Implement #370 for File Info Extension

Author: duckontheweb

pystac/extensions/file.py

@@ -199,7 +199,13 @@ def ext(cls, obj: pystac.Asset) -> "FileExtension":
 
         This extension can be applied to instances of :class:`~pystac.Asset`.
         """
-        return cls(obj)
+        if isinstance(obj, pystac.Asset):
+            cls.validate_has_extension(obj)
+            return cls(obj)
+        else:
+            raise pystac.ExtensionTypeError(
+                f"File Info extension does not apply to type {type(obj)}"
+            )
 
 
 class FileExtensionHooks(ExtensionHooks):

tests/extensions/test_file.py

@@ -50,6 +50,7 @@ def test_apply(self) -> None:
 class FileTest(unittest.TestCase):
     FILE_ITEM_EXAMPLE_URI = TestCases.get_path("data-files/file/item.json")
     FILE_COLLECTION_EXAMPLE_URI = TestCases.get_path("data-files/file/collection.json")
+    PLAIN_ITEM = TestCases.get_path("data-files/item/sample-item.json")
 
     def setUp(self) -> None:
         self.maxDiff = None
@@ -188,3 +189,25 @@ def test_migrates_old_checksum(self) -> None:
             FileExtension.ext(item.assets["noises"]).checksum,
             "90e40210a30d1711e81a4b11ef67b28744321659",
         )
+
+    def test_extension_type_error(self) -> None:
+        item = pystac.Item.from_file(self.FILE_ITEM_EXAMPLE_URI)
+
+        with self.assertRaises(pystac.ExtensionTypeError):
+            _ = FileExtension.ext(item)  # 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.PLAIN_ITEM)
+        asset = item.assets["thumbnail"]
+
+        with self.assertRaises(pystac.ExtensionNotImplemented):
+            _ = FileExtension.ext(asset)
+
+        # Should succeed if Asset has no owner
+        stac_io = pystac.StacIO.default()
+        item_dict = stac_io.read_json(self.FILE_ITEM_EXAMPLE_URI)
+        asset_dict = item_dict["assets"]["measurement"]
+        ownerless_asset = pystac.Asset.from_dict(asset_dict)
+
+        _ = FileExtension.ext(ownerless_asset)