Flask-Login

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

app = Flask(__name__)
app.config['SECRET_KEY'] = os.environ.get('FLASK_SECRET_KEY', 'a_secret_key_for_dev_only') # IMPORTANT: Change in production!

login_manager = LoginManager()
login_manager.init_app(app)
login_manager.login_view = 'login'

# A simple 'database' for demonstration
USERS = {
    "testuser": {"password": "testpass", "id": "1"},
    "another": {"password": "anotherpass", "id": "2"}
}

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

    def get_id(self):
        return str(self.id)

@login_manager.user_loader
def load_user(user_id):
    # This callback is used to reload the user object from the user ID stored in the session.
    # It should take the unicode ID of a user, and return the corresponding user object.
    for username, data in USERS.items():
        if data['id'] == user_id:
            return User(data['id'], data['password'], username)
    return None

@app.route('/')
def index():
    if current_user.is_authenticated:
        return f'Hello, {current_user.username}! <a href="{url_for("protected")}">Protected Page</a> | <a href="{url_for("logout")}">Logout</a>'
    return '<a href="/login">Login</a>'

@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']
        if username in USERS and USERS[username]['password'] == password:
            user_data = USERS[username]
            user = User(user_data['id'], user_data['password'], username)
            login_user(user)
            next_page = request.args.get('next')
            return redirect(next_page or url_for('index'))
        return 'Invalid credentials'
    return render_template_string('''
        <form method="post">
            <p><input type=text name=username placeholder="Username"></p>
            <p><input type=password name=password placeholder="Password"></p>
            <p><input type=submit value=Login></p>
        </form>
    ''')

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

@app.route('/protected')
@login_required
def protected():
    return f'This is a protected page. Only {current_user.username} can see this! <a href="{url_for("index")}">Home</a>'

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