Implement #370 for View Geometry Extension

Author: duckontheweb

pystac/extensions/view.py

@@ -143,8 +143,8 @@ def sun_elevation(self, v: Optional[float]) -> None:
     def get_schema_uri(cls) -> str:
         return SCHEMA_URI
 
-    @staticmethod
-    def ext(obj: T) -> "ViewExtension[T]":
+    @classmethod
+    def ext(cls, obj: T) -> "ViewExtension[T]":
         """Extends the given STAC Object with properties from the :stac-ext:`View
         Geometry Extension <scientific>`.
 
@@ -156,8 +156,10 @@ def ext(obj: T) -> "ViewExtension[T]":
             pystac.ExtensionTypeError : If an invalid object type is passed.
         """
         if isinstance(obj, pystac.Item):
+            cls.validate_has_extension(obj)
             return cast(ViewExtension[T], ItemViewExtension(obj))
         elif isinstance(obj, pystac.Asset):
+            cls.validate_has_extension(obj)
             return cast(ViewExtension[T], AssetViewExtension(obj))
         else:
             raise pystac.ExtensionTypeError(

tests/extensions/test_view.py

@@ -220,6 +220,24 @@ def test_sun_elevation(self) -> None:
         # Validate
         view_item.validate()
 
+    def test_extension_not_implemented(self) -> None:
+        # Should raise exception if Item does not include extension URI
+        item = pystac.Item.from_file(self.example_uri)
+        item.stac_extensions.remove(ViewExtension.get_schema_uri())
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = ViewExtension.ext(item)
+
+        # Should raise exception if owning Item does not include extension URI
+        asset = item.assets["blue"]
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = ViewExtension.ext(asset)
+
+        # Should succeed if Asset has no owner
+        ownerless_asset = pystac.Asset.from_dict(asset.to_dict())
+        _ = ViewExtension.ext(ownerless_asset)
+
 
 class ViewSummariestest(unittest.TestCase):
     def setUp(self) -> None: