Inconsistent on_change callback timing for st_folium compared to native Streamlit widgets

[yun881201]

I’ve noticed that st_folium’s on_change callback fires at a different point in the execution flow than native Streamlit widgets. My understanding of Streamlit widget callbacks is that they follow this order:

The widget’s value in st.session_state is updated

Its on_change callback (if any) is executed

The page reruns, with the widget function returning the new value

By contrast, it appears that st_folium only executes its on_change callback when the code reaches the st_folium(...) line during the rerun, rather than immediately after updating session_state. This discrepancy breaks assumptions that “callback runs as a prefix to the script rerunning” and complicates synchronizing map state with custom session-state management.

Can you please clarify:

Is the st_folium callback intentionally designed to fire only at its invocation line, rather than immediately upon state change?

If so, is there a recommended pattern for guaranteeing that on_change callbacks see and update the very latest map state, in the same way native widgets do?

Thank you!

[blackary]

Hi @yun881201,

That's not the intended behavior, and it doesn't seem to be the case in this simple test:

from datetime import datetime

import folium
import streamlit as st
from streamlit_folium import st_folium

m = folium.Map(location=[39.949610, -75.150282], zoom_start=16)

st.write("Hi 1", datetime.now())


def on_change():
    st.write("on_change", datetime.now())


st.write("Hi 2", datetime.now())

st_data = st_folium(m, width=725, on_change=on_change)

st.write("Hi 3", datetime.now())

In this simple test, on_change is printed first in the app, before any of the other st.write statements, and datetime.now() is the earliest

Do you happen to have a simple example that shows the callback happening later in the script? It could be happening in some circumstances, but that's not the intended behavior.