main.py

from flask import Flask, render_template, redirect, url_for, flash
from flask_sqlalchemy import SQLAlchemy
from flask_login import UserMixin, LoginManager, login_user, login_required, current_user, logout_user
from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, SubmitField
from wtforms.validators import ValidationError, InputRequired, Length
from flask_bcrypt import Bcrypt

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///database.db'
db = SQLAlchemy(app)
app.config['SECRET_KEY'] = 'YOUR_SECRET_KEY'
bcrypt = Bcrypt(app)

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

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


class User(db.Model, UserMixin):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(20), nullable=False, unique=True)
    password = db.Column(db.String(80), nullable=False)

class RegistrationForm(FlaskForm):
    username = StringField(validators=[InputRequired(), Length(min=4, max=20)], render_kw={"placeholder": "Username"})
    password = PasswordField(validators=[InputRequired(), Length(min=4, max=20)], render_kw={"placeholder": "Password"})
    submit = SubmitField("Register")

    def validate_username(self, username):
        existing_user_username = User.query.filter_by(username=username.data).first()
        if existing_user_username:
            raise ValidationError("That username already exists! Try a different one.")

class LoginForm(FlaskForm):
    username = StringField(validators=[InputRequired(), Length(min=4, max=20)], render_kw={"placeholder": "Username"})
    password = PasswordField(validators=[InputRequired(), Length(min=4, max=20)], render_kw={"placeholder": "Password"})
    submit = SubmitField("Login")

@app.route('/')
def home():
    return render_template('home.html')

@app.route('/dashboard', methods=["GET", "POST"])
@login_required
def dashboard():
    return render_template('dashboard.html')

@app.route('/logout', methods=["GET", "POST"])
@login_required
def logout():
    logout_user()
    return redirect(url_for('login'))

@app.route('/login', methods=["GET", "POST"])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('dashboard'))

    form = LoginForm()

    if form.validate_on_submit():
        user = User.query.filter_by(username=form.username.data).first()
        if user:
            if bcrypt.check_password_hash(user.password, form.password.data):
                login_user(user)
                return redirect(url_for('dashboard'))
            else:
                flash("Incorrect Password! Please Try again")
        else:
            flash("This email is not registered. Try Signing Up!")

    return render_template('login.html', form=form)

@app.route('/register', methods=["GET", "POST"])
def register():
    if current_user.is_authenticated:
        return redirect(url_for('dashboard'))
    form = RegistrationForm()

    if form.validate_on_submit():
        hashed_password = bcrypt.generate_password_hash(form.password.data)
        new_user = User(username=form.username.data, password=hashed_password)
        db.session.add(new_user)
        db.session.commit()
        return redirect(url_for('login'))

    return render_template('register.html', form=form)

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