Example application to debug unexpected connection closures when running the code from the Bocadillo tutorial.

Start the server:

uvicorn app:app --log-level=debug
# OR, if you have Heroku CLI:
heroku local

the following clients are available:

• asyncio + websockets:

  • Start the client:

  python -m clients.asyncio ws://localhost:8000/echo
  

  • To send messages, type them in the REPL and hit Enter.

• asyncio + websockets + running input() in the threadpool:

  • Start the client:

  python -m clients.asyncio_threadpool ws://localhost:8000/echo
  

  • To send messages, type them in the REPL and hit Enter.

• trio + trio-websocket:

  • Create a messages file: touch messages.
  • Start the client:

  python -m clients.trio ws://localhost:8000/echo
  

  • To send messages:

  echo "Hello" >> messages

  • Messages are read asynchronously from the file, so the WebSocket client can send and receive pings and pongs as designed, and the connection stays open.

• A browser-based JavaScript client: open your browser at http://localhost:8000.

The client code in the asyncio + websockets client is using input() to get user input. This is a blocking call, which results in other tasks running on the event loop from running. In particular, this prevents the client from exchanging ping and pong messages with the server. After a while, either the server or the client time out waiting for a pong, and the connection is shut down.

Key learning point: don't block the event loop. Make sure all I/O is asynchronous. This isn't always easy, especially when reading from files or stdin. Alternatives to asyncio such as trio have more tools built-in to help you achieve full-async I/O handling.