Login form and verification in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a user submits the login form with correct credentials?

Consider a Flask login form that checks username and password against stored values. What is the typical behavior after a successful login?

Flask

from flask import Flask, request, redirect, url_for, render_template_string
app = Flask(__name__)

users = {'alice': 'wonderland'}

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        if username in users and users[username] == password:
            return redirect(url_for('dashboard'))
        else:
            return 'Login Failed', 401
    return render_template_string('''<form method='post'>
        <input name='username'>
        <input name='password' type='password'>
        <input type='submit'>
    </form>''')

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

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

A The user is redirected to the dashboard page with a welcome message.

B The login form reloads with an error message about invalid credentials.

C The server returns a 404 Not Found error.

D The user sees the login form again without any message.

💡 Hint

Think about what redirect(url_for('dashboard')) does after a successful login.

When the username and password match, the code redirects the user to the dashboard route, which shows a welcome message.

📝 Syntax

intermediate

Identify the syntax error in this Flask login route

Which option correctly fixes the syntax error in this Flask login route code?

Flask

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

users = {'alice': 'wonderland'}

@app.route('/login', methods=['POST'])
def login():
    username = request.form['username']
    password = request.form['password']
    if username in users and users[username] == password
        return 'Logged in'
    else:
        return 'Failed'

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

A Change 'request.form' to 'request.forms'.

B Indent the return statements one level less.

C Add a colon ':' at the end of the if statement line.

D Remove the else block entirely.

💡 Hint

Python requires a colon at the end of control statements like if.

The if statement is missing a colon at the end, causing a syntax error. Adding ':' fixes it.

❓ state_output

advanced

What is the output after submitting invalid credentials?

Given this Flask login snippet, what response does the server send when the password is wrong?

Flask

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

users = {'bob': 'builder'}

@app.route('/login', methods=['POST'])
def login():
    username = request.form['username']
    password = request.form['password']
    if username in users and users[username] == password:
        return 'Success'
    else:
        return 'Unauthorized', 401

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

A A redirect to the login page

B 'Unauthorized' with HTTP status code 401

C 'Success' with HTTP status code 200

D A server error 500

💡 Hint

Look at the else block's return statement and its status code.

If the password is wrong, the server returns the string 'Unauthorized' with status code 401, indicating access denied.

🔧 Debug

advanced

Why does this Flask login form always fail to authenticate?

Examine the code below. Why does the login always fail even with correct credentials?

Flask

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

users = {'eve': 'secret'}

@app.route('/login', methods=['POST'])
def login():
    username = request.form.get('username')
    password = request.form.get('password')
    if username == users.get(username) and password == users.get(username):
        return 'Logged in'
    else:
        return 'Failed'

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

A The route does not accept POST requests.

B The form data keys are incorrect; should use 'user' and 'pass'.

C The users dictionary is empty, so no match is possible.

D The condition compares username to password instead of checking password correctness.

💡 Hint

Look carefully at the if condition comparing username and password.

The condition wrongly compares username to users.get(username) (which returns the password), so username == password is checked, which fails. It should check password == users.get(username).

🧠 Conceptual

expert

What is the main security risk of this simple Flask login implementation?

Consider a Flask login system that stores passwords in plain text in a dictionary and checks them directly. What is the biggest security risk?

Flask

users = {'admin': '12345'}

@app.route('/login', methods=['POST'])
def login():
    username = request.form['username']
    password = request.form['password']
    if username in users and users[username] == password:
        return 'Welcome'
    else:
        return 'Denied', 401

A Passwords are stored in plain text, risking exposure if the code or data leaks.

B The login form uses POST instead of GET, which is insecure.

C The code does not use HTTPS, causing syntax errors.

D The username is not validated for length, causing runtime errors.

💡 Hint

Think about how passwords should be stored securely.

Storing passwords in plain text is dangerous because if anyone accesses the data, they see all passwords. Passwords should be hashed and salted.

Practice

1. What is the main purpose of a login form in a Flask web application?

easy

A. To create database tables

B. To display images on the webpage

C. To collect username and password from users

D. To send emails automatically

Solution

1. Step 1: Understand the role of a login form

   A login form is designed to collect user credentials like username and password.

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

   In Flask, the login form collects these details to verify user identity before granting access.

