Fix not able to modify init args for callable with class return and default class

CHANGELOG.rst

@@ -28,6 +28,8 @@ Fixed
   space (`#499 <https://github.com/omni-us/jsonargparse/pull/499>`__).
 - ``format_usage()`` not working (`#501
   <https://github.com/omni-us/jsonargparse/issues/501>`__).
+- Not able to modify init args for callable with class return and default class
+  (`#5?? <https://github.com/omni-us/jsonargparse/pull/5??>`__).
 
 
 v4.28.0 (2024-04-17)

jsonargparse/_typehints.py

@@ -980,6 +980,8 @@ def subclass_spec_as_namespace(val, prev_val=None):
         val = Namespace({root_key: val})
         if isinstance(prev_val, str):
             prev_val = Namespace(class_path=prev_val)
+        elif inspect.isclass(prev_val):
+            prev_val = Namespace(class_path=get_import_path(prev_val))
     if isinstance(val, dict):
         val = Namespace(val)
     if "init_args" in val and isinstance(val["init_args"], dict):

jsonargparse_tests/test_typehints.py

@@ -792,6 +792,13 @@ def test_callable_multiple_args_return_type_class(parser, subtests):
             assert f"{__name__}.{name}" in help_str
 
 
+def test_callable_return_class_default_class_override_init_arg(parser):
+    parser.add_argument("--optimizer", type=Callable[[List[float]], Optimizer], default=SGD)
+    cfg = parser.parse_args(["--optimizer.momentum=0.5", "--optimizer.lr=0.05"])
+    assert cfg.optimizer.class_path == f"{__name__}.SGD"
+    assert cfg.optimizer.init_args == Namespace(lr=0.05, momentum=0.5)
+
+
 class StepLR:
     def __init__(self, optimizer: Optimizer, last_epoch: int = -1):
         self.optimizer = optimizer