geojson_feature()

altair/utils/tests/test_geojson.py

@@ -151,3 +151,21 @@ def test_geo_pandas():
     assert dct['data']['format'] == {'type':'json','property':'features'}
     assert (gpd.GeoDataFrame.from_features(dct['data']['values']) == data).all().all()
     assert dct['encoding'] == {'fill': {'field': 'properties.prop', 'type': 'quantitative'}}
+
+def test_geojson_feature():
+    Chart = lambda data,**arg: alt.Chart(alt.geojson_feature(data,'test_prop')
+                                        ).mark_geoshape().project().encode(**arg)
+
+    # Fake GeoInterface
+    data = _create_fake_geo_interface() 
+    dct = Chart(data).to_dict() 
+
+    assert dct['data']['format'] == {'type':'json','property':'test_prop'}
+    assert dct['data']['values'] == data.__geo_interface__
+
+    # url
+    data = "url.json"
+    dct = Chart(data).to_dict() 
+
+    assert dct['data']['format'] == {'type':'json','property':'test_prop'}
+    assert dct['data']['url'] == data

altair/vegalite/v2/api.py

@@ -1295,3 +1295,31 @@ def topo_feature(url, feature, **kwargs):
     """
     return core.UrlData(url=url, format=core.TopoDataFormat(type='topojson',
                                                          feature=feature, **kwargs))
+def geojson_feature(data, feature, **kwargs):
+    """A convenience function for extracting features from a geojson object or url
+
+    Parameters
+    ----------
+    data : anyOf(string, geojson.GeoJSON)
+        string is interpreted as URL from which to load the data set.
+        geojson.GeoJSON is interpreted as data set itself.
+
+    feature : string
+        The JSON property containing the GeoJSON object set to convert to 
+        a GeoJSON feature collection. For example ``features[0].geometry``.
+
+    **kwargs :
+        additional keywords passed to JsonDataFormat
+
+    """
+    if isinstance(data, six.string_types):
+        return core.UrlData(url=data, format=core.JsonDataFormat(type='json',
+                                                         property=feature, **kwargs))
+    elif hasattr(data,'__geo_interface__'):
+        return core.InlineData(values=data.__geo_interface__ , format=core.JsonDataFormat(type='json',
+                                                         property=feature, **kwargs))
+    else:
+        warnings.warn("data of type {0} not recognized".format(type(data)))
+        return data
+
+