Before_request hooks in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a before_request hook in Flask?

A before_request hook is a function that runs before each request is processed. It lets you run code like checking user login or setting up data before the main view runs.

beginner

How do you register a before_request function in Flask?

You use the @app.before_request decorator above a function. Flask calls this function before every request automatically.

intermediate

Can a before_request function stop a request from continuing?

Yes. If the before_request function returns a response (like a redirect or error), Flask stops and sends that response immediately, skipping the view function.

beginner

Why use before_request hooks instead of putting code inside each view?

They help avoid repeating code in every view. For example, checking if a user is logged in or setting up common data can be done once in a before_request hook.

intermediate

What happens if multiple before_request functions are registered?

Flask runs all before_request functions in the order they were registered. If any returns a response, the rest are skipped.

What decorator do you use to create a before_request hook in Flask?

A @app.after_request

B @app.teardown_request

C @app.route

D @app.before_request

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

What happens if a before_request function returns a response?

A The response is sent immediately, skipping the view function.

B The response is ignored and the view runs anyway.

C Flask raises an error.

D The response is logged but the view runs.

Returning a response in before_request stops further processing and sends that response.

Which of these is a common use for before_request hooks?

A Serving static files

B Rendering HTML templates

C Checking if a user is logged in

D Handling form submissions

before_request hooks are often used to check user login status before views run.

If you register multiple before_request functions, in what order do they run?

A In reverse order

B In the order they were registered

C Random order

D Alphabetical order by function name

Flask runs before_request functions in the order they were added.

Can before_request hooks access the request data?

A Yes, they can access request data like headers and form inputs

B No, request data is not available yet

C Only URL parameters are accessible

D Only after the view runs

before_request hooks run after Flask receives the request, so they can access all request data.

Explain what a before_request hook is and how it affects the request flow in Flask.

Think about what happens before your page loads.

Describe a practical example where using a before_request hook improves your Flask app.

Imagine you want to protect many pages from unauthorized access.

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