Before_request hooks in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a before_request hook returns a response?

Consider a Flask app with a before_request hook that returns a response object. What will Flask do next?

Flask

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

@app.before_request
def check():
    return jsonify({'error': 'blocked'})

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

# What will be the output when accessing '/'?

A The 'home' route runs and returns 'Welcome!'

B The before_request response is sent; 'home' route is skipped

C Flask raises an error because before_request returned a response

D The before_request hook runs after the 'home' route

💡 Hint

Think about what happens if a before_request returns a response instead of None.

If a before_request hook returns a response, Flask stops processing further and sends that response immediately. The route handler is skipped.

📝 Syntax

intermediate

Identify the syntax error in this before_request hook

Which option contains a syntax error in defining a Flask before_request hook?

Flask

from flask import Flask
app = Flask(__name__)

@app.before_request
def check():
    print('Checking request')

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

A

@app.before_request
def check():
    print('Checking request')

B

@app.before_request()
def check():
    print('Checking request')

C

@app.before_request
def check:
    print('Checking request')

D

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💡 Hint

Check function definition syntax carefully.

Option C misses parentheses in the function definition (def check:), causing a syntax error.

❓ state_output

advanced

How many times does the before_request hook run on multiple requests?

Given this Flask app, how many times will the before_request hook run after 3 separate HTTP GET requests to '/'?

Flask

from flask import Flask
app = Flask(__name__)
counter = 0

@app.before_request
def increment():
    global counter
    counter += 1

@app.route('/')
def home():
    return f'Count: {counter}'

A 3 times total, once per request

B 1 time total, because the app runs once

C 0 times, before_request hooks don't run automatically

D Depends on the number of routes, so 1 time

💡 Hint

Think about when before_request hooks run in relation to each HTTP request.

The before_request hook runs before every request, so it runs 3 times for 3 requests.

🔧 Debug

advanced

Why does this before_request hook not block unauthorized access?

This Flask app tries to block access if a header is missing, but unauthorized users still get through. Why?

Flask

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

@app.before_request
def check_auth():
    if 'X-Auth' not in request.headers:
        abort(403)

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

A abort() is not imported from flask

B abort(403) is called but the route still runs

C before_request hook is not registered correctly

D abort(403) raises an exception that stops the request

💡 Hint

Consider what abort(403) does in Flask.

Calling abort(403) raises an exception that immediately stops the request and returns a 403 response. The route does not run.

🧠 Conceptual

expert

Order of execution with multiple before_request hooks

Given two before_request hooks in this Flask app, in what order do they run?

Flask

from flask import Flask
app = Flask(__name__)

@app.before_request
def first():
    print('First hook')

@app.before_request
def second():
    print('Second hook')

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

A First hook runs, then second hook

B Only the last defined hook runs

C Hooks run in random order

D Second hook runs, then first hook

💡 Hint

Think about the order decorators are applied in Python.

Flask runs before_request hooks in the order they are defined in the code, so 'first' runs before 'second'.