Implement #370 for Projection extension

Author: duckontheweb

pystac/extensions/projection.py

@@ -237,8 +237,8 @@ def transform(self, v: Optional[List[float]]) -> None:
     def get_schema_uri(cls) -> str:
         return SCHEMA_URI
 
-    @staticmethod
-    def ext(obj: T) -> "ProjectionExtension[T]":
+    @classmethod
+    def ext(cls, obj: T) -> "ProjectionExtension[T]":
         """Extends the given STAC Object with properties from the :stac-ext:`Projection
         Extension <projection>`.
 
@@ -250,12 +250,14 @@ def ext(obj: T) -> "ProjectionExtension[T]":
             pystac.ExtensionTypeError : If an invalid object type is passed.
         """
         if isinstance(obj, pystac.Item):
+            cls.validate_has_extension(obj)
             return cast(ProjectionExtension[T], ItemProjectionExtension(obj))
         elif isinstance(obj, pystac.Asset):
+            cls.validate_has_extension(obj)
             return cast(ProjectionExtension[T], AssetProjectionExtension(obj))
         else:
             raise pystac.ExtensionTypeError(
-                f"File extension does not apply to type {type(obj)}"
+                f"Projection extension does not apply to type {type(obj)}"
             )
 
     @staticmethod

tests/extensions/test_projection.py

@@ -406,6 +406,24 @@ def test_transform(self) -> None:
         # Validate
         proj_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(ProjectionExtension.get_schema_uri())
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = ProjectionExtension.ext(item)
+
+        # Should raise exception if owning Item does not include extension URI
+        asset = item.assets["B8"]
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = ProjectionExtension.ext(asset)
+
+        # Should succeed if Asset has no owner
+        ownerless_asset = pystac.Asset.from_dict(asset.to_dict())
+        _ = ProjectionExtension.ext(ownerless_asset)
+
 
 class ProjectionSummariesTest(unittest.TestCase):
     def setUp(self) -> None: