Introduction to Basic User Authentication

Welcome to another FastAPI course, where we will delve into creating a robust authentication system. Authentication is vital in any web application, as it helps ensure that only authorized users can access certain features or data. Today, we'll set up a basic login endpoint to validate user credentials, giving us a solid foundation to build more advanced authentication systems in future lessons.

User authentication is the process of verifying the identity of a user attempting to access a system. The most basic form involves checking whether a submitted username and password match those stored in a database. For this lesson, we will use a simple dictionary as our user database. This allows us to focus on the concepts without worrying about database setup.

Before we proceed with building our authentication system, let's set the groundwork for our FastAPI project. To kick things off, we’ll create a FastAPI app instance and set up a mock database to store user credentials.

from fastapi import FastAPI

app = FastAPI()

# Mock database with usernames and passwords
users_db = {"user1": "pass1", "user2": "pass2"}

We begin by importing the necessary components from FastAPI. Next, we instantiate our FastAPI app and define users_db, a simple dictionary that acts as our mock database containing usernames and passwords.

Next, we need to create a function to authenticate users by verifying their username and password.

Let's define the function:

from fastapi import HTTPException

# Function to check if user exists in the database
def authenticate_user(username: str, password: str):
    # Check if user exists in the mock database
    if users_db.get(username) == password:
        return {"username": username}
    # Raise an HTTP 401 Unauthorized error if credentials are incorrect
    raise HTTPException(status_code=401, detail="Incorrect username or password")

This function, authenticate_user, accepts a username and a password. It checks if the provided username exists in our users_db and if the password matches. If they do, it returns a dictionary with the username.

If the credentials are incorrect, it raises an HTTPException with a 401 status code, indicating that the authentication failed. Remember, a 401 Unauthorized status code means that the request lacks valid authentication credentials to proceed.