app.py

from flask import Flask, render_template, request, redirect, url_for, session
from flask_mysqldb import MySQL
#
app = Flask(__name__)
app.config['MYSQL_HOST'] = 'mysql'
app.config['MYSQL_USER'] = 'root'
app.config['MYSQL_PASSWORD'] = 'password'
app.config['MYSQL_DB'] = 'login_demo'

mysql = MySQL(app)
#
@app.route('/')
def index():
    if 'username' in session:
        username = session['username']
        return f'Logged in as {username} | <a href="/logout">Logout</a> | <a href="/dashboard">Dashboard</a>'
    return 'You are not logged in | <a href="/login">Login</a>'

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        cursor = mysql.connection.cursor()
        cursor.execute('SELECT * FROM users WHERE username = %s AND password = %s', (username, password))
        user = cursor.fetchone()
        cursor.close()
        if user:
            session['username'] = username
            return redirect(url_for('index'))
        else:
            return 'Login failed. Invalid username or password.'
    return render_template('login.html')

@app.route('/dashboard')
def dashboard():
    if 'username' in session:
        return render_template('dashboard.html', username=session['username'])
    return redirect(url_for('login'))

@app.route('/logout')
def logout():
    session.pop('username', None)
    return redirect(url_for('index'))

if __name__ == '__main__':
    app.run(debug=True, host='0.0.0.0')