Before_request hooks in Flask - Mini Project: Build & Apply

Using Before_request Hooks in Flask

📖 Scenario: You are building a simple Flask web app that needs to check if a user is logged in before allowing access to certain pages.To do this, you will use a before_request hook to run a function before every request. This function will check a variable to decide if the user is logged in.

🎯 Goal: Create a Flask app that uses a before_request hook to check a login status before serving pages.If the user is not logged in, the app should redirect them to a login page.

📋 What You'll Learn

Create a Flask app instance

Define a global variable logged_in set to False

Use @app.before_request to define a function that checks logged_in

Redirect to /login if logged_in is False

Create two routes: / for the home page and /login for the login page

💡 Why This Matters

🌍 Real World

Before_request hooks are used in web apps to run code before every page loads, such as checking if a user is logged in or setting up data.

💼 Career

Understanding before_request hooks is important for backend web developers to control access and prepare requests in Flask applications.

1

Create the Flask app and login status variable

Import Flask and create an app instance called app. Then create a variable called logged_in and set it to False.

Flask

from flask import Flask

# Your code here

Hint

Use app = Flask(__name__) to create the app.

Set logged_in = False as a global variable.

2

Add the before_request hook to check login

Use the @app.before_request decorator to create a function called check_login. Inside it, check if logged_in is False. If so, import and use redirect and request from Flask to redirect to /login unless the current path is already /login.

Flask

from flask import Flask

app = Flask(__name__)
logged_in = False

# Your code here

Hint

Use @app.before_request above the function.

Inside the function, check if not logged_in and request.path != '/login'.

Return redirect('/login') in that case.

3

Create the home page route

Create a route for / using @app.route('/'). Define a function called home that returns the string 'Welcome to the home page!'.

Flask

from flask import Flask, redirect, request

app = Flask(__name__)
logged_in = False

@app.before_request
def check_login():
    if not logged_in and request.path != '/login':
        return redirect('/login')

# Your code here

Hint

Use @app.route('/') to create the home page route.

Return the welcome string from the home function.

4

Create the login page route

Create a route for /login using @app.route('/login'). Define a function called login that returns the string 'Please log in.'.

Flask

from flask import Flask, redirect, request

app = Flask(__name__)
logged_in = False

@app.before_request
def check_login():
    if not logged_in and request.path != '/login':
        return redirect('/login')

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

# Your code here

Hint

Use @app.route('/login') to create the login page route.

Return the login prompt string from the login function.

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