Show HN: Pynecone – Web Apps in Pure Python
A comment at the bottom of the thread (at the time of writing) explains how it works:  In Pynecone, the frontend compiles down to a NextJS/React app, and the backend is a FastAPI server. Events are sent as network calls that update the app state, and the state delta is sent back to the React app, which updates the UI.

EDIT: The gallery is also useful: https://pynecone.io/docs/gallery