Introduction

In this tutorial, we will discuss how to implement user authentication in a Flask web application using Flask-Login. User authentication is a crucial feature of any web application that requires users to log in to access certain functionalities.

What you will Learn

• How to set up Flask-Login for user authentication
• How to log users in and out
• How to remember user sessions

Prerequisites

• Basic knowledge of Python
• A basic understanding of Flask
• Python and Flask installed on your machine

Step-by-Step Guide

Flask-Login provides user session management for Flask. It handles the common tasks of logging in, logging out, and remembering users' sessions over extended periods.

Installation

To get started, you need to install Flask-Login. It can be installed via pip:

pip install flask-login

Code Examples

Now let's proceed with a step-by-step guide on how to implement user authentication.

1. Setting up Flask-Login

First, you need to setup Flask-Login in your application.

from flask import Flask
from flask_login import LoginManager

app = Flask(__name__)
login_manager = LoginManager()
login_manager.init_app(app)

2. User Loader Function

Flask-Login uses a user loader function that you provide to reload the user from the ID stored in the session. Here is an example:

from flask_login import UserMixin

class User(UserMixin):
    pass

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

3. Logging Users In and Out

To log a user in, you should call login_user(). To log out a user, call logout_user().

from flask_login import login_user, logout_user

# somewhere in your login route
@login.route("/login", methods=["GET", "POST"])
def login():
    # assume user authentication has been done
    user = User()
    login_user(user)

# somewhere in your logout route
@login.route("/logout")
def logout():
    logout_user()

4. Remembering Users

If you want users to stay logged in after browser closes, you can pass remember=True to the login_user() function.

login_user(user, remember=True)

Summary

In this tutorial, we've learned how to implement user authentication in a Flask web application using Flask-Login. We've covered how to set up Flask-Login, how to load users, how to log users in and out, and how to remember user sessions.

Practice Exercises

1. Exercise 1: Create a Flask application and set up Flask-Login.
2. Exercise 2: Create a login route that logs a user in and redirects to a protected page.
3. Exercise 3: Create a logout route that logs the user out and redirects to the login page.

Solutions

# Solution for Exercise 1
from flask import Flask
from flask_login import LoginManager

app = Flask(__name__)
login_manager = LoginManager()
login_manager.init_app(app)

# Solution for Exercise 2
from flask import redirect, url_for
from flask_login import login_user

@app.route("/login", methods=["GET", "POST"])
def login():
    # Assuming user authentication is done
    user = User()
    login_user(user)
    return redirect(url_for('protected'))

# Solution for Exercise 3
from flask_login import logout_user

@app.route("/logout")
def logout():
    logout_user()
    return redirect(url_for('login'))

Keep practicing and exploring more about Flask-Login to gain a deeper understanding. Happy coding!