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Starlette includes an application class Starlette that nicely ties together all of its other functionality.

from starlette.applications import Starlette
from starlette.responses import PlainTextResponse
from starlette.routing import Route, Mount, WebSocketRoute
from starlette.staticfiles import StaticFiles

def homepage(request):
    return PlainTextResponse('Hello, world!')

def user_me(request):
    username = "John Doe"
    return PlainTextResponse('Hello, %s!' % username)

def user(request):
    username = request.path_params['username']
    return PlainTextResponse('Hello, %s!' % username)

async def websocket_endpoint(websocket):
    await websocket.accept()
    await websocket.send_text('Hello, websocket!')
    await websocket.close()

def startup():
    print('Ready to go')

routes = [
    Route('/', homepage),
    Route('/user/me', user_me),
    Route('/user/{username}', user),
    WebSocketRoute('/ws', websocket_endpoint),
    Mount('/static', StaticFiles(directory="static")),

app = Starlette(debug=True, routes=routes, on_startup=[startup])

Instantiating the application

class starlette.applications.Starlette(debug=False, routes=None, middleware=None, exception_handlers=None, on_startup=None, on_shutdown=None, lifespan=None)

Creates an application instance.


  • debug - Boolean indicating if debug tracebacks should be returned on errors.
  • routes - A list of routes to serve incoming HTTP and WebSocket requests.
  • middleware - A list of middleware to run for every request. A starlette application will always automatically include two middleware classes. ServerErrorMiddleware is added as the very outermost middleware, to handle any uncaught errors occurring anywhere in the entire stack. ExceptionMiddleware is added as the very innermost middleware, to deal with handled exception cases occurring in the routing or endpoints.
  • exception_handlers - A mapping of either integer status codes, or exception class types onto callables which handle the exceptions. Exception handler callables should be of the form handler(request, exc) -> response and may be be either standard functions, or async functions.
  • on_startup - A list of callables to run on application startup. Startup handler callables do not take any arguments, and may be be either standard functions, or async functions.
  • on_shutdown - A list of callables to run on application shutdown. Shutdown handler callables do not take any arguments, and may be be either standard functions, or async functions.
  • lifespan - A lifespan context function, which can be used to perform startup and shutdown tasks. This is a newer style that replaces the on_startup and on_shutdown handlers. Use one or the other, not both.

Storing state on the app instance

You can store arbitrary extra state on the application instance, using the generic app.state attribute.

For example:

app.state.ADMIN_EMAIL = ''

Accessing the app instance

Where a request is available (i.e. endpoints and middleware), the app is available on