Before_request hooks in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to register a function that runs before each request in Flask.

Flask

from flask import Flask
app = Flask(__name__)

@app.[1]
def before():
    print("This runs before each request")

Drag options to blanks, or click blank then click option'

Abefore_request

Broute

Cteardown_request

Dafter_request

Attempts:

3 left

💡 Hint

Common Mistakes

Using @app.route instead of @app.before_request

Using @app.after_request which runs after requests

The @app.before_request decorator registers a function to run before each request.

2fill in blank

medium

Complete the code to abort the request with a 403 error inside a before_request function.

Flask

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

@app.before_request
def check():
    if not authorized():
        [1](403)

Drag options to blanks, or click blank then click option'

Aredirect

Babort

Craise

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using return 403 which does not stop the request properly

Using raise without an exception

The abort(403) function stops the request and returns a 403 Forbidden error.

3fill in blank

hard

Fix the error in the before_request function to correctly access the request path.

Flask

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

@app.before_request
def log_path():
    print("Request path:", request.[1])

Drag options to blanks, or click blank then click option'

Apath

Burl

Cfull_path

Dendpoint

Attempts:

3 left

💡 Hint

Common Mistakes

Using request.url which includes the full URL

Using request.endpoint which is the function name

The request.path attribute gives the URL path of the current request.

4fill in blank

hard

Fill both blanks to create a before_request function that sets a global variable and checks a header.

Flask

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

@app.before_request
def before():
    g.user = request.headers.get([1])
    if g.user is None:
        [2](401)

Drag options to blanks, or click blank then click option'

A"X-User"

Babort

C"Authorization"

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using return instead of abort to stop the request

Using wrong header names

The header X-User is checked, and abort(401) stops unauthorized requests.

5fill in blank

hard

Fill all three blanks to create a before_request hook that logs the method, path, and aborts if method is POST.

Flask

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

@app.before_request
def check_post():
    print("Method:", request.[1])
    print("Path:", request.[2])
    if request.method == [3]:
        abort(405)

Drag options to blanks, or click blank then click option'

Amethod

Bpath

C"POST"

D"GET"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'GET' instead of 'POST' to check method

Using request.url instead of request.path

This code prints the HTTP method and path, then aborts with 405 if the method is POST.

Practice

1. What is the main purpose of a before_request hook in Flask?

easy

A. To run code before every request to prepare or check something

B. To define the main route of the application

C. To handle errors after a request is processed

D. To send the final response to the client

Solution

1. Step 1: Understand the role of before_request

   The before_request hook runs before each request to prepare data or check conditions.

2. Step 2: Compare with other options

   Defining routes, handling errors, or sending responses happen elsewhere, not in before_request.

3. Final Answer:

   To run code before every request to prepare or check something -> Option A

4. Quick Check:

   before_request runs before request = C [OK]

Hint: before_request runs code before each request [OK]

Common Mistakes:

• Confusing before_request with route handlers
• Thinking it handles errors or sends responses
• Assuming it runs after the request

2. Which of the following is the correct way to register a before_request hook in Flask that does not short-circuit the request?

easy

A. @app.before_request\nclass BeforeHook: pass

B. @app.before_request()\ndef before(): pass

C. @app.before_request\ndef before(): return 'Hello'

D. @app.before_request\ndef before(): pass

Solution

1. Step 1: Recall the decorator syntax

   The before_request decorator is used without parentheses: @app.before_request.

2. Step 2: Check function definition

   The hook must be a function, not a class, and must not return a response to continue to the route.

3. Final Answer:

   @app.before_request\ndef before(): pass -> Option D

4. Quick Check:

   Decorator without () and function = D [OK]

Hint: Use @app.before_request without parentheses [OK]

Common Mistakes:

• Adding parentheses after @app.before_request
• Defining a class instead of a function
• Returning a value when not needed

3. Given this Flask code, what will be the output when accessing /?

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

@app.before_request
def check():
    return redirect('/login')

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

@app.route('/login')
def login():
    return 'Login Page'

medium

A. Home Page

B. Login Page

C. Redirect loop error

D. 404 Not Found

Solution

1. Step 1: Analyze the before_request hook

   The check function unconditionally returns a redirect to /login for every request.

2. Step 2: Determine the final response

   Request to / redirects to /login, but /login also triggers before_request which redirects again, causing an infinite loop.

3. Final Answer:

   Redirect loop error -> Option C

4. Quick Check:

   Always redirect to /login = C [OK]

Hint: Unconditional return in before_request causes loop [OK]

Common Mistakes:

• Expecting 'Login Page' without considering loop
• Thinking before_request does not run on /login
• Ignoring that every request triggers the hook

4. What is wrong with this Flask code snippet?

from flask import Flask
app = Flask(__name__)

@app.before_request
def before():
    print('Before request')
    return None

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

medium

A. before_request must return a response always

B. Nothing is wrong; code works fine

C. Printing inside before_request is not allowed

D. Returning None in before_request causes an error

Solution

1. Step 1: Understand return behavior in before_request

   Returning None means no response is sent, so Flask continues to route handler.

2. Step 2: Check if printing is allowed

   Printing is allowed and useful for debugging; it does not cause errors.

3. Final Answer:

   Nothing is wrong; code works fine -> Option B

4. Quick Check:

   None return means continue normal route = A [OK]

Hint: Return None or nothing to continue request normally [OK]

Common Mistakes:

• Thinking return None causes error
• Believing before_request must always return response
• Assuming print statements break Flask hooks

5. You want to use a before_request hook to block access to all routes except /login unless a user is logged in. Which code snippet correctly implements this?

hard

A. from flask import request, redirect, session
@app.before_request
def block():
if request.path != '/login' and not session.get('user'):
return redirect('/login')

B. from flask import request, redirect
@app.before_request
def block():
if request.path == '/login':
return redirect('/login')

C. from flask import session
@app.before_request
def block():
if not session.get('user'):
return 'Access Denied'

D. from flask import request
@app.before_request
def block():
if request.path == '/login' or session.get('user'):
return redirect('/login')

Solution

1. Step 1: Identify the condition to block access

   We want to redirect to /login if the path is not /login and user is not logged in.

2. Step 2: Check each option logic

   from flask import request, redirect, session @app.before_request def block(): if request.path != '/login' and not session.get('user'): return redirect('/login') correctly checks path and session, redirecting only when needed. Others either redirect wrongly or deny access without redirect.

3. Final Answer:

   from flask import request, redirect, session @app.before_request def block(): if request.path != '/login' and not session.get('user'): return redirect('/login') -> Option A

4. Quick Check:

   Check path and session, redirect if not logged in = A [OK]

Hint: Check path and session, redirect if not logged in [OK]

Common Mistakes:

• Redirecting /login to itself causing loop
• Not checking path before redirect
• Returning string instead of redirect response