Implement #370 for Label Extension

Author: duckontheweb

pystac/extensions/label.py

@@ -645,7 +645,13 @@ def ext(cls, obj: pystac.Item) -> "LabelExtension":
 
         This extension can be applied to instances of :class:`~pystac.Item`.
         """
-        return cls(obj)
+        if isinstance(obj, pystac.Item):
+            cls.validate_has_extension(obj)
+            return cls(obj)
+        else:
+            raise pystac.ExtensionTypeError(
+                f"Label extension does not apply to type {type(obj)}"
+            )
 
 
 class LabelExtensionHooks(ExtensionHooks):

tests/extensions/test_label.py

@@ -361,3 +361,18 @@ def test_merge_label_overviews_error(self) -> None:
 
         with self.assertRaises(AssertionError):
             _ = overview_1.merge_counts(overview_2)
+
+    def test_extension_type_error(self) -> None:
+        collection = pystac.Collection.from_file(
+            TestCases.get_path("data-files/collections/with-assets.json")
+        )
+        with self.assertRaises(pystac.ExtensionTypeError):
+            _ = LabelExtension.ext(collection)  # type: ignore
+
+    def test_extension_not_implemented(self) -> None:
+        # Should raise exception if Item does not include extension URI
+        item = pystac.Item.from_file(self.label_example_1_uri)
+        item.stac_extensions.remove(LabelExtension.get_schema_uri())
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = LabelExtension.ext(item)