Triggering a client callback before the end of the event handler

[faraplay]

The docs say that to trigger a client callback in a server's event handler, you return the list of values to pass to the client. But what if I want to do some other things in the event handler after triggering the callback?
I want something like this:

@sio.event
def my_event(sid, data):
    value_to_return = do_something_with_data(data)

    ### returning the value as in
    # return value_to_return
    ### would end the function here
    trigger_callback(value_to_return)

    do_more_things_with_data(data)

[miguelgrinberg]

The callback is a special signal that tells the other side that the event handler it requested ran to completion, it cannot be used for other purposes.

If you want to send something to the other side while the event handler is running, you can use emit() or call().

[faraplay]

In that case is there a good way to notify the client that the event it emitted has been merely received, and then continue to process the data?

E.g. in the official Javascript library one could write something like the following for the server:

io.on("my_event", (data, callback) => {
    store_data(data);
    callback();
    do_more_things_with_data(data);
});

(one could modify this to pass a value to the callback too)

I suppose I could make a workaround by assigning a unique id to each client event emit, but I wanted to check if I could do something analogous to the above in Python.

[miguelgrinberg]

You can do this:

def my_event(data):
    store_data(data)
    sio.start_background_task(process_data, data)
    return callback_data

Then you write the data processing part in a separate process_data function.