Comment by ironmagma

The trouble is though that you also have state in Python which can’t necessarily be represented in Javascript, such as a database connection. You could pickle it, but accepting a user-provided pickled value is a security risk. Also, doing so would leak database credentials, and recreating the state on the server every event is expensive anyway (especially when the state is something like a pandas dataframe). It’s better to just have one dedicated machine per application session that holds the state for the duration of the app.