Setting and reading cookies in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a cookie in web development?

A cookie is a small piece of data stored on the user's browser. It helps websites remember information like login status or preferences.

beginner

How do you set a cookie in Flask?

You set a cookie by creating a response object and using its set_cookie() method with a key and value.

beginner

How do you read a cookie value in Flask?

You read a cookie from the request.cookies dictionary using the cookie's key name.

intermediate

What is the purpose of the max_age parameter when setting a cookie?

It sets how long the cookie will last in seconds before it expires and is deleted from the browser.

intermediate

Why should you use cookies carefully in web apps?

Because cookies store data on the user's device, they can affect privacy and security. Sensitive data should not be stored in cookies.

Which Flask object do you use to set a cookie?

A App object

B Response object

C Session object

D Request object

Cookies are set on the response sent back to the browser, so you use the Response object.

How do you access cookies sent by the browser in Flask?

A request.cookies

B response.cookies

C session.cookies

D app.cookies

Cookies sent by the browser are available in the request.cookies dictionary.

What does the max_age parameter control when setting a cookie?

A Cookie size limit

B Cookie encryption

C Cookie domain

D Cookie expiration time

max_age sets how long the cookie stays valid before expiring.

Which of these is NOT a good practice when using cookies?

A Tracking user sessions

B Storing user preferences

C Storing sensitive passwords

D Setting cookie expiration

Sensitive data like passwords should never be stored in cookies for security reasons.

What type of data structure is request.cookies in Flask?

A Dictionary

B List

C Tuple

D Set

request.cookies behaves like a dictionary where cookie names are keys.

Explain how to set and read cookies in a Flask web application.

Think about how the server sends cookies and how it reads them back from the browser.

Describe why cookie expiration is important and how to control it in Flask.

Consider what happens if cookies never expire.

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