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Authentication

Starlette offers a simple but powerful interface for handling authentication and permissions. Once you've installed AuthenticationMiddleware with an appropriate authentication backend the request.user and request.auth interfaces will be available in your endpoints.

from starlette.authentication import (
    AuthenticationBackend, AuthenticationError, SimpleUser, UnauthenticatedUser,
    AuthCredentials
)
from starlette.middleware.authentication import AuthenticationMiddleware
from starlette.responses import PlainTextResponse
import base64
import binascii


class BasicAuthBackend(AuthenticationBackend):
    async def authenticate(self, request):
        if "Authorization" not in request.headers:
            return

        auth = request.headers["Authorization"]
        try:
            scheme, credentials = auth.split()
            if scheme.lower() != 'basic':
                return
            decoded = base64.b64decode(credentials).decode("ascii")
        except (ValueError, UnicodeDecodeError, binascii.Error) as exc:
            raise AuthenticationError('Invalid basic auth credentials')

        username, _, password = decoded.partition(":")
        # TODO: You'd want to verify the username and password here,
        #       possibly by installing `DatabaseMiddleware`
        #       and retrieving user information from `request.database`.
        return AuthCredentials(["authenticated"]), SimpleUser(username)


app = Starlette()
app.add_middleware(AuthenticationMiddleware, backend=BasicAuthBackend())


@app.route('/')
async def homepage(request):
    if request.user.is_authenticated:
        return PlainTextResponse('hello, ' + request.user.display_name)
    return PlainTextResponse('hello, you')

Users

Once AuthenticationMiddleware is installed the request.user interface will be available to endpoints or other middleware.

This interface should subclass BaseUser, which provides two properties, as well as whatever other information your user model includes.

  • .is_authenticated
  • .display_name

Starlette provides two built-in user implementations: UnauthenticatedUser(), and SimpleUser(username).

AuthCredentials

It is important that authentication credentials are treated as separate concept from users. An authentication scheme should be able to restrict or grant particular privileges independently of the user identity.

The AuthCredentials class provides the basic interface that request.auth exposes:

  • .scopes

Permissions

Permissions are implemented as an endpoint decorator, that enforces that the incoming request includes the required authentication scopes.

from starlette.authentication import requires


@app.route('/dashboard')
@requires('authenticated')
async def dashboard(request):
    ...

You can include either one or multiple required scopes:

from starlette.authentication import requires


@app.route('/dashboard')
@requires(['authenticated', 'admin'])
async def dashboard(request):
    ...

By default 403 responses will be returned when permissions are not granted. In some cases you might want to customize this, for example to hide information about the URL layout from unauthenticated users.

from starlette.authentication import requires


@app.route('/dashboard')
@requires(['authenticated', 'admin'], status_code=404)
async def dashboard(request):
    ...

Alternatively you might want to redirect unauthenticated users to a different page.

from starlette.authentication import requires


@app.route('/homepage')
async def homepage(request):
    ...


@app.route('/dashboard')
@requires('authenticated', redirect='homepage')
async def dashboard(request):
    ...

For class-based endpoints, you should wrap the decorator around a method on the class.

@app.route("/dashboard")
class Dashboard(HTTPEndpoint):
    @requires("authenticated")
    async def get(self, request):
        ...

Custom authentication error responses

You can customise the error response sent when a AuthenticationError is raised by an auth backend:

def on_auth_error(request: Request, exc: Exception):
    return JSONResponse({"error": str(exc)}, status_code=401)

app.add_middleware(AuthenticationMiddleware, backend=BasicAuthBackend(), on_error=on_auth_error)