I am using Streamlit to create a Plotly Express choropleth map of counties in Georgia (USA) with shading based on the county population. I also have a slider widget above the map for the user to change the opacity of the map. Here is a full working example of the app, including URL for reading in the data:

import streamlit as st
import geopandas as gpd
import plotly.express as px

# set slider for map layer opacity
opacity = st.slider(
    label='Set map layer opacity',
    min_value=0.2,
    max_value=1.0,
    value=0.6,
    step=0.1
)

# Define a function to fetch and cache the data
@st.cache_data
def fetch_data(url):

    # clean the input URL
    url = url.replace(" ", "%20")

    # Read the GeoJSON data from the URL
    gdf = gpd.read_file(url)

    # Select the required columns for choropleth map
    gdf = gdf[['GEOID', 'TotPop_e22', 'geometry']]

    return gdf

# read in population, geometry of Georgia counties
url = "https://services1.arcgis.com/Ug5xGQbHsD8zuZzM/arcgis/rest/services/ACS 2022 Demographic Population GeoSplitJoined/FeatureServer/9/query?outFields=*&where=1%3D1&f=geojson"
gdf = fetch_data(url)

# create mapping figure
fig = px.choropleth_mapbox(
    gdf,
    geojson=gdf.geometry,
    locations=gdf.index,
    color='TotPop_e22',
    center={"lat": 32.90, "lon": -83.50},
    mapbox_style='carto-positron',
    zoom=6,
    opacity=opacity,
    height=650
    )

# plot the mapping figure
st.plotly_chart(
    fig, 
    theme='streamlit',
    use_container_width=True
    )

The app works fine, but I can’t figure out why the entire application / map re-renders each time the slider changes. As the slider only impacts the visual style of the map, not the underlying data structure or fig object, I am unsure how to keep this from happening.

Desired outcome: I would like the app to only load the data once, when the app initially opens, by using the st.cache_data decorator. Once the app is open, I would like to interact with the map by zooming, panning, etc. and then be able to change the opacity via the slider widget WITHOUT the entire application reloading & the map resetting each time the slider widget gets a new value for the opacity. Thank you for the help.