Fix subclass defaults incorrectly taken from base class

CHANGELOG.rst

@@ -12,13 +12,22 @@ The semantic versioning only considers the public API as described in
 paths are considered internals and can change in minor and patch releases.
 
 
+v4.40.2 (2025-07-??)
+--------------------
+
+Fixed
+^^^^^
+- Subclass defaults incorrectly taken from base class (`#743
+  <https://github.com/omni-us/jsonargparse/pull/743>`__).
+
+
 v4.40.1 (2025-07-24)
 --------------------
 
 Fixed
 ^^^^^
-- ``print_shtab`` incorrectly parsed from environment variable (`#725
-  <https://github.com/omni-us/jsonargparse/pull/725>`__).
+- ``print_shtab`` incorrectly parsed from environment variable (`#726
+  <https://github.com/omni-us/jsonargparse/pull/726>`__).
 - ``adapt_class_type`` used a locally defined `partial_instance` wrapper
   function that is not pickleable (`#728
   <https://github.com/omni-us/jsonargparse/pull/728>`__).

jsonargparse/_typehints.py

@@ -1081,7 +1081,8 @@ def adapt_typehints(
             )
 
         try:
-            val_class = import_object(resolve_class_path_by_name(typehint, val["class_path"]))
+            class_path = resolve_class_path_by_name(typehint, val["class_path"])
+            val_class = import_object(class_path)
             if is_instance_or_supports_protocol(val_class, typehint):
                 return val_class  # importable instance
             if is_protocol(val_class):
@@ -1098,10 +1099,14 @@ def adapt_typehints(
             elif prev_implicit_defaults:
                 inner_parser = ActionTypeHint.get_class_parser(typehint, sub_add_kwargs)
                 prev_val.init_args = inner_parser.get_defaults()
+                if prev_val.class_path != class_path:
+                    inner_parser = ActionTypeHint.get_class_parser(val_class, sub_add_kwargs)
+                    for key in inner_parser.get_defaults().keys():
+                        prev_val.init_args.pop(key, None)
             if not_subclass:
                 msg = "implement protocol" if is_protocol(typehint) else "correspond to a subclass of"
                 raise_unexpected_value(f"Import path {val['class_path']} does not {msg} {typehint.__name__}")
-            val["class_path"] = get_import_path(val_class)
+            val["class_path"] = class_path
             val = adapt_class_type(val, serialize, instantiate_classes, sub_add_kwargs, prev_val=prev_val)
         except (ImportError, AttributeError, AssertionError, ArgumentError) as ex:
             class_path = val if isinstance(val, str) else val["class_path"]

jsonargparse_tests/test_subclasses.py

@@ -51,6 +51,24 @@ def test_subclass_basics(parser, type):
     assert init["op"] is None
 
 
+class BaseClassDefault:
+    def __init__(self, param: str = "base_default"):
+        self.param = param
+
+
+class SubClassDefault(BaseClassDefault):
+    def __init__(self, param: str = "sub_default"):
+        super().__init__(param=param)
+
+
+def test_subclass_defaults(parser):
+    parser.add_subclass_arguments(BaseClassDefault, "cls")
+    cfg = parser.parse_args(["--cls=BaseClassDefault"])
+    assert cfg.cls.init_args.param == "base_default"
+    cfg = parser.parse_args(["--cls=SubClassDefault"])
+    assert cfg.cls.init_args.param == "sub_default"
+
+
 def test_subclass_init_args_in_subcommand(parser, subparser):
     subparser.add_subclass_arguments(Calendar, "cal", default=lazy_instance(Calendar))
     subcommands = parser.add_subcommands()