Support base layers in GroupedLayerControl

GroupedLayerControl always passed `null` as the base layers argument of
`L.control.groupedLayers`, so base layers could only go in a separate
regular LayerControl (#1876). An unknown `base_layers` keyword was also
silently forwarded into the JS options.

Add a `base_layers` argument: the given layers are rendered as the base
layer radio buttons of the grouped control and get `control = False`, so
they no longer also show up in a regular LayerControl. Which base layer is
visible on opening still follows each layer's `show`, as with LayerControl.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: adarshsm

folium/plugins/groupedlayercontrol.py

@@ -23,6 +23,10 @@ class GroupedLayerControl(JSCSSMixin, MacroElement):
     exclusive_groups : bool, default True
         Whether to use radio buttons (default) or checkboxes.
         If you want to use both, use two separate instances of this class.
+    base_layers : list, default None
+        Layer objects, typically tile layers, to show as base layers in this
+        control. These are removed from the regular LayerControl. Use the
+        layers' `show` argument to choose which one is shown on opening.
     **kwargs
         Additional (possibly inherited) options. See
         https://leafletjs.com/reference.html#control-layers
@@ -45,7 +49,15 @@ class GroupedLayerControl(JSCSSMixin, MacroElement):
         {% macro script(this,kwargs) %}
 
             L.control.groupedLayers(
+                {%- if this.base_layers %}
+                {
+                    {%- for key, val in this.base_layers.items() %}
+                    {{ key|tojson }} : {{val}},
+                    {%- endfor %}
+                },
+                {%- else %}
                 null,
+                {%- endif %}
                 {
                     {%- for group_name, overlays in this.grouped_overlays.items() %}
                     {{ group_name|tojson }} : {
@@ -65,12 +77,16 @@ class GroupedLayerControl(JSCSSMixin, MacroElement):
         {% endmacro %}
         """)
 
-    def __init__(self, groups, exclusive_groups=True, **kwargs):
+    def __init__(self, groups, exclusive_groups=True, base_layers=None, **kwargs):
         super().__init__()
         self._name = "GroupedLayerControl"
         self.options = remove_empty(**kwargs)
         if exclusive_groups:
             self.options["exclusiveGroups"] = list(groups.keys())
+        self.base_layers = {}
+        for element in base_layers or []:
+            self.base_layers[element.layer_name] = element.get_name()
+            element.control = False
         self.layers_untoggle = set()
         self.grouped_overlays = {}
         for group_name, sublist in groups.items():

tests/plugins/test_grouped_layer_control.py

@@ -39,3 +39,35 @@ def test_grouped_layer_control():
          {fg2.get_name()}.remove();
     """)
     assert expected in out
+
+
+def test_grouped_layer_control_base_layers():
+    m = folium.Map([40.0, 70.0], zoom_start=6, tiles=None)
+    osm = folium.TileLayer("OpenStreetMap", name="osm").add_to(m)
+    topo = folium.TileLayer("OpenTopoMap", name="topo", show=False).add_to(m)
+    fg1 = folium.FeatureGroup(name="g1").add_to(m)
+    fg2 = folium.FeatureGroup(name="g2").add_to(m)
+    groupedlayercontrol.GroupedLayerControl(
+        groups={"groups1": [fg1, fg2]}, base_layers=[osm, topo]
+    ).add_to(m)
+    lc = folium.LayerControl().add_to(m)
+    out = normalize(m._parent.render())
+
+    expected = normalize(f"""
+        L.control.groupedLayers(
+            {{
+                "osm" : {osm.get_name()},
+                "topo" : {topo.get_name()},
+            }},
+            {{
+                "groups1" : {{
+                    "g1" : {fg1.get_name()},
+                    "g2" : {fg2.get_name()},
+                }},
+            }},
+            {{"exclusiveGroups": ["groups1",],}},
+         ).addTo({m.get_name()});
+    """)
+    assert expected in out
+    # The base layers are no longer listed in the regular LayerControl.
+    assert not lc.base_layers