User registration in Flask - frontend, part 2 of 2

Posted on 2020-06-02T06:30:03Z by Kevin Foong

This is the second part of my tutorial to create a fully functioning user registration and login form in Flask. In the first part of the tutorial we looked at the backend side especially Flask SQLAlchemy and the database. I assume you have already set up everything from the first part and this will just be a continuation of it. In this part we will focus on the Flask side, in particular Flask WTF and Flask Login. Part 1 is avaiable here.

1. First install flask-wtf and flask-login in your project's virtual environment. "wtforms[email]" is just an extra package from wtforms which provides email validation which we will use later.

pip install flask-wtf
pip install wtforms[email]
pip install flask-login

2. Create forms.py under your "app" folder.

Flask-WTF is a great package in helping you build forms in Flask. It also helps you perform validation checking on individual fields. Lets take a look at one example to see how it works:

password = PasswordField('Password', validators=[InputRequired()])

We specify that we want a password field. This will be rendered as an HTML <input type="password"> element. The validators parameter takes a list of validations you want to perform on the field. In this case we are saying that it is a mandatory field (InputRequired). 

password2 = PasswordField('Repeat Password', validators=[InputRequired(), EqualTo('password')])

This is the second password field in which there is a validator to ensure that it matches the first password field.

A few other points include,

• In LoginForm we have added a UserMixin class. This provides helpful properties that we can use in our Flask app such as is_authenticated to tell us if the current is logged in.
• In RegistrationForm there is a validate_email function. This is a custom validator provided by fask-wtf which enables you to add your own validation code. In this case we use it to check the database to see if the email already exists.
• User.query.filter_by(email=email.data).first() is the equivalent to SQL

SELECT TOP(1)
FROM user
WHERE email = email.data

The source code for forms.py is below.

from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, BooleanField, SubmitField
from wtforms.validators import InputRequired, Email, EqualTo, ValidationError
from flask_login import UserMixin
from .models import User

class LoginForm(FlaskForm, UserMixin):
    email = StringField('Username (email)', validators=[InputRequired()])
    password = PasswordField('Password', validators=[InputRequired()])
    remember_me = BooleanField('Remember Me')
    submit = SubmitField('Login')

class RegistrationForm(FlaskForm):
    email = StringField('Email', validators=[InputRequired(), Email()])
    password = PasswordField('Password', validators=[InputRequired()])
    password2 = PasswordField('Repeat Password', validators=[InputRequired(), EqualTo('password')])
    submit = SubmitField('Register')

    # flask-wtf takes validate_<field_name> as custom validators
    # so the below validator gets invoked on username
    def validate_email(self, email):
        user = User.query.filter_by(email=email.data).first()
        if user is not None:
            raise ValidationError('Email has previously registered')

3. Create routes.py under your "app" folder.

Most of this is quite standard Flask. A few key points regarding Flask-login to note:

• Flask-login provides a login_user and logout_user function. login-user works by storing the User object in the client session and logout-user removes it from the session. You can then check if the user is logged in through current_user.is_authenticated. This method is available to us because we enabled it by adding the UserMixin class (also provided by flask-login) to our User model.
• Flask-login also manages the "remember me" functionality. You just need to pass remember=true to login_user, and a cookie will be set on the user's PC. According to the Flask-login documentation "A cookie is saved on the user’s computer, and then Flask-Login will automatically restore the user ID from that cookie if it is not in the session".
• Flask-login provides a @login-required decorator which you can put on top of any function that requires the user to be logged in. If the user is not logged in the function decorated with @login_manager.unauthorized_handler will be called which in our case is the unauthorized function.
• Flask-login requires you to always have a callback decorated with @login_manager.user_loader. This function returns the user object, given the user_id.

from flask import render_template, redirect, url_for, flash, request
from flask_login import current_user, login_user, logout_user, login_required
from . import app, db, login_manager
from .forms import LoginForm, RegistrationForm
from .models import User

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

@app.route('/login', methods=['GET', 'POST'])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    next_page = request.args.get('next')
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(email=form.email.data).first()
        if user is None or not user.check_password(form.password.data):
            flash('Invalid username or password')
            return redirect(url_for('login', next=next_page))

        login_user(user, remember=form.remember_me.data)
        flash('You are now logged in')

        if not next_page:
            return redirect(url_for('index'))
        return redirect(url_for(next_page))

