What tool or service would you recommend for implementing user authentication in an application?

To implement user authentication in a Python application, the Django Allauth library (if using Django) or Flask-Login for Flask are often used.

If a more universal and scalable system is needed, consider authentication services like Auth0 or Firebase Authentication, which offer ready-made solutions supporting OAuth, social logins, and multi-factor authentication.

Example with Flask-Login:

from flask import Flask, request, redirect, url_for
from flask_login import LoginManager, UserMixin, login_user, login_required, logout_user

app = Flask(__name__)
app.secret_key = 'secret'
login_manager = LoginManager()
login_manager.init_app(app)

class User(UserMixin):
    def __init__(self, id):
        self.id = id

@login_manager.user_loader
def load_user(user_id):
    return User(user_id)

@app.route('/login', methods=['POST'])
def login():
    user = User(request.form['user_id'])
    login_user(user)
    return redirect(url_for('protected'))

@app.route('/protected')
@login_required
def protected():
    return 'Logged in successfully'

@app.route('/logout')
def logout():
    logout_user()
    return 'Logged out'

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