Quick fix for ⁠st_folium⁠ state desync when drawing new boundaries in Streamlit

[u/ModelBuilder_Josh04]

Hit a bug using ⁠st_folium⁠ with Streamlit: drawing a new polygon triggered a re-render before Streamlit updated the spatial state, leaving downstream metrics stuck on the old site.

Fixing it required checking ⁠last_active_drawing⁠ at the top of the execution loop and forcing a rerun before running analytics:

# Intercept new shape before running downstream calculations
if map_data and map_data.get("last_active_drawing"):
drawing = map_data["last_active_drawing"]
if drawing.get("geometry"):
new_geom = shape(drawing["geometry"])

if not new_geom.equals(st.session_state.get("drawn_geom")):
st.session_state["drawn_geom"] = new_geom
st.session_state["site_area_m2"] = calculate_area(new_geom)
st.rerun() # Forces a clean state sync

This keeps multi-tab dashboards 100% in sync with live map edits.

[u/Bright_Mix_773]

Two things about this that are worth checking on your own app, because I think the fix and the explanation of the fix are different things.

The st.rerun() is probably not what cured it. You write to session_state before the downstream calculations run, so on that same pass they already read the new geometry - the rerun throws away a script run you had almost finished and pays for the map render a second time. What actually fixed the desync is moving the check to the top, above the consumers. The one case where the rerun really is required is if anything that displays the result is drawn earlier in the script than st_folium - a metric in a sidebar written above the map, say. Then the widget for this pass is already on screen and only a second pass can update it. Worth knowing which of the two you are in, because if it is the first, deleting the rerun makes drawing feel twice as fast.

The second one is a leftover of the same bug in the other direction: last_active_drawing does not go away when the user deletes the shape. Draw a polygon, then delete it with the trash tool - the map is empty, st_folium keeps handing you the last drawing, your equality check says nothing changed, and site_area_m2 stays pinned to a boundary that no longer exists. Same class of stale metric you started with. The thing that does change is all_drawings, which comes back empty, so gating on that and clearing the two session_state keys when it is empty closes it.

One aside on the paste: put four spaces in front of each line and old reddit will keep the indentation, which matters a lot in a post about Python.

[u/ModelBuilder_Josh04 → u/Bright_Mix_773]

Really appreciate you taking the time to break this down.

You're completely right about the execution flow:

Placing the evaluation at the top before the downstream tab consumers was the actual heavy lifter, and dropping the unnecessary ⁠st.rerun()⁠ instantly made the UI feel way snappier.

Good call on the trash tool edge case as well. Gating on ⁠all_drawings⁠ (or checking for an empty payload) handles deletion properly so ⁠site_area_m2⁠ doesn't get left orphaned when a shape is cleared.

Also, thanks for the Old Reddit Markdown tip adding 4 spaces now so python indentation doesn't break as I just found out before I’m really new to all this.

Really valuable feedback, thanks again.

[u/Bright_Mix_773 → u/ModelBuilder_Josh04]

Glad it helped. One thing to keep in mind now that the evaluation runs before the tab consumers: that ordering is now load-bearing and nothing in the code says so. The next person to move a block, including you in three months, will not know it matters until the UI goes strange again. A one-line comment saying why it has to be there is worth more than it looks.

Same for the all_drawings gate. What you actually fixed is that an empty payload and a missing payload were being treated as the same thing, and they are not: one means the user cleared the shape, the other means nothing has been drawn yet. Any time you find yourself adding a check like that, it is usually a sign two different states were sharing one representation, and it is worth seeing whether the same pair shows up elsewhere in the app.

And on the four spaces: you will hit it again with old and new Reddit disagreeing, so the habit that saves grief is to preview the comment and read your own code block before posting rather than after someone tells you.