Write a pure ASGI Middleware to redirect requests

[cyberksh]

Could someone verify if this is the right thing to do? It works as expected but I wanted to ensure that there are no pitfalls to this approach.

import pathlib
import fastapi
import starlette.types as types
import starlette.responses as responses


class StaticRedirectMiddleware:
    def __init__(self, app: fastapi.FastAPI, static_dir: pathlib.Path):
        self.app: fastapi.FastAPI = app
        self.static_dir: pathlib.Path = static_dir

    async def __call__(self, scope: types.Scope, receive: types.Receive, send: types.Send):
        if scope["type"] != "http":
            return await self.app(scope, receive, send)

        response_sent = False

        async def send_wrapper(message: types.Message):
            nonlocal response_sent

            if response_sent:
                return

            if message["type"] == "http.response.start":
                status_code = message["status"]
                if status_code == fastapi.status.HTTP_404_NOT_FOUND:
                    response = responses.FileResponse(self.static_dir / "index.html")
                    await response(scope, receive, send)
                    response_sent = True
                else:
                    await send(message)
            elif message["type"] == "http.response.body":
                if not response_sent:
                    await send(message)

        await self.app(scope, receive, send_wrapper)

[adriangb]

At a glance it looks good to me! My onbly doubt would be handling of if response_sent:. Are you sure you want to do nothing? Not call send()? Raise an error?

[jhominal]

1. They cannot call send because in that case, the FileResponse object will already have returned a full response content - there are very few messages that would be valid to send in such a context;
2. A send in that case is not an error condition, because the underlying app is probably sending a http.response.body message for that 404 response.

[jhominal]

I would only have two notes:

1. You should replace the fastapi.FastAPI with starlette.types.ASGIApp (ASGIApp represents the interface that ASGI apps should conform to, of which FastAPI is an implementation) - and then by replacing fastapi.status with starlette.status you would have only imports from starlette;
2. Instead of replacing the 404 with the content of the index page (which means that any time someone hits you with a request to /anything/not_existing.html, you return the content of index.html), I would have returned a redirection status code (e.g. 302). The difference can be quite meaningful in cases of content indexation by a crawler (and cache), or if there is CSS or JS content on the index page that expects a given URL;

[cyberksh]

Thanks I'll do these.