Implement #370 for Timestamps Extension

Author: duckontheweb

pystac/extensions/timestamps.py

@@ -116,11 +116,13 @@ def unpublished(self, v: Optional[Datetime]) -> None:
     def get_schema_uri(cls) -> str:
         return SCHEMA_URI
 
-    @staticmethod
-    def ext(obj: T) -> "TimestampsExtension[T]":
+    @classmethod
+    def ext(cls, obj: T) -> "TimestampsExtension[T]":
         if isinstance(obj, pystac.Item):
+            cls.validate_has_extension(obj)
             return cast(TimestampsExtension[T], ItemTimestampsExtension(obj))
         elif isinstance(obj, pystac.Asset):
+            cls.validate_has_extension(obj)
             return cast(TimestampsExtension[T], AssetTimestampsExtension(obj))
         else:
             raise pystac.ExtensionTypeError(

tests/extensions/test_timestamps.py

@@ -185,3 +185,21 @@ def test_unpublished(self) -> None:
 
         # Validate
         timestamps_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(TimestampsExtension.get_schema_uri())
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = TimestampsExtension.ext(item)
+
+        # Should raise exception if owning Item does not include extension URI
+        asset = item.assets["blue"]
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = TimestampsExtension.ext(asset)
+
+        # Should succeed if Asset has no owner
+        ownerless_asset = pystac.Asset.from_dict(asset.to_dict())
+        _ = TimestampsExtension.ext(ownerless_asset)