How To Add Authentication to Your App with Flask-Login

How To Add Authentication to Your App with Flask-Login

Flask-Login provides user session management for Flask. It handles the common tasks of logging in, logging out, and remembering users' sessions over extended periods of time.

Here's a basic guide to adding authentication to your Flask app using Flask-Login:

Step 1: Install Flask-Login

First, you need to install the Flask-Login extension:

pip install flask-login

Step 2: Set Up User Loader

After installation, set up the user loader callback. This callback is used to reload the user object from the user ID stored in the session.

from flask_login import LoginManager, UserMixin

login_manager = LoginManager()
login_manager.init_app(app)  # Initialize it with your Flask app instance.

class User(UserMixin):
    # Create a User class with UserMixin that includes generic implementations
    # that are appropriate for most use cases.

@login_manager.user_loader
def load_user(user_id):
    # Given *user_id*, return the associated User object.
    return User.get(user_id)

Step 3: Set Up User Class

Flask-Login expects your user class to have certain properties and methods (is_authenticated, is_active, is_anonymous, get_id). UserMixin provides these default implementations, which should suffice for most use cases.

class User(UserMixin):
    def __init__(self, username, password, id, active=True):
        self.id = id
        self.username = username
        self.password = password
        self.active = active

    # method to retrieve a user from the database
    @staticmethod
    def get(user_id):
        # For the purpose of this example, we'll use a predefined user.
        # In a real application, you'd query the database here.
        if user_id == '1':
            return User('user', 'password', 1)
        else:
            return None

Step 4: Set Up Routes for Login and Logout

Now, create routes for login and logout functionality.

from flask import Flask, redirect, url_for, request
from flask_login import current_user, login_user, logout_user

@app.route('/login', methods=['GET', 'POST'])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        user = User.get(1)  # Get user from database, here we're just using a mock
        if user and user.password == password:
            login_user(user)
            return redirect(url_for('index'))
        else:
            return 'Invalid username or password'
    return 'Login form here'

@app.route('/logout')
def logout():
    logout_user()
    return redirect(url_for('index'))

Step 5: Require Authentication in Protected Routes

To protect a route, use the login_required decorator.

from flask_login import login_required

@app.route('/protected')
@login_required
def protected():
    return 'Logged in as: ' + current_user.username

Step 6: Initialize the App with LoginManager

Make sure to initialize LoginManager with your app object.

if __name__ == '__main__':
    # The login manager contains the code that lets your application and Flask-Login work together.
    login_manager.init_app(app)
    app.run(debug=True)

Remember that this is just a simple example to show you the basics of Flask-Login. In a real-world application, you would also need to:

  • Use a database to store user information.
  • Handle passwords securely (never store them in plaintext, use a hashing library like bcrypt).
  • Provide user feedback and HTML templates for your login forms.
  • Configure the login view (login_manager.login_view = 'login') so Flask-Login knows where to redirect users for login.
  • Set up a user registration system to allow new users to sign up.

By following these steps, you'll integrate Flask-Login into your Flask application and provide a working authentication system.

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