Middleware can be used to wrap a Quart app instance and alter the ASGI process. A very simple example would be to reject requests based on the presence of a header,

class RejectMiddleware:

    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if "headers" not in scope:
            return await self.app(scope, receive, send)

        for header, value in scope['headers']:
            if header.lower() == b'x-secret' and value == b'very-secret':
                return await self.app(scope, receive, send)

        return await self.error_response(receive, send)

    async def error_response(self, receive, send):
        await send({
            'type': 'http.response.start',
            'status': 401,
            'headers': [(b'content-length', b'0')],
        await send({
            'type': 'http.response.body',
            'body': b'',
            'more_body': False,

Whilst middleware can always be used as a wrapper around the app instance, it is best to assign to and wrap the asgi_app attribute,

quart_app.asgi_app = RejectMiddleware(quart_app.asgi_app)

as this ensures that the middleware is applied in any test code.

You can combine multiple middleware wrappers,

quart_app.asgi_app = RejectMiddleware(quart_app.asgi_app)
quart_app.asgi_app = AdditionalMiddleware(quart_app.asgi_app)

and use any ASGI middleware.


Middleware runs before any Quart code, which means that if the middleware returns a response no Quart functionality nor any Quart extensions will run.