validate basemap style is a url (#497)

### Change list

- Add validation that the basemap style input is a HTTP url

Closes #485

lonboard/_map.py

@@ -14,7 +14,7 @@
 from lonboard._layer import BaseLayer
 from lonboard._viewport import compute_view
 from lonboard.basemap import CartoBasemap
-from lonboard.traits import DEFAULT_INITIAL_VIEW_STATE, ViewStateTrait
+from lonboard.traits import DEFAULT_INITIAL_VIEW_STATE, BasemapUrl, ViewStateTrait
 from lonboard.types.map import MapKwargs
 
 if TYPE_CHECKING:
@@ -156,13 +156,13 @@ def __init__(
     - Default: `5`
     """
 
-    basemap_style = traitlets.Unicode(CartoBasemap.PositronNoLabels).tag(sync=True)
+    basemap_style = BasemapUrl(CartoBasemap.PositronNoLabels)
     """
-    A MapLibre-compatible basemap style.
+    A URL to a MapLibre-compatible basemap style.
 
     Various styles are provided in [`lonboard.basemap`](https://developmentseed.org/lonboard/latest/api/basemap/).
 
-    - Type: `str`
+    - Type: `str`, holding a URL hosting a basemap style.
     - Default
       [`lonboard.basemap.CartoBasemap.PositronNoLabels`][lonboard.basemap.CartoBasemap.PositronNoLabels]
     """

lonboard/traits.py

@@ -8,6 +8,7 @@
 
 import warnings
 from typing import Any, List, Optional, Set, Tuple, Union, cast
+from urllib.parse import urlparse
 
 import matplotlib as mpl
 import numpy as np
@@ -968,3 +969,26 @@ def validate(
 
         self.error(obj, value)
         assert False
+
+
+class BasemapUrl(traitlets.Unicode):
+    def __init__(
+        self: TraitType,
+        *args,
+        **kwargs: Any,
+    ) -> None:
+        super().__init__(*args, **kwargs)
+        self.tag(sync=True)
+
+    def validate(self, obj: Any, value: Any) -> Any:
+        value = super().validate(obj, value)
+
+        try:
+            parsed = urlparse(value)
+        except:  # noqa
+            self.error(obj, value, info="to be a URL")
+
+        if not parsed.scheme.startswith("http"):
+            self.error(obj, value, info="to be a HTTP(s) URL")
+        else:
+            return value

tests/test_map.py

@@ -1,6 +1,7 @@
 import geopandas as gpd
 import pytest
 import shapely
+from traitlets import TraitError
 
 from lonboard import Map, ScatterplotLayer, SolidPolygonLayer
 
@@ -27,3 +28,8 @@ def allow_single_layer():
 
     layer = SolidPolygonLayer.from_geopandas(gdf)
     _m = Map(layer)
+
+
+def test_map_basemap_non_url():
+    with pytest.raises(TraitError, match=r"expected to be a HTTP\(s\) URL"):
+        _m = Map([], basemap_style="hello world")