Class Starlette

starlette/applications.py:19–118

Creates an Starlette application.

class Starlette:
    """Creates an Starlette application."""

    def __init__(
        self: AppType,
        debug: bool = False,
        routes: Sequence[BaseRoute] | None = None,
        middleware: Sequence[Middleware] | None = None,
        exception_handlers: Mapping[Any, ExceptionHandler] | None = None,
        lifespan: Lifespan[AppType] | None = None,
    ) -> None:
        """Initializes the application.

        Parameters:
            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 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.
        """
        self.debug = debug
        self.state = State()
        self.router = Router(routes, lifespan=lifespan)
        self.exception_handlers = {} if exception_handlers is None else dict(exception_handlers)
        self.user_middleware = [] if middleware is None else list(middleware)
        self.middleware_stack: ASGIApp | None = None

    def build_middleware_stack(self) -> ASGIApp:
        debug = self.debug
        error_handler = None
        exception_handlers: dict[Any, ExceptionHandler] = {}

        for key, value in self.exception_handlers.items():
            if key in (500, Exception):
                error_handler = value
            else:
                exception_handlers[key] = value

        middleware = (
            [Middleware(ServerErrorMiddleware, handler=error_handler, debug=debug)]
            + self.user_middleware
            + [Middleware(ExceptionMiddleware, handlers=exception_handlers, debug=debug)]
        )

        app = self.router
        for cls, args, kwargs in reversed(middleware):
            app = cls(app, *args, **kwargs)