3. Final Answer:

   To collect username and password from users -> Option C

4. Quick Check:

   Login form = collect credentials [OK]

Hint: Login forms always gather user credentials [OK]

Common Mistakes:

• Confusing login form with other features like email or images
• Thinking login form creates database tables
• Assuming login form sends emails automatically

2. Which Flask code snippet correctly defines a route to display a login form using GET method?

easy

A. app.route('/login', methods=['GET'])\ndef login():\n return render_template('login.html')

B. app.route('/login', methods=['POST'])\ndef login():\n return render_template('login.html')

C. app.route('/login')\ndef login():\n return redirect('/home')

D. app.route('/login', methods=['DELETE'])\ndef login():\n return 'Delete method not allowed'

Solution

1. Step 1: Identify the correct HTTP method for showing a form

   Login forms are usually shown with GET requests, so methods=['GET'] is correct.

2. Step 2: Check the route and function return

   The route '/login' with methods=['GET'] returns the login.html template to display the form.

3. Final Answer:

   app.route('/login', methods=['GET'])\ndef login():\n return render_template('login.html') -> Option A

4. Quick Check:

   Show form = GET method [OK]

Hint: Use GET method to display forms in Flask routes [OK]

Common Mistakes:

• Using POST method to show the form instead of GET
• Not specifying methods list defaults to GET but unclear
• Using DELETE method which is invalid for forms

3. Given this Flask login verification code snippet, what will be the output if username='admin' and password='1234' are submitted?

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

@app.route('/login', methods=['POST'])
def login():
    user = request.form.get('username')
    pwd = request.form.get('password')
    if user == 'admin' and pwd == '1234':
        return 'Login successful'
    else:
        return 'Login failed'

medium

A. Login successful

B. Login failed

C. SyntaxError

D. No output

Solution

1. Step 1: Check the submitted username and password

   The code compares user to 'admin' and pwd to '1234'. Both match the submitted values.

2. Step 2: Determine the returned response

   Since both match, the function returns 'Login successful'.

3. Final Answer:

   Login successful -> Option A

4. Quick Check:

   Correct credentials = Login successful [OK]

Hint: Match input values exactly to get success message [OK]

Common Mistakes:

• Confusing POST and GET methods for form data
• Assuming wrong output when credentials match
• Ignoring case sensitivity in username/password

4. Identify the error in this Flask login route code:

@app.route('/login', methods=['POST'])
def login():
    username = request.form['username']
    password = request.form['password']
    if username = 'user' and password = 'pass':
        return 'Welcome!'
    else:
        return 'Invalid credentials'

medium

A. No return statement in the function

B. Missing 'methods' argument in @app.route decorator

C. Incorrect use of request.args instead of request.form

D. Using '=' instead of '==' in the if condition

Solution

1. Step 1: Check the if condition syntax

   The code uses single '=' which is assignment, not comparison. It should be '==' to compare values.

2. Step 2: Verify other parts of the code

   The methods argument is present, request.form is correctly used, and return statements exist.

3. Final Answer:

   Using '=' instead of '==' in the if condition -> Option D

4. Quick Check:

   Comparison needs '==' not '=' [OK]

Hint: Use '==' for comparisons, '=' is assignment [OK]

Common Mistakes:

• Using '=' instead of '==' in conditions
• Forgetting to specify POST method in route
• Using request.args for POST form data

5. You want to create a Flask login system that locks out a user after 3 failed attempts. Which approach correctly combines login verification and attempt tracking?

hard

A. Use a global variable to count attempts and reset on successful login

B. Store attempt count in session and check before verifying credentials

C. Check attempts only after successful login to reset count

D. Ignore attempt count and always allow login attempts

Solution

1. Step 1: Understand where to store attempt count

   Using session storage per user is best to track attempts individually without global conflicts.

2. Step 2: Implement logic to check attempts before verifying credentials

   Check if attempts exceed 3 before allowing login verification to prevent further tries.

3. Step 3: Reset attempt count on successful login

   Resetting attempts after success avoids locking out valid users.

4. Final Answer:

   Store attempt count in session and check before verifying credentials -> Option B

5. Quick Check:

   Session tracks attempts per user [OK]

Hint: Use session to track attempts and block after limit [OK]

Common Mistakes:

• Using global variables causing shared count for all users
• Resetting attempts only after success but not checking before login
• Ignoring attempt limits and allowing unlimited tries