errorhandler('403') with quotes.

D The handler function must accept an error parameter but 'forbidden' does not.

💡 Hint

Check the function signature of the error handler.

Error handler functions must accept the error argument to work properly. Missing it causes Flask to ignore the handler.

❓ state_output

advanced

What is the response status code after raising a custom exception handled by an error handler?

Consider this Flask app code. What status code will the client receive when '/fail' is accessed?

Flask

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

class CustomError(Exception):
    pass

@app.errorhandler(CustomError)
def handle_custom_error(e):
    return jsonify({'message': 'Custom error occurred'}), 418

@app.route('/fail')
def fail():
    raise CustomError()

A 418

B 500

C 200

D 404

💡 Hint

Look at the status code returned by the error handler.

The error handler returns status code 418, so the client receives that code when the exception is raised.

🧠 Conceptual

expert

Which statement about Flask error handler decorators is true?

Select the correct statement about how Flask error handler decorators work.

A Error handlers can catch both HTTP status codes and custom exception classes.

B Error handlers must always return plain text responses, JSON is not supported.

C Multiple error handlers for the same error code can be defined and all will run.

D Error handlers only work for errors raised inside route functions, not middleware.

💡 Hint

Think about what types of errors Flask errorhandler can catch.

Flask error handlers can catch both HTTP status codes like 404 and custom exception classes.