Login form and verification in Flask

Introduction

A login form lets users enter their username and password to access a website securely. Verification checks if the entered details are correct.

When you want to restrict parts of your website to registered users only.

When you need to personalize user experience after they sign in.

When you want to protect sensitive information behind a secure login.

When you want to track user activity after they log in.

Syntax

Flask

from flask import Flask, render_template, request, redirect, url_for

app = Flask(__name__)

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        # Verify username and password here
        if username == 'user' and password == 'pass':
            return redirect(url_for('welcome'))
        else:
            return 'Login Failed'
    return render_template('login.html')

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

The @app.route decorator defines the URL path and allowed methods.

request.form gets data submitted from the form.

Examples

Basic login route that checks username and password and returns a message.

Flask

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        # Check credentials
        if username == 'admin' and password == '1234':
            return 'Logged in!'
        else:
            return 'Wrong credentials'
    return render_template('login.html')

Checks if username and password are entered before proceeding.

Flask

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form.get('username')
        password = request.form.get('password')
        if username and password:
            # Add real verification here
            return redirect(url_for('dashboard'))
        else:
            return 'Please enter username and password'
    return render_template('login.html')

Sample Program

This Flask app shows a login form. When the user submits the form, it checks if the username is 'admin' and password is 'secret'. If correct, it redirects to a welcome page. If wrong, it shows an error message on the same page.

Flask

from flask import Flask, render_template_string, request, redirect, url_for

app = Flask(__name__)

login_form = '''
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Login</title>
</head>
<body>
    <h2>Login Form</h2>
    <form method="POST" action="/login">
        <label for="username">Username:</label><br>
        <input type="text" id="username" name="username" required><br><br>
        <label for="password">Password:</label><br>
        <input type="password" id="password" name="password" required><br><br>
        <button type="submit">Login</button>
    </form>
    {% if error %}
    <p style="color:red;">{{ error }}</p>
    {% endif %}
</body>
</html>
'''

@app.route('/login', methods=['GET', 'POST'])
def login():
    error = None
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        # Simple verification: username is 'admin' and password is 'secret'
        if username == 'admin' and password == 'secret':
            return redirect(url_for('welcome'))
        else:
            error = 'Invalid username or password'
    return render_template_string(login_form, error=error)

@app.route('/welcome')
def welcome():
    return '<h1>Welcome, admin!</h1>'

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

Important Notes

Never store passwords as plain text in real apps; use hashing.

Use HTTPS to keep login data safe during transmission.

Flask's render_template_string is used here for simplicity; normally use separate HTML files.

Summary

A login form collects username and password from users.

Verification checks if the entered details match expected values.

Flask routes handle showing the form and processing the login data.

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