[adarshsm]

Closes #1876.

The problem

GroupedLayerControl always rendered L.control.groupedLayers(null, {...groups...}, options), so the base layers argument was always null. To get base layers you had to keep a regular LayerControl next to the grouped one, which gives two separate controls. The issue asks for both in one control. The Leaflet plugin already supports this through its first argument, as @hansthen said in the issue.

Passing base_layers= today doesn't raise. It falls into **kwargs and ends up in the JS options object as "base_layers": [...], where the plugin ignores it.

The change

• New base_layers argument (list of layers, default None). The layers are rendered as the first argument of L.control.groupedLayers ({"name": layer, ...}). Without base_layers the output is unchanged (null).
• Like the grouped overlays, those layers get control = False, so a regular LayerControl on the same map doesn't list them a second time.
• Which base layer is visible on opening follows each layer's show, the same as with LayerControl. Pass show=False to all but one base layer.
• Docstring updated.

m = folium.Map(tiles=None)
osm = folium.TileLayer("OpenStreetMap", name="osm").add_to(m)
topo = folium.TileLayer("OpenTopoMap", name="topo", show=False).add_to(m)
...
GroupedLayerControl(groups={"groups1": [fg1, fg2]}, base_layers=[osm, topo]).add_to(m)

Tests

• New test_grouped_layer_control_base_layers checks the rendered L.control.groupedLayers({...base layers...}, {...groups...}, ...) call, and checks that a regular LayerControl added afterwards has no base layers left. It fails on main and passes with this change.
• I also rendered the example map above and ran the generated <script> blocks through node --check to make sure the JS parses.
• pytest tests/plugins tests/test_map.py: 95 passed. The 2 failures (test_timedynamic_geo_json without geodatasets, and test_icon_invalid_marker_colors) also fail on unmodified main in my environment. black --check and ruff check pass on the touched files.

I haven't tested this in a browser.

🤖 Generated with Claude Code

[hansthen]

@adarshsm Looks good! Thanks for picking this up.