How does ServerErrorMiddleware#__call__ send its response

[chrisinmtown]

Would you please try to find a minute for an ELI5 question? I'm trying to understand the __call__ method of ServerErrorMiddleware. As far as I can tell, if an exception is raised, this method tries to report it back out to the caller. It calls either function debug_response or error_response, both of which build a PlainTextResponse. What I cannot explain is that this method then awaits the object returned by one of those functions. I don't have a SSCCE yet, in my tests Python complains something like TypeError: object is not callable.

I found these related discussions but they don't touch on this issue:

• Unexpected behavior in custom error handers registered for `Exception` class #2933
• Cannot add a custom handler for HTTPExceptions with status_code=500 #2499

Here's the permalink where the method starts: https://github.com/encode/starlette/blob/fa5355442753f794965ae1af0f87f9fec1b9a3de/starlette/middleware/errors.py#L149

I posted an excerpt of code, hope you don't mind, so I could annotate the line that I don't understand.

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        if scope["type"] != "http":
            await self.app(scope, receive, send)
            return

        response_started = False

        async def _send(message: Message) -> None:
            nonlocal response_started, send

            if message["type"] == "http.response.start":
                response_started = True
            await send(message)

        try:
            await self.app(scope, receive, _send)
        except Exception as exc:
            request = Request(scope)
            if self.debug:
                # In debug mode, return traceback responses.
                response = self.debug_response(request, exc)
            elif self.handler is None:
                # Use our default 500 error handler.
                response = self.error_response(request, exc)
            else:
                # Use an installed 500 error handler.
                if is_async_callable(self.handler):
                    response = await self.handler(request, exc)
                else:
                    response = await run_in_threadpool(self.handler, request, exc)

            if not response_started:
                await response(scope, receive, send)                <----- THIS is the line I'm asking about

            # We always continue to raise the exception.
            # This allows servers to log the error, or allows test clients
            # to optionally raise the error within the test case.
            raise exc

Am I missing something really obvious about how the response is sent? Why is the send parameter passed to the freshly created response object? It just seems to me that something is missing, like maybe the line should be something like await send(response, scope, receive). Thanks in advance.

[Kludex]

Would you please try to find a minute for an ELI5 question?

Sure.

As far as I can tell, if an exception is raised, this method tries to report it back out to the caller.

Correct.

TypeError: object is not callable.

Do you have the traceback?

---

Why is the send parameter passed to the freshly created response object?

Because a Response object is an ASGI application by itself, and to run it, you need to pass scope, receive, and send.

It just seems to me that something is missing, like maybe the line should be something like await send(response, scope, receive)

This question makes me think that you don't understand the ASGI protocol itself. I recommend reading about it a bit, but to try to help... Every Response class in Starlette is an app by itself. You can do things like:

from starlette.responses import Response

app = Response(content="Hello world!", status_code=200)

And run with uvicorn main:app.

An app, in ASGI terms, is a callable that receives scope (connection info), receive (callable to receive data from the ASGI server), send (callable to send data to the ASGI server).

[chrisinmtown]

Thank you so much for the INSTANT response. I will keep reading and learning. You can close this, if that's the usual practice.

[Kludex]

Reading the Starlette documentation, and this: https://asgi.readthedocs.io/en/latest/, helps.