Repeated saves duplicate HTML content and increase file size

[banderlog]

Saving folium.Map to HTML file via map.save('out.html') adds iteratively proportionally as many lines as many objects you have.

I.E. on a second map.save() will be added N strings, on third save will be added 2xN and so on.
N is number of objects + number of tiles + empty strings.
For 10 object it will be 32 strings.

It is a problem for maps with thousands of objects, when you can get +200KB for output file each time.

To Reproduce

import folium


mymap = folium.Map(location=[40.75, -73.98], zoom_start=13)

# 10 objects
locations = [
    [40.7829, -73.9654, "Central Park"],
    [40.7484, -73.9857, "Empire State Building"],
    [40.7580, -73.9855, "Times Square"],
    [40.7061, -73.9969, "Brooklyn Bridge"],
    [40.6892, -74.0445, "Statue of Liberty"],
    [40.7587, -73.9787, "Rockefeller Center"],
    [40.7128, -74.0060, "City Hall"],
    [40.7308, -73.9973, "Washington Square Park"],
    [40.7420, -74.0048, "The High Line"],
    [40.7118, -74.0131, "One World Trade Center"]
]

for lat, lon, name in locations:
    folium.Marker(
        location=[lat, lon],
        icon=folium.Icon(color="blue", icon="info-sign")
    ).add_to(mymap)

#  Save the as HTML file twice in a row
mymap.save("1.html")
mymap.save("2.html")

Output of `diff 1.html 2.html `

579a580,612
>     
>             tile_layer_052f99b21ed1f9bb1c251666826b3405.addTo(map_5c1c64a03c53392f2db23886f23f12a3);
>         
>     
>                 marker_63f8ecad580ed4635b59dcfaf6c3e61f.setIcon(icon_beb42bcc1ea445f94958395ebfe206ea);
>             
>     
>                 marker_b160f28fb6d129ef2f92cdf20ea3e4d6.setIcon(icon_e75ca799208df661741ebbf502942fca);
>             
>     
>                 marker_53b65bb222dd30de59a2e63245de6307.setIcon(icon_68748d7f297e38784cecd742f031d88b);
>             
>     
>                 marker_25b58c032f3524af0221ac1207168844.setIcon(icon_32131f76c1960e3d1549864864743cd2);
>             
>     
>                 marker_334210d9a39502bfdfed7afdbd9e79c3.setIcon(icon_79f631e614c35fa625cb9ebf66332aa3);
>             
>     
>                 marker_da7f6dc7f513fca1304a2326ea2f0816.setIcon(icon_cbf658a28b584b3cc19f9f6bb4e79bd2);
>             
>     
>                 marker_bf430c5d7c07635d674ed2a334bb8e75.setIcon(icon_2a596a031d157fecbc56bf8fb6cc6780);
>             
>     
>                 marker_bdd2c45a9c4ea0d913663870e08555da.setIcon(icon_b46c071e79f3899b2fb392d7bc213dab);
>             
>     
>                 marker_9dd46997657a85db4ffe1fd23d60ceb2.setIcon(icon_6defd56ba4021afd9de3f8e7297d13eb);
>             
>     
>                 marker_9623e338468155be7219352ee3f2ba7a.setIcon(icon_dd2bb17bb8228efb1570ab10408dc3a2);
>         

Expected behavior

No extra strings added

Environment (please complete the following information):

• jupyter/cli both
• Python version 3.12
• folium version 0.20.0
• branca version 0.8.2

[vraj2131]

Hi, I’d like to work on this if it’s still available.

I reproduced the issue locally on folium 0.20.0 / branca 0.8.2 with Python 3.12 (CLI). Saving the same folium.Map twice adds duplicate HTML on the second save — with 10 markers, diff 1.html 2.html shows ~32 extra lines, matching what you described.

My plan is to trace Map.save() and the HTML rendering path to see why content is being appended instead of regenerated on each save, then add a test to ensure repeated saves produce identical output.

Happy to open a PR once I’ve confirmed no one else is already working on it. Thanks!