Differentiate between failed registry lookups which call retrieve.

Author: Julian

referencing/_core.py

@@ -219,7 +219,7 @@ def __getitem__(self, uri: URI) -> Resource[D]:
             except exceptions.NoSuchResource:
                 raise
             except Exception:
-                raise exceptions.NoSuchResource(ref=uri)
+                raise exceptions.Unretrievable(ref=uri)
 
     def __iter__(self) -> Iterator[URI]:
         """
@@ -414,6 +414,8 @@ def lookup(self, ref: URI) -> Resolved[D]:
                 resource = registry[uri]
             except exceptions.NoSuchResource:
                 raise exceptions.Unresolvable(ref=ref) from None
+            except exceptions.Unretrievable:
+                raise exceptions.Unresolvable(ref=ref)
 
         if fragment.startswith("/"):
             return resource.pointer(

referencing/exceptions.py

@@ -17,6 +17,20 @@ class NoSuchResource(KeyError):
 
     ref: URI
 
+    def __eq__(self, other: Any) -> bool:
+        if self.__class__ is not other.__class__:
+            return NotImplemented
+        return attrs.astuple(self) == attrs.astuple(other)
+
+
+@frozen
+class Unretrievable(KeyError):
+    """
+    The given URI is not present in a registry, and retrieving it failed.
+    """
+
+    ref: URI
+
 
 @frozen
 class CannotDetermineSpecification(Exception):

referencing/tests/test_core.py

@@ -174,6 +174,12 @@ def test_dict_conversion(self):
             .crawl()
         )
 
+    def test_no_such_resource(self):
+        registry = Registry()
+        with pytest.raises(exceptions.NoSuchResource) as e:
+            registry["urn:bigboom"]
+        assert e.value == exceptions.NoSuchResource(ref="urn:bigboom")
+
     def test_combine(self):
         one = Resource.opaque(contents={})
         two = ID_AND_CHILDREN.create_resource({"foo": "bar"})
@@ -388,7 +394,7 @@ def retrieve(uri):
 
         registry = Registry(retrieve=retrieve)
         assert registry["urn:succeed"] == {}
-        with pytest.raises(exceptions.NoSuchResource):
+        with pytest.raises(exceptions.Unretrievable):
             registry["urn:uhoh"]
 
     def test_retrieve_already_available_resource(self):