feat: add an AsIsLayoutStrategy

This simply passes through the existing hrefs, raising a value error if one
doesn't exist.

pystac/layout.py

@@ -517,3 +517,41 @@ def get_item_href(self, item: "Item_Type", parent_dir: str) -> str:
         item_root = join_path_or_url(join_type, parent_dir, "{}".format(item.id))
 
         return join_path_or_url(join_type, item_root, "{}.json".format(item.id))
+
+
+class AsIsLayoutStrategy(HrefLayoutStrategy):
+    """Layout strategy that simply preserves the current href of all objects.
+
+    If any object doesn't have a self href, a ValueError is raised.
+    """
+
+    def get_catalog_href(
+        self, cat: "Catalog_Type", parent_dir: str, is_root: bool
+    ) -> str:
+        href = cat.self_href
+        if href is None:
+            raise ValueError(
+                f"Catalog is missing href, required for AsIsLayoutStrategy: {cat}"
+            )
+        else:
+            return href
+
+    def get_collection_href(
+        self, col: "Collection_Type", parent_dir: str, is_root: bool
+    ) -> str:
+        href = col.self_href
+        if href is None:
+            raise ValueError(
+                f"Collection is missing href, required for AsIsLayoutStrategy: {col}"
+            )
+        else:
+            return href
+
+    def get_item_href(self, item: "Item_Type", parent_dir: str) -> str:
+        href = item.self_href
+        if href is None:
+            raise ValueError(
+                f"Item is missing href, required for AsIsLayoutStrategy: {item}"
+            )
+        else:
+            return href

tests/test_layout.py

@@ -10,6 +10,7 @@
     CustomLayoutStrategy,
     TemplateLayoutStrategy,
     BestPracticesLayoutStrategy,
+    AsIsLayoutStrategy,
     TemplateError,
 )
 from tests.utils import TestCases, ARBITRARY_GEOM, ARBITRARY_BBOX
@@ -420,3 +421,41 @@ def test_produces_layout_for_item(self) -> None:
         href = self.strategy.get_href(item, parent_dir="http://example.com")
         expected = "http://example.com/{}/{}.json".format(item.id, item.id)
         self.assertEqual(href, expected)
+
+
+class AsIsLayoutStrategyTest(unittest.TestCase):
+    def setUp(self) -> None:
+        self.strategy = AsIsLayoutStrategy()
+
+    def test_catalog(self) -> None:
+        cat = pystac.Catalog(id="test", description="test desc")
+        with self.assertRaises(ValueError):
+            self.strategy.get_href(cat, parent_dir="http://example.com", is_root=True)
+        cat.set_self_href("/an/href")
+        href = self.strategy.get_href(
+            cat, parent_dir="https://example.com", is_root=True
+        )
+        self.assertEqual(href, "/an/href")
+
+    def test_collection(self) -> None:
+        collection = TestCases.test_case_8()
+        collection.set_self_href(None)
+        with self.assertRaises(ValueError):
+            self.strategy.get_href(
+                collection, parent_dir="http://example.com", is_root=True
+            )
+        collection.set_self_href("/an/href")
+        href = self.strategy.get_href(
+            collection, parent_dir="https://example.com", is_root=True
+        )
+        self.assertEqual(href, "/an/href")
+
+    def test_item(self) -> None:
+        collection = TestCases.test_case_8()
+        item = next(iter(collection.get_all_items()))
+        item.set_self_href(None)
+        with self.assertRaises(ValueError):
+            self.strategy.get_href(item, parent_dir="http://example.com")
+        item.set_self_href("/an/href")
+        href = self.strategy.get_href(item, parent_dir="http://example.com")
+        self.assertEqual(href, "/an/href")