Merge pull request #419 from duckontheweb/fix/369-idempotent-add-to

Make ExtensionManagementMixin.add_to idempotent

Author: Jon Duckworth

CHANGELOG.md

@@ -26,6 +26,8 @@
 - `EOExtension.get_bands` returns `None` for asset without EO bands ([#406](https://github.com/stac-utils/pystac/pull/406))
 - `identify_stac_object_type` returns `None` and `identify_stac_object` raises `STACTypeError` for non-STAC objects
   ([#402](https://github.com/stac-utils/pystac/pull/402))
+- `ExtensionManagementMixin.add_to` is now idempotent (only adds schema URI to
+  `stac_extensions` once per `Item` regardless of the number of calls) ([#419](https://github.com/stac-utils/pystac/pull/419))
 
 ### Removed
 

pystac/extensions/base.py

@@ -100,10 +100,11 @@ def get_schema_uri(cls) -> str:
     @classmethod
     def add_to(cls, obj: S) -> None:
         """Add the schema URI for this extension to the
-        :attr:`pystac.STACObject.stac_extensions` list for the given object."""
+        :attr:`~pystac.STACObject.stac_extensions` list for the given object, if it is
+        not already present."""
         if obj.stac_extensions is None:
             obj.stac_extensions = [cls.get_schema_uri()]
-        else:
+        elif cls.get_schema_uri() not in obj.stac_extensions:
             obj.stac_extensions.append(cls.get_schema_uri())
 
     @classmethod

tests/extensions/test_eo.py

@@ -43,6 +43,7 @@ class EOTest(unittest.TestCase):
     )
     EO_COLLECTION_URI = TestCases.get_path("data-files/eo/eo-collection.json")
     S2_ITEM_URI = TestCases.get_path("data-files/eo/eo-sentinel2-item.json")
+    PLAIN_ITEM = TestCases.get_path("data-files/item/sample-item.json")
 
     def setUp(self) -> None:
         self.maxDiff = None
@@ -52,6 +53,24 @@ def test_to_from_dict(self) -> None:
             item_dict = json.load(f)
         assert_to_from_dict(self, Item, item_dict)
 
+    def test_add_to(self) -> None:
+        item = Item.from_file(self.PLAIN_ITEM)
+        self.assertNotIn(EOExtension.get_schema_uri(), item.stac_extensions)
+
+        # Check that the URI gets added to stac_extensions
+        EOExtension.add_to(item)
+        self.assertIn(EOExtension.get_schema_uri(), item.stac_extensions)
+
+        # Check that the URI only gets added once, regardless of how many times add_to
+        # is called.
+        EOExtension.add_to(item)
+        EOExtension.add_to(item)
+
+        eo_uris = [
+            uri for uri in item.stac_extensions if uri == EOExtension.get_schema_uri()
+        ]
+        self.assertEqual(len(eo_uris), 1)
+
     def test_validate_eo(self) -> None:
         item = pystac.Item.from_file(self.LANDSAT_EXAMPLE_URI)
         item2 = pystac.Item.from_file(self.BANDS_IN_ITEM_URI)