So that's what we do. Have a function on the server, decorate it with @anvil.server.callable, and call it from the client with a function call.

Now we can pass arguments (even keyword arguments), and return values from the function. We don't have to marshal everything into an HTTP request any more — it's just a Python function call.

(The data still passes over HTTP, of course. Actually, we use an encrypted WebSocket.)

OK, so the next question is, "What sorts of data should you be able to pass into, or return out of, these functions?"

Well, strings, numbers, dicts, and lists are easy. That's just JSON. But we want to avoid translations, so we want to pass proper objects from the server to the client.

Unfortunately, this is a web server, serving lots of clients, so it has to be stateless: the server just can't afford to keep these objects in RAM.