Fix handling of empty node lists

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Python JSONPath Change Log
 
+## Version 0.10.3 (unreleased)
+
+**Fixes**
+
+- Fixed handling of relative and root queries when used as arguments to filter functions. Previously, when those queries resulted in an empty node list, we were converting them to an empty regular list before passing it to functions that accept _ValueType_ arguments. Now, in such cases, we convert empty node lists to the special result _Nothing_, which is required by the spec.
+
 ## Version 0.10.2
 
 **Fixes**

jsonpath/filter.py

@@ -623,7 +623,19 @@ def _unpack_node_lists(
  if func.arg_types[idx] != ExpressionType.NODES and isinstance(
  arg, NodeList
  ):
- _args.append(arg.values_or_singular())
+ if len(arg) == 0:
+ # If the query results in an empty nodelist, the
+ # argument is the special result Nothing.
+ _args.append(UNDEFINED)
+ elif len(arg) == 1:
+ # If the query results in a nodelist consisting of a
+ # single node, the argument is the value of the node
+ _args.append(arg[0].obj)
+ else:
+ # This should not be possible as a non-singular query
+ # would have been rejected when checking function
+ # well-typedness.
+ _args.append(arg)
  else:
  _args.append(arg)
  return _args

jsonpath/function_extensions/length.py

@@ -1,7 +1,9 @@
 """The standard `length` function extension."""
 from collections.abc import Sized
-from typing import Optional
+from typing import Union
 
+from jsonpath.filter import UNDEFINED
+from jsonpath.filter import _Undefined
 from jsonpath.function_extensions import ExpressionType
 from jsonpath.function_extensions import FilterFunction
 
@@ -12,9 +14,13 @@ class Length(FilterFunction):
  arg_types = [ExpressionType.VALUE]
  return_type = ExpressionType.VALUE
 
- def __call__(self, obj: Sized) -> Optional[int]:
- """Return an object's length, or `None` if the object does not have a length."""
+ def __call__(self, obj: Sized) -> Union[int, _Undefined]:
+ """Return an object's length.
+
+ If the object does not have a length, the special _Nothing_ value is
+ returned.
+ """
  try:
  return len(obj)
  except TypeError:
- return None
+ return UNDEFINED