Don't wrap CannotDetermineSpecification exceptions in retrievers.

They indicate that a resource was indeed retrieved, just we don't
know how to interpret it, so Unretrievable is a lie.

They still will get wrapped by Unresolvable exceptions when used
with Resolver.lookup.

Author: Julian

referencing/_core.py

@@ -249,7 +249,10 @@ def __getitem__(self, uri: URI) -> Resource[D]:
         except LookupError:
             try:
                 return self._retrieve(uri)
-            except exceptions.NoSuchResource:
+            except (
+                exceptions.CannotDetermineSpecification,
+                exceptions.NoSuchResource,
+            ):
                 raise
             except Exception:
                 raise exceptions.Unretrievable(ref=uri)

referencing/tests/test_core.py

@@ -403,7 +403,7 @@ def test_retrieve(self):
         registry = Registry(retrieve=lambda uri: foo)
         assert registry["urn:example"] == foo
 
-    def test_retrieve_error(self):
+    def test_retrieve_arbitrary_exception(self):
         def retrieve(uri):
             if uri == "urn:succeed":
                 return {}
@@ -414,6 +414,25 @@ def retrieve(uri):
         with pytest.raises(exceptions.Unretrievable):
             registry["urn:uhoh"]
 
+    def test_retrieve_no_such_resource(self):
+        def retrieve(uri):
+            if uri == "urn:succeed":
+                return {}
+            raise exceptions.NoSuchResource(ref=uri)
+
+        registry = Registry(retrieve=retrieve)
+        assert registry["urn:succeed"] == {}
+        with pytest.raises(exceptions.NoSuchResource):
+            registry["urn:uhoh"]
+
+    def test_retrieve_cannot_determine_specification(self):
+        def retrieve(uri):
+            return Resource.from_contents({})
+
+        registry = Registry(retrieve=retrieve)
+        with pytest.raises(exceptions.CannotDetermineSpecification):
+            registry["urn:uhoh"]
+
     def test_retrieve_already_available_resource(self):
         def retrieve(uri):
             raise Exception("Oh no!")