Flask session object - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask session code?

Consider this Flask route that sets and reads session data. What will be the printed output when a user visits the route twice in the same browser session?

Flask

from flask import Flask, session
app = Flask(__name__)
app.secret_key = 'secret'

@app.route('/')
def index():
    if 'visits' in session:
        session['visits'] += 1
    else:
        session['visits'] = 1
    print(f"Visits: {session['visits']}")
    return f"You have visited {session['visits']} times."

A Visits: 1 on first visit, Visits: 2 on second visit

B KeyError on second visit

C Visits: 1 on both visits

D Visits: 0 on first visit, Visits: 1 on second visit

💡 Hint

Think about how session data persists across requests in the same browser session.

The session object stores data per user session. On first visit, 'visits' is set to 1. On second visit, it increments to 2. So the printed output reflects the count increasing.

📝 Syntax

intermediate

Which option correctly sets a value in Flask session?

You want to store the username 'alice' in the Flask session. Which code snippet does this correctly?

A session.set('username', 'alice')

B session['username'] = 'alice'

C session.add('username', 'alice')

D session.username = 'alice'

💡 Hint

Remember, Flask session behaves like a dictionary.

Flask session is a dictionary-like object. You set values by assigning to keys like session['key'] = value. Methods like set() or add() do not exist.

🔧 Debug

advanced

What error occurs with this Flask session code?

This Flask route tries to increment a session counter but raises an error. What is the error and why?

Flask

from flask import Flask, session
app = Flask(__name__)
app.secret_key = 'secret'

@app.route('/')
def index():
    session['count'] += 1
    return f"Count is {session['count']}"

A KeyError because 'count' key does not exist initially

B TypeError because session is not subscriptable

C RuntimeError because secret_key is missing

D No error, outputs 'Count is 1' on first visit

💡 Hint

What happens if you try to increment a key that is not yet in the session?

The code tries to increment session['count'] without initializing it first. Since 'count' key is missing, a KeyError is raised.

❓ state_output

advanced

What is the session content after these requests?

Given this Flask app, what is the content of session after visiting /set and then /increment?

Flask

from flask import Flask, session
app = Flask(__name__)
app.secret_key = 'secret'

@app.route('/set')
def set_value():
    session['counter'] = 5
    return 'Counter set to 5'

@app.route('/increment')
def increment():
    session['counter'] += 1
    return f"Counter incremented to {session['counter']}"

A {'counter': 5}

B {} (empty session)

C KeyError on /increment

D {'counter': 6}

💡 Hint

Think about the order of requests and how session data persists.

Visiting /set sets session['counter'] to 5. Then visiting /increment adds 1, making it 6. So session contains {'counter': 6}.

🧠 Conceptual

expert

Why must Flask app have a secret_key to use session?

Flask sessions store data client-side in cookies. Why is setting app.secret_key required for sessions to work securely?

A secret_key stores the session data on the server securely

B secret_key encrypts the session data so users cannot read it

C secret_key is used to cryptographically sign session cookies to prevent tampering

D secret_key is only needed for HTTPS connections

💡 Hint

Think about how Flask protects session data stored in cookies.

Flask signs session cookies with secret_key to ensure data is not altered by clients. It does not encrypt data by default, nor store it server-side.