Fix Union with path type and non-path value incorrectly dumped as string

CHANGELOG.rst

@@ -11,14 +11,18 @@ The semantic versioning only considers the public API as described in
 :ref:`api-ref`. Components not mentioned in :ref:`api-ref` or different import
 paths are considered internals and can change in minor and patch releases.
 
+
 v4.42.1 (unreleased)
 --------------------
 
 Fixed
 ^^^^^
 - Prevent extra environment variables in helptext when default_env=True, for
-  version actions and subcommands(`#787
+  version actions and subcommands (`#787
   <https://github.com/omni-us/jsonargparse/pull/787>`__).
+- ``Union`` with path type and non-path value incorrectly dumped as string
+  (`#789 <https://github.com/omni-us/jsonargparse/pull/789>`__).
+
 
 v4.42.0 (2025-10-14)
 --------------------

jsonargparse/typing.py

@@ -220,6 +220,8 @@ def _is_path_type(value, type_class):
 
 
 def _serialize_path(path: Path):
+    if not isinstance(path, Path):
+        raise ValueError("Expected a Path instance.")
     if path_dump_preserve_relative.get() and path.relative != path.absolute:
         return {
             "relative": path._relative,

jsonargparse_tests/test_typehints.py

@@ -1590,3 +1590,9 @@ def mocked_get_import_path(cls):
             subclass_paths = get_all_subclass_paths(ImportClass)
     assert "Failed to import ImportClass" in str(w[0].message)
     assert subclass_paths == []
+
+
+def test_non_path_dump(parser):
+    parser.add_argument("--data", type=Union[Path_fr, Dict[str, List[str]]])
+    cfg = parser.parse_args(['--data={"key": ["value"]}'])
+    assert json_or_yaml_load(parser.dump(cfg)) == {"data": {"key": ["value"]}}