Setting and reading cookies in Flask

Introduction

Cookies help websites remember you. You can save small bits of information on a visitor's browser and read it later.

Remembering a user's login status so they don't have to log in every time.

Saving user preferences like theme color or language choice.

Tracking items in a shopping cart before checkout.

Counting how many times a user visits a page.

Storing temporary data between page requests.

Syntax

Flask

from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/set')
def set_cookie():
    resp = make_response('Cookie is set')
    resp.set_cookie('key', 'value')
    return resp

@app.route('/get')
def get_cookie():
    value = request.cookies.get('key')
    return f'Cookie value is {value}'

Use make_response() to create a response object before setting cookies.

Read cookies from request.cookies dictionary.

Examples

Sets a cookie named username with value alice.

Flask

resp.set_cookie('username', 'alice')

Reads the value of the cookie named username. Returns None if not found.

Flask

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

Sets a cookie that expires after 1 hour (3600 seconds).

Flask

resp.set_cookie('session_id', 'abc123', max_age=3600)

Sample Program

This Flask app has two routes: /set sets a cookie named favorite_color with value blue that lasts one day. /get reads and shows the cookie value if it exists.

Flask

from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/set')
def set_cookie():
    resp = make_response('Cookie has been set!')
    resp.set_cookie('favorite_color', 'blue', max_age=60*60*24)  # 1 day
    return resp

@app.route('/get')
def get_cookie():
    color = request.cookies.get('favorite_color')
    if color:
        return f'Your favorite color is {color}'
    else:
        return 'No favorite color cookie found.'

Important Notes

Cookies are stored on the user's browser and sent with every request to your site.

Be careful not to store sensitive information in cookies because users can see and change them.

Use max_age or expires to control how long cookies last.

Summary

Cookies let you save small data on a user's browser to remember things.

Use set_cookie on a response to save a cookie.

Use request.cookies.get() to read a cookie value.

Practice

1. What is the main purpose of using cookies in a Flask web application?

easy

A. To increase the speed of the Flask server

B. To run server-side scripts automatically

C. To change the HTML structure of a webpage

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

Solution

1. Step 1: Understand what cookies do

   Cookies store small data on the user's browser to remember information like login status or preferences.

2. Step 2: Identify the correct purpose in Flask context

   Flask uses cookies to save data on the client side, not to run server scripts or change server speed.

3. Final Answer:

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

4. Quick Check:

   Cookies save data on browser = C [OK]

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

Common Mistakes:

• Thinking cookies run server scripts
• Confusing cookies with server speed optimization
• Believing cookies change webpage HTML structure

2. Which of the following is the correct way to set a cookie named user_id with value 123 in Flask?

easy

A. response.cookies.set('user_id', 123)

B. response.set_cookie('user_id', '123')

C. request.set_cookie('user_id', '123')

D. request.cookies.set('user_id', '123')

Solution

1. Step 1: Identify the correct method to set cookies

   Cookies are set on the response object using set_cookie method.

2. Step 2: Check the value type and object used

   The value must be a string, so '123' is correct. Also, setting cookies on request object is invalid.

3. Final Answer:

   response.set_cookie('user_id', '123') -> Option B

4. Quick Check:

   Use response.set_cookie with string value = A [OK]

Hint: Use response.set_cookie with string values [OK]

Common Mistakes:

• Using request object to set cookies
• Passing integer instead of string as cookie value
• Calling set_cookie on request.cookies

3. Given this Flask route code, what will be the output when accessing /get_cookie after setting the cookie username to alice?

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

@app.route('/set_cookie')
def set_cookie():
    resp = make_response('Cookie Set')
    resp.set_cookie('username', 'alice')
    return resp

@app.route('/get_cookie')
def get_cookie():
    user = request.cookies.get('username')
    return f'User: {user}'

medium

A. User: alice

B. User: None

C. User: username

D. Error: Cookie not found

Solution

1. Step 1: Understand cookie setting route

   The /set_cookie route sets a cookie named 'username' with value 'alice' on the response.

2. Step 2: Understand cookie reading route

   The /get_cookie route reads the 'username' cookie from the request and returns it in the string.

3. Final Answer:

   User: alice -> Option A

4. Quick Check:

   Cookie 'username' = 'alice' read correctly = A [OK]

Hint: Set cookie first, then read with request.cookies.get() [OK]

Common Mistakes:

• Expecting cookie value before setting it
• Confusing cookie name with value
• Assuming error if cookie missing without checking

4. Identify the error in this Flask code snippet that tries to read a cookie named session_id:

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

@app.route('/check')
def check():
    session = request.cookies['session_id']
    return f'Session: {session}'

medium

A. Should use response.cookies instead of request.cookies

B. Cookie name must be capitalized as 'Session_id'

C. Using square brackets to access cookie may cause KeyError if cookie missing

D. Flask does not support reading cookies

Solution

1. Step 1: Check how cookie is accessed

   The code uses request.cookies['session_id'] which raises KeyError if cookie is missing.

2. Step 2: Suggest safer method

   Using request.cookies.get('session_id') returns None if cookie missing, avoiding error.

3. Final Answer:

   Using square brackets to access cookie may cause KeyError if cookie missing -> Option C

4. Quick Check:

   Access cookie safely with get() to avoid errors = B [OK]

Hint: Use request.cookies.get() to avoid KeyError [OK]

Common Mistakes:

• Using response.cookies to read cookies
• Assuming cookie names are case-insensitive
• Ignoring possibility of missing cookies causing errors

5. You want to create a Flask route that sets a cookie named theme with value dark that expires in 7 days. Which code snippet correctly does this?

hard

A. resp.set_cookie('theme', 'dark', max_age=7*24*60*60)

B. resp.set_cookie('theme', 'dark', expires=7)

C. resp.set_cookie('theme', 'dark', max_age=7)

D. resp.set_cookie('theme', 'dark', expires='7 days')

Solution

1. Step 1: Understand cookie expiration parameters

   To set expiration, use max_age in seconds or expires as a datetime object.

2. Step 2: Calculate 7 days in seconds and check syntax

   7 days = 7*24*60*60 seconds. resp.set_cookie('theme', 'dark', max_age=7*24*60*60) uses max_age correctly with this calculation.

3. Final Answer:

   resp.set_cookie('theme', 'dark', max_age=7*24*60*60) -> Option A

4. Quick Check:

   Use max_age in seconds for 7 days = D [OK]

Hint: Use max_age in seconds for cookie expiry [OK]

Common Mistakes:

• Passing expires as integer days instead of seconds or datetime
• Using string '7 days' which is invalid
• Confusing max_age and expires parameters