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

@app.route('/logout')
@login_required
def logout():
    logout_user()
    flash('You are now logged out')
    return redirect(url_for('index'))

@app.route('/register', methods=['GET', 'POST'])
def register():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = RegistrationForm()
    if form.validate_on_submit():
        user = User(email=form.email.data)
        user.set_password(form.password.data)
        db.session.add(user)
        db.session.commit()
        flash('Registration successful')
        return redirect(url_for('login'))
    return render_template('register.html', form=form)

# handler when you are trying to access a page but you are not logged in
@login_manager.unauthorized_handler
def unauthorized():
    flash('You must be logged in to view this page')
    return redirect(url_for('login', next=request.endpoint))

# Callback used by flask-login
# This callback is used to reload the user object from the user ID stored in the session
@login_manager.user_loader
def load_user(user_id):
    return User.query.get(user_id)

4. Create a "templates" sub-folder under your "app" folder. In the templates folder create index.html, login.html and logout.html

Here I provided basic templates for index, login and registration. They are basic HTML without any CSS to keep things simple.

index.html

<body>
    {% with messages = get_flashed_messages(with_categories=true) %}
        {% if messages %}
            {% for category, message in messages %}
                <div style="color:red;">{{ message }}</div>
            {% endfor %}
        {% endif %}
    {% endwith %}
    <h1>User Registration</h1>
    <p>Welcome to my page</p>
    <a href="{{ url_for('login') }}">Login</a>
    <a href="{{ url_for('logout') }}">Logout</a>
    <a href="{{ url_for('register') }}">Register</a>
</body>

login.html

<body>
    {% with messages = get_flashed_messages(with_categories=true) %}
        {% if messages %}
            {% for category, message in messages %}
                <div style="color:red;">{{ message }}</div>
            {% endfor %}
        {% endif %}
    {% endwith %}
    <h2>Login</h2>
    <form action="" method="post">
        {{ form.csrf_token }}
        <div>
            {{ form.email.label }}
            {{ form.email }}
            {% for error in form.email.errors %}
            <p style="color:red;">{{ error }}</p>
            {% endfor %}
        </div>

        <div>
            {{ form.password.label }}
            {{ form.password }}
            {% for error in form.password.errors %}
            <p style="color:red;">{{ error }}</p>
            {% endfor %}
        </div>

        <div>
            {{ form.remember_me }} Remember me
        </div>

        <div>
            {{ form.submit }}
        </div>
    </form>
</body>

register.html

<body>
    {% with messages = get_flashed_messages(with_categories=true) %}
        {% if messages %}
            {% for category, message in messages %}
                <div style="color:red;">{{ message }}</div>
            {% endfor %}
        {% endif %}
    {% endwith %}
    <h2>Register</h2>
    <form action="" method="post">
        {{ form.csrf_token }}
        <div>
            {{ form.email.label }}
            {{ form.email }}
            {% for error in form.email.errors %}
            <p style="color:red;">{{ error }}</p>
            {% endfor %}
        </div>

        <div>
            {{ form.password.label }}
            {{ form.password }}
            {% for error in form.password.errors %}
            <p style="color:red;">{{ error }}</p>
            {% endfor %}
        </div>
        <div>
            {{ form.password2.label }}
            {{ form.password2 }}
            {% for error in form.password2.errors %}
            <p style="color:red;">{{ error }}</p>
            {% endfor %}
        </div>

        <div>
            {{ form.submit }}
        </div>
    </form>
</body>

5. Update __init__.py

Lastly we need to update __init__.py to ensure we initialise flask-login and import all our Python files.

from flask import Flask
from config import Config
from flask_sqlalchemy import SQLAlchemy
from flask_migrate import Migrate
from flask_login import LoginManager

app = Flask(__name__)

app.config.from_object(Config)
db = SQLAlchemy(app)
migrate = Migrate(app, db, compare_type=True)
login_manager = LoginManager(app)

from . import models, routes, forms

The final project directory should now look like this.

Run Flask, go to http://localhost:5000 and you should be able to register as a new user into your database and perform login and logout.

Enjoy!