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 == 'X-Secret' and value == '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,

quart_app.asgi_app = RejectMiddleware(quart_app.asgi_app)

This can then be extended and used with any ASGI Middleware and served with any ASGI server.


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