How cookies work in HTTP in Flask

Introduction

Cookies help websites remember you by storing small pieces of information on your browser. This makes your experience smoother and personalized.

Remembering a user login so they don't have to sign in every time.

Keeping track of items in a shopping cart on an online store.

Saving user preferences like language or theme.

Tracking user visits to show customized content.

Managing sessions securely between the browser and server.

Syntax

Flask

from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/')
def index():
    username = request.cookies.get('username')
    if username:
        return f'Hello, {username}!'
    else:
        resp = make_response('Hello, Guest!')
        resp.set_cookie('username', 'Alice')
        return resp

Use request.cookies.get('cookie_name') to read a cookie sent by the browser.

Use response.set_cookie('cookie_name', 'value') to send a cookie to the browser.

Examples

This sets a cookie named theme with the value dark in the user's browser.

Flask

resp.set_cookie('theme', 'dark')

This reads the username cookie sent by the browser, if it exists.

Flask

username = request.cookies.get('username')

This deletes the username cookie from the browser.

Flask

resp.delete_cookie('username')

Sample Program

This Flask app checks if the browser sent a cookie named username. If yes, it greets the user by name. If not, it sets the cookie with the value Alice and welcomes the new visitor.

Flask

from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/')
def index():
    username = request.cookies.get('username')
    if username:
        return f'Welcome back, {username}!'
    else:
        resp = make_response('Hello, new visitor! Setting your cookie now.')
        resp.set_cookie('username', 'Alice')
        return resp

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Cookies are stored on the user's browser and sent with every request to the server for that domain.

Cookies can have expiration times, making them temporary or persistent.

Always be careful with sensitive data; do not store passwords in cookies.

Summary

Cookies store small data on the browser to remember users.

Flask uses request.cookies to read and response.set_cookie() to write cookies.

Cookies improve user experience by saving preferences and login info.

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