Exception handling in routes in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a route raises a handled exception?

Consider this Flask route that raises a ValueError which is caught by a custom error handler. What will the client receive as a response?

Flask

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

@app.route('/test')
def test_route():
    raise ValueError('Invalid value')

@app.errorhandler(ValueError)
def handle_value_error(e):
    return jsonify({'error': str(e)}), 400

# Assume app.run() is called elsewhere

A Default Flask 500 Internal Server Error page

B Empty response with status code 200

C {"error": "Invalid value"} with status code 400

D Flask debug traceback page

💡 Hint

Think about what happens when an exception is raised and a matching error handler exists.

The ValueError raised in the route is caught by the @app.errorhandler(ValueError) decorated function. This handler returns a JSON response with the error message and a 400 status code. So the client receives the JSON error message with status 400.

📝 Syntax

intermediate

Which option correctly registers an error handler for HTTP 404 errors?

In Flask, you want to create a custom response for 404 Not Found errors. Which code snippet correctly registers this handler?

A

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

B

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

C

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

D

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

💡 Hint

Check the type of argument @app.errorhandler expects for HTTP status codes.

The @app.errorhandler decorator accepts an integer HTTP status code like 404 to register a handler for that error. Option B uses 404 as an integer, which is correct. Option B uses a string '404' which is invalid. Option B registers a route, not an error handler. Option B uses a generic exception class, which is not specific to 404.

🔧 Debug

advanced

Why does this Flask app catch the custom exception in the route?

Given this Flask app code, the custom exception MyError raised in the route is caught by the error handler. Why?

Flask

from flask import Flask
app = Flask(__name__)

class MyError(Exception):
    pass

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

@app.errorhandler(Exception)
def handle_exception(e):
    return 'Handled error', 500

# app.run() elsewhere

A The error handler catches all exceptions, so it should catch MyError

B The custom exception is not derived from Exception, so it is not caught

C The error handler is registered after the route, so it is ignored

D Flask only catches HTTPException subclasses by default, so custom exceptions need explicit handling

💡 Hint

Consider Python inheritance and Flask's error handling behavior.

Since MyError inherits from Exception, and the error handler is registered for Exception, it will catch MyError. The order of registration does not affect catching. Flask catches all exceptions raised in routes and passes them to matching handlers. So option A is correct.

❓ state_output

advanced

What is the response status code when a route raises an unhandled exception?

In this Flask app, the route raises a KeyError which has no registered error handler. What status code does the client receive?

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/key')
def key_route():
    raise KeyError('missing key')

# No error handler for KeyError
# app.run() elsewhere

A 500 Internal Server Error

B 404 Not Found

C 200 OK with empty body

D 400 Bad Request

💡 Hint

Think about Flask's default behavior when exceptions are not handled.

When a route raises an exception that is not caught by any error handler, Flask returns a 500 Internal Server Error response by default. So the client receives status code 500.

🧠 Conceptual

expert

How does Flask differentiate between HTTPException and other exceptions in error handling?

Flask has special handling for exceptions derived from werkzeug.exceptions.HTTPException. Which statement best describes this behavior?

A Flask converts all exceptions to HTTPException before handling

B HTTPException subclasses are ignored by Flask and always cause 500 errors

C Only exceptions not derived from HTTPException trigger error handlers

D HTTPException subclasses automatically generate HTTP responses with their code and description unless overridden

💡 Hint

Recall how Flask uses HTTPException to create HTTP error responses.

Flask treats exceptions derived from HTTPException specially by automatically creating HTTP responses using their status code and description. This means if you raise a 404 Not Found exception, Flask sends a 404 response unless you override it with a custom handler. Other exceptions default to 500 errors unless handled.