Part II: Hello World

Let’s build a simple ASGI app - one that returns “Hello, World!” for all requests and paths.

async def app(scope, receive, send):
    if scope["type"] == "http":
        await send({
            "type": "http.response.start",
            "status": 200,
            "headers": [(b"content-type", b"text/plain")],
        })
        await send({
            "type": "http.response.body",
            "body": b"Hello, World!",
        })

That’s the complete ASGI application. As mentioned in the previous lesson, ASGI is event-based - http.response.start and http.response.body are two such event types.

As highlighted in Foundation, the scope represents request info that doesn’t change for the overall lifecycle of that request. The scope is a dict that has many items, but for now let’s only worry about type which can be one of the following - http, lifespan, or websocket.

A small tangent here - what’s the difference between WSGI and ASGI? WSGI is synchronous - one request in, one response out. ASGI is async and event-based, which allows it to handle long-lived connections like WebSockets.

The Response Events

We send two events to complete an HTTP response:

http.response.start - Sends the status code and headers. Must be sent first.

status - HTTP status code (200, 404, 500, etc.)
headers - list of (name, value) tuples, both as bytes

http.response.body - Sends the actual content.

body - the response body as bytes

That’s the minimum. Status and headers first, then body.

Running It

Save the code in a file called main.py and run:

uvicorn main:app --reload

Open http://127.0.0.1:8000 in your browser or use curl:

curl http://127.0.0.1:8000

You should see:

Hello, World!

Try any path - /foo, /bar/baz - you’ll get the same response. We’re not doing any routing yet.

What’s Next

This works, but it’s not very useful. In the next lesson, we’ll look at what’s inside scope and how to read the request path, method, and headers.