The Three Python ASGI Servers

Three prominent ASGI servers are all good options for testing and running your ASGI app: Uvicorn, Hypercorn, and Daphne. This article gives a brief synopsis of each, with examples for command-line invocation.

An ASGI app

First, let's make a simple ASGI app, and save it in the current directory with the filename simpleasgi.py.

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

This app sends the raw path bytes as the HTTP response body. When served with an ASGI server, you should be able to point your browser at something like http://localhost:8000/some/path/to/where/we/want/to/be/ and then view the path, in this case "/some/path/to/where/we/want/to/be/".

Let's try serving this with the various servers.

Setup a virtual environment and install the server(s)

In your favorite terminal, change to the working directory you want to use (create the directory first if necessary), then create a virtual environment like so:

python -m venv venv

Use the Python executable you want to use in the virtual environment. That might be called python, as above, or it might be python3 or python3.8 or python3.9. As seen above, I usually use the name venv for the virtual environment directory name, which happens to be the same name as the venv Python module we are calling.

Now activate the virtual environment with

source ./venv/bin/activate

or, on Windows:

.\venv\Scripts\Activate.ps1

If not using Bash or Powershell, you might look in the ./venv/bin or .\venv\Scripts directory to see other options for CMD, fish, or csh.

Now we can install any or all of the ASGI servers you want to try.

pip install uvicorn
pip install hypercorn
pip install daphne

Or just pip install uvicorn hypercorn daphne and be done with it.

Ready to experiment.

Uvicorn

Uvicorn is the first server I usually choose. It is fast, using uvloop if it can. Uvicorn supports HTTP/1.1 and WebSockets, but does not (yet) support HTTP/2.

Install Uvicorn with pip, as detailed above.

The, run the app with

uvicorn --host 0.0.0.0 --port 8000 simpleasgi:app

Try it out in your browser, changing the path and refreshing as many times as you like.

Hypercorn

Historically, Hypercorn is a gift from the Quart async web framework. Unlike Uvicorn, Hypercorn does indeed support HTTP/2 right now. It can use uvloop as Uvicorn does, or use other event loops, even the one from the less common yet curious (ha!) trio library.

Install Hypercorn with pip, as detailed above.

Now run the app with

hypercorn --bind 0.0.0.0:8000 simpleasgi:app

Try it out in your browser, and prosper.

Daphne

Daphne was the first, if I understand correctly, and comes to us from the Django Channels project. It is implemented using Twisted. Like Hypercorn, Daphne does indeed have current support for HTTP/2, provided you install the necessary Twisted tls and http2 dependencies.

Daphne is stable, and serves as a reference implementation for ASGI.

Install Daphne with pip, as detailed above.

Now run the app with

daphne --bind 0.0.0.0 --port 8000 simpleasgi:app

Fire up your web client, point it at localhost, and enjoy.

Help

uvicorn --help
hypercorn --help
daphne --help