Raise PointerToNowhere for non-integer array-index pointer segments

referencing/_core.py

@@ -264,7 +264,14 @@ def pointer(self, pointer: str, resolver: Resolver[D]) -> Resolved[D]:
         segments: list[int | str] = []
         for segment in unquote(pointer[1:]).split("/"):
             if isinstance(contents, Sequence):
-                segment = int(segment)
+                try:
+                    segment = int(segment)
+                except ValueError as value_error:
+                    error = exceptions.PointerToNowhere(
+                        ref=pointer,
+                        resource=self,
+                    )
+                    raise error from value_error
             else:
                 segment = segment.replace("~1", "/").replace("~0", "~")
             try:

referencing/tests/test_core.py

@@ -762,6 +762,18 @@ def test_lookup_non_existent_pointer_to_array_index(self):
             resource=resource,
         )
 
+    def test_lookup_non_integer_pointer_to_array_index(self):
+        resource = Resource.opaque([1, 2, 4, 8])
+        resolver = Registry({"http://example.com/1": resource}).resolver()
+        ref = "http://example.com/1#/foo"
+        with pytest.raises(exceptions.Unresolvable) as e:
+            resolver.lookup(ref)
+        assert e.value == exceptions.PointerToNowhere(
+            ref="/foo",
+            resource=resource,
+        )
+        assert str(e.value) == "'/foo' does not exist within [1, 2, 4, 8]"
+
     def test_lookup_pointer_to_empty_string(self):
         resolver = Registry().resolver_with_root(Resource.opaque({"": {}}))
         assert resolver.lookup("#/").contents == {}