Authentication

Authentication verifies who the user is.

A common API authentication flow is:

1. The user provides an email and password.
2. The API verifies the credentials.
3. If the credentials are valid, the user is authenticated.
4. If the credentials are invalid, the API returns an authentication error.

In FastAPI, HTTPException can be used to return an error when authentication fails.

Code

from fastapi import FastAPI, HTTPException

app = FastAPI()


def authenticate_user(
    email: str,
    password: str
):
    if email != "tarun@example.com":
        raise HTTPException(
            status_code=401,
            detail="Invalid credentials"
        )

    if password != "123456":
        raise HTTPException(
            status_code=401,
            detail="Invalid credentials"
        )

    return {
        "id": 1,
        "email": email
    }

Understanding the Code

The authenticate_user() function accepts two values:

def authenticate_user(
    email: str,
    password: str
):

The email parameter is expected to be a string, and the password parameter is also expected to be a string.

Checking the Email

The email is checked first:

if email != "tarun@example.com":
    raise HTTPException(
        status_code=401,
        detail="Invalid credentials"
    )

If the email does not match, FastAPI raises an HTTP exception with status code 401.

Checking the Password

The password is then checked:

if password != "123456":
    raise HTTPException(
        status_code=401,
        detail="Invalid credentials"
    )

If the password is incorrect, the API again returns a 401 error.

Returning the User

If both credentials are valid, the function returns the user’s information:

return {
    "id": 1,
    "email": email
}

Successful Output

When valid credentials are provided, the function returns:

{
    "id": 1,
    "email": "tarun@example.com"
}

Authentication Flow

User
  ↓
Email + Password
  ↓
Authenticate User
  ↓
Check Email
  ↓
Check Password
  ↓
Valid Credentials
  ↓
Return User

If the credentials are invalid:

Invalid Credentials
        ↓
HTTPException
        ↓
Status Code: 401

Summary

Authentication is used to verify the identity of a user.