Quote GeoJson identifier properties that aren't valid JS identifiers (#2296)

When GeoJson features have no unique `id`, `find_identifier` picks a
property with unique values and emits it with dot notation in the styler
and highlighter `switch`. A key like "Time (UTC)" became
`feature.properties.Time (UTC)`, which Javascript runs as a call and throws
`ReferenceError: UTC is not defined`, so nothing in the layer was drawn
(#1782). A key like "property-key" turned into a subtraction, and a key
with a dot ("osm.id") made `get_feature_id` raise `KeyError` while
rendering.

Keys that aren't identifiers now use bracket notation with an escaped
JSON string, and `get_feature_id` reads that form back.

Co-authored-by: adarshsm <24850536+adarshsm@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: adarshsm

folium/features.py

@@ -33,7 +33,7 @@
 from folium.elements import JSCSSMixin
 from folium.folium import Map
 from folium.map import Class, FeatureGroup, Icon, Layer, Marker, Popup, Tooltip
-from folium.template import Template
+from folium.template import Template, _to_escaped_json
 from folium.utilities import (
     JsCode,
     TypeBoundsReturn,
@@ -829,7 +829,11 @@ def find_identifier(self) -> str:
                     if isinstance(feat["properties"].get(key, None), (str, int))
                 }
                 if len(unique_values) == len(feats):
-                    return f"feature.properties.{key}"
+                    if key.isidentifier():
+                        return f"feature.properties.{key}"
+                    # Keys like "Time (UTC)" or "@id" need bracket notation
+                    # to be valid Javascript.
+                    return f"feature.properties[{_to_escaped_json(key)}]"
         # We add an 'id' field with a unique value to the data.
         if self.embed:
             for i, feature in enumerate(feats):
@@ -910,8 +914,13 @@ def _create_mapping(self, func: Callable, switch: str) -> TypeStyleMapping:
 
     def get_feature_id(self, feature: dict) -> Union[str, int]:
         """Return a value identifying the feature."""
-        fields = self.feature_identifier.split(".")[1:]
-        value = functools.reduce(operator.getitem, fields, feature)
+        prefix = "feature.properties["
+        if self.feature_identifier.startswith(prefix):
+            key = json.loads(self.feature_identifier[len(prefix) : -1])
+            value = feature["properties"][key]
+        else:
+            fields = self.feature_identifier.split(".")[1:]
+            value = functools.reduce(operator.getitem, fields, feature)
         assert isinstance(value, (str, int))
         return value
 

tests/test_features.py

@@ -262,20 +262,28 @@ def _assert_id_got_added(data):
     assert geojson.find_identifier() == "feature.id"
     assert geojson.data["features"][0]["id"] == "this-is-an-id"
 
+    data_with_unique_properties = _create(
+        {"property_key": "some-value"},
+        {"property_key": "another-value"},
+    )
+    geojson = GeoJson(data_with_unique_properties)
+    assert geojson.find_identifier() == "feature.properties.property_key"
+
+    # Keys that aren't valid Javascript identifiers need bracket notation.
     data_with_unique_properties = _create(
         {"property-key": "some-value"},
         {"property-key": "another-value"},
     )
     geojson = GeoJson(data_with_unique_properties)
-    assert geojson.find_identifier() == "feature.properties.property-key"
+    assert geojson.find_identifier() == 'feature.properties["property-key"]'
 
     data_with_unique_properties = _create(
         {"property-key": 42},
         {"property-key": 43},
         {"property-key": "or a string"},
     )
     geojson = GeoJson(data_with_unique_properties)
-    assert geojson.find_identifier() == "feature.properties.property-key"
+    assert geojson.find_identifier() == 'feature.properties["property-key"]'
 
     # The test cases below have no id field or unique property,
     # so an id will be added to the data.
@@ -344,6 +352,35 @@ def _assert_id_got_added(data):
     assert geojson.data["features"][0]["id"] == "0"
 
 
+@pytest.mark.parametrize("key", ["Time (UTC)", "@id", "osm.id"])
+def test_geojson_style_identifier_with_special_characters(key):
+    """Assert #1782 is fixed"""
+    m = Map()
+    data = {
+        "type": "FeatureCollection",
+        "features": [
+            {
+                "type": "Feature",
+                "properties": {key: time},
+                "geometry": {"type": "Point", "coordinates": [0, 0]},
+            }
+            for time in ["10:00", "11:00", "12:00"]
+        ],
+    }
+    geojson = GeoJson(
+        data,
+        style_function=lambda x: {
+            "color": "red" if x["properties"][key] == "10:00" else "blue"
+        },
+    ).add_to(m)
+    rendered = m.get_root().render()
+    assert f"switch(feature.properties[{json.dumps(key)}])" in rendered
+    assert geojson.style_map == {
+        '{"color": "red"}': ["10:00"],
+        "default": '{"color": "blue"}',
+    }
+
+
 def test_geojson_empty_features_with_styling():
     # test we don't fail style function validation when there are no features
     m = Map()