Abstract:

This article is a step-by-step tutorial on building a simple, secure, session-based user authentication system from scratch using the Python library FastHTML. The guide aims to demystify the process for developers who may be hesitant to implement their own auth solutions. It covers setting up the project, creating a SQLite database with fastlite, and building registration and login forms using FastHTML's Python-based syntax for generating HTML. Key security practices are emphasized, particularly the hashing of user passwords with the passlib library. The tutorial walks through handling form submissions, verifying credentials, managing user sessions to track login status, and protecting specific routes (like a user dashboard) with a custom Python decorator. Finally, it implements a simple logout function, providing a complete, foundational user management system.

Building a Simple Login System in FastHTML: A Step-by-Step Guide

• Project Goal: The article demonstrates how to build a complete, session-based user login and registration system using the FastHTML Python library, making authentication less intimidating for developers.
• Initial Setup: The process begins by installing the necessary packages, python-fasthtml and passlib. A basic FastHTML application is initialized, and a SQLite database is set up using the built-in fastlite utility to create a users table.
• Secure Password Handling: A critical takeaway is the importance of never storing passwords in plain text. The tutorial uses the passlib library to securely hash passwords with the bcrypt algorithm before saving them to the database.
• User Registration: A registration form is created using FastHTML's Python functions, which map directly to HTML elements. The corresponding backend handler checks if a user already exists, hashes the new password, and inserts the new user record into the database.
• Login and Verification: The login handler retrieves a user by their email, then uses passlib's verify function to compare the provided password against the stored hash. This ensures the password is correct without ever exposing the plain-text version.
• Session Management: Upon successful login, the user's email is stored in a cryptographically encrypted session object. This session persists across requests and is used to identify the user as authenticated.
• Protecting Routes with Decorators: To secure pages like a dashboard, a custom Python decorator (@basic_auth) is created. This decorator wraps the route handler, checks if an auth key exists in the session, and returns a "Not Authorized" error if the user is not logged in.
• Logout Functionality: Logging out is achieved by simply deleting the auth key from the user's session and then redirecting them back to the login page using an htmx header.