How cookies work in HTTP in Flask - Try It Yourself

How cookies work in HTTP with Flask

📖 Scenario: You are building a simple web app using Flask. You want to remember a visitor's name using cookies so that when they come back, the app greets them personally.

🎯 Goal: Create a Flask app that sets a cookie with the visitor's name and reads it back to greet them on return visits.

📋 What You'll Learn

Create a Flask app with one route '/'

Set a cookie named 'username' with value 'Alice' when the visitor first arrives

Read the 'username' cookie on subsequent visits

Display a greeting message using the cookie value if it exists

Use Flask's make_response to set cookies

💡 Why This Matters

🌍 Real World

Cookies are used by websites to remember users, keep them logged in, and personalize their experience.

💼 Career

Understanding cookies is essential for web developers to manage user sessions and preferences securely.

1

Set up the Flask app and route

Import Flask and request from flask. Create a Flask app called app. Define a route for '/' with a function called index that returns the string 'Welcome!'.

Flask

from flask import Flask, request

app = Flask(__name__)

@app.route('/')
def index():
    # Your code here

Hint

Start by importing Flask and request, then create the app and a simple route that returns a welcome message.

2

Add a variable to hold the cookie name

Create a variable called cookie_name and set it to the string 'username'.

Flask

from flask import Flask, request

app = Flask(__name__)

cookie_name = ''  # Your code here

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

Hint

This variable will store the name of the cookie we use to remember the visitor.

3

Check for the cookie and set it if missing

In the index function, import make_response from flask. Use request.cookies.get(cookie_name) to get the cookie value and store it in username. If username is None, create a response with the text 'Hello, new visitor!' and set a cookie named cookie_name with value 'Alice'. Otherwise, create a response with the text f'Welcome back, {username}!'. Return the response.

Flask

from flask import Flask, request, make_response

app = Flask(__name__)

cookie_name = 'username'

@app.route('/')
def index():
    # Your code here

Hint

Use request.cookies.get() to read the cookie. Use make_response() to create a response and set_cookie() to add a cookie.

4

Add the app run command

Add the standard Flask app run block: if __name__ == '__main__': and inside it call app.run(debug=True).

Flask

from flask import Flask, request, make_response

app = Flask(__name__)

cookie_name = 'username'

@app.route('/')
def index():
    username = request.cookies.get(cookie_name)
    if username is None:
        response = make_response('Hello, new visitor!')
        response.set_cookie(cookie_name, 'Alice')
    else:
        response = make_response(f'Welcome back, {username}!')
    return response

# Your code here

Hint

This block runs the Flask app when you start the script.

Practice

1. What is the main purpose of cookies in HTTP when using Flask?

easy

A. To store small pieces of data on the user's browser for later use

B. To increase the speed of the Flask server

C. To encrypt user data on the server

D. To manage database connections automatically

Solution

1. Step 1: Understand what cookies do in HTTP

   Cookies store small data on the user's browser to remember information between requests.

2. Step 2: Identify Flask's use of cookies

   Flask uses cookies to save user preferences or login info on the client side.

3. Final Answer:

   To store small pieces of data on the user's browser for later use -> Option A

4. Quick Check:

   Cookies store data on browser = D [OK]

Hint: Cookies save small data on browser, not server speed [OK]

Common Mistakes:

• Thinking cookies speed up the server
• Confusing cookies with encryption
• Assuming cookies manage databases

2. Which Flask method is used to set a cookie in the HTTP response?

easy

A. request.set_cookie()

B. request.cookies()

C. response.set_cookie()

D. response.cookies()

Solution

1. Step 1: Recall Flask's cookie setting method

   Cookies are set on the response object, not the request.

2. Step 2: Identify the correct method

   The method to set a cookie is set_cookie() on the response object.

3. Final Answer:

   response.set_cookie() -> Option C

4. Quick Check:

   Set cookie on response = C [OK]

Hint: Set cookies on response, read from request [OK]

Common Mistakes:

• Using request.set_cookie() instead of response.set_cookie()
• Confusing request.cookies() as a setter
• Using response.cookies() which does not exist

3. Given this Flask code snippet, what will be printed when the client sends a cookie named 'user' with value 'Alice'?

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

@app.route('/')
def index():
    user = request.cookies.get('user')
    return f'Hello, {user}!'

medium

A. Hello, Alice!

B. Hello, !

C. Hello, None!

D. Hello, user!

Solution

1. Step 1: Understand how Flask reads cookies

   The code uses request.cookies.get('user') to get the cookie value named 'user'.

2. Step 2: Check the cookie value sent by client

   The client sends a cookie 'user' with value 'Alice', so user variable will be 'Alice'.

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   Cookie 'user' = Alice, output = A [OK]

Hint: request.cookies.get('name') returns cookie value or None [OK]

Common Mistakes:

• Assuming cookie value is None if present
• Forgetting to use .get() method
• Confusing cookie name with literal string

4. What is wrong with this Flask code that tries to set a cookie?

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

@app.route('/')
def index():
    response = 'Hello'
    response.set_cookie('theme', 'dark')
    return response

medium

A. The cookie name 'theme' is invalid

B. You cannot set cookies on a string response object

C. The set_cookie method requires a third argument

D. Flask does not support cookies

Solution

1. Step 1: Identify the response object type

   The variable response is a string, not a Flask response object.

2. Step 2: Understand how to set cookies in Flask

   Cookies can only be set on a response object, like one created by make_response(), not on a plain string.

3. Final Answer:

   You cannot set cookies on a string response object -> Option B

4. Quick Check:

   Set cookies on response object, not string = B [OK]

Hint: Use make_response() to set cookies, not plain strings [OK]

Common Mistakes:

• Trying to set cookies on string instead of response object
• Thinking cookie names must be special
• Believing set_cookie needs extra arguments always

5. You want to create a Flask route that sets a cookie named 'visited' with value 'yes' that expires in 1 day. Which code correctly does this?

hard

A. from flask import make_response
@app.route('/')
def index():
resp = make_response('Welcome')
resp.cookies['visited'] = 'yes'
return resp

B. @app.route('/')
def index():
response = 'Welcome'
response.set_cookie('visited', 'yes', max_age=86400)
return response

C. from flask import request
@app.route('/')
def index():
request.set_cookie('visited', 'yes', max_age=86400)
return 'Welcome'

D. from flask import make_response
@app.route('/')
def index():
resp = make_response('Welcome')
resp.set_cookie('visited', 'yes', max_age=86400)
return resp

Solution

1. Step 1: Identify how to create a response object

   Use make_response() to create a response object to set cookies.

2. Step 2: Set cookie with expiration

   Use set_cookie() with max_age=86400 (seconds in 1 day) to set expiration.

3. Step 3: Check each option

   from flask import make_response @app.route('/') def index(): resp = make_response('Welcome') resp.set_cookie('visited', 'yes', max_age=86400) return resp correctly uses make_response and set_cookie with max_age. Others either set cookie on string or request or use wrong syntax.

4. Final Answer:

   resp.set_cookie('visited', 'yes', max_age=86400) -> Option D

5. Quick Check:

   Use make_response + set_cookie + max_age = A [OK]

Hint: Use make_response and set_cookie with max_age for expiry [OK]

Common Mistakes:

• Setting cookie on string instead of response object
• Trying to set cookie on request object
• Using resp.cookies dict instead of set_cookie method