from shiny import reactive
from shiny.express import session, ui

@reactive.effect
async def _():
    if session:
        x = {"message": "Hello from Python!"}
        await session.send_custom_message("send_alert", x)

ui.tags.script(
    """
    Shiny.addCustomMessageHandler("send_alert", function(x) {
        document.body.innerHTML = x.message;
    });
    """
)

from shiny import reactive, session
from shiny.express import ui

ui.input_slider("n", "N", 1, 100, 50)

@reactive.effect
def _():
    ui.update_slider("n", value=10)