Fix: Callable that returns class not using required parameter default from lambda

CHANGELOG.rst

@@ -30,6 +30,8 @@ Fixed
   <https://github.com/omni-us/jsonargparse/issues/516>`__).
 - ``--print_config`` failing in some cases (`#517
   <https://github.com/omni-us/jsonargparse/issues/517>`__).
+- Callable that returns class not using required parameter default from lambda
+  (`#523 <https://github.com/omni-us/jsonargparse/pull/523>`__).
 
 
 v4.29.0 (2024-05-24)

jsonargparse/_core.py

@@ -1326,12 +1326,14 @@ def merge_config(self, cfg_from: Namespace, cfg_to: Namespace) -> Namespace:
         Returns:
             A new object with the merged configuration.
         """
-        cfg = cfg_to.clone()
+        cfg_from = cfg_from.clone()
+        cfg_to = cfg_to.clone()
         with parser_context(parent_parser=self):
-            ActionTypeHint.discard_init_args_on_class_path_change(self, cfg, cfg_from)
-        cfg.update(cfg_from)
-        ActionTypeHint.apply_appends(self, cfg)
-        return cfg
+            ActionTypeHint.discard_init_args_on_class_path_change(self, cfg_to, cfg_from)
+        ActionTypeHint.delete_init_args_required_none(cfg_from, cfg_to)
+        cfg_to.update(cfg_from)
+        ActionTypeHint.apply_appends(self, cfg_to)
+        return cfg_to
 
     def _check_value_key(self, action: argparse.Action, value: Any, key: str, cfg: Optional[Namespace]) -> Any:
         """Checks the value for a given action.

jsonargparse/_typehints.py

@@ -396,6 +396,21 @@ def discard_init_args_on_class_path_change(parser_or_action, prev_cfg, cfg):
                     keys = keys[: num + 1] + [k for k in keys[num + 1 :] if not k.startswith(key + ".")]
             num += 1
 
+    @staticmethod
+    def delete_init_args_required_none(cfg_from, cfg_to):
+        for key, val in cfg_from.items(branches=True):
+            if isinstance(val, Namespace) and val.get("class_path") and val.get("init_args"):
+                skip_keys = [
+                    k
+                    for k, v in val.init_args.__dict__.items()
+                    if v is None and cfg_to.get(f"{key}.init_args.{k}") is not None
+                ]
+                if skip_keys:
+                    parser = ActionTypeHint.get_class_parser(val.class_path)
+                    for skip_key in skip_keys:
+                        if skip_key in parser.required_args:
+                            del val.init_args[skip_key]
+
     @staticmethod
     @contextmanager
     def subclass_arg_context(parser):

jsonargparse_tests/test_typehints.py

@@ -816,9 +816,10 @@ def __init__(self, optimizer: Optimizer, last_epoch: int = -1):
 
 
 class ReduceLROnPlateau:
-    def __init__(self, optimizer: Optimizer, monitor: str):
+    def __init__(self, optimizer: Optimizer, monitor: str, factor: float = 0.1):
         self.optimizer = optimizer
         self.monitor = monitor
+        self.factor = factor
 
 
 def test_callable_args_return_type_union_of_classes(parser, subtests):
@@ -846,7 +847,7 @@ def test_callable_args_return_type_union_of_classes(parser, subtests):
         }
         cfg = parser.parse_args([f"--scheduler={value}"])
         assert f"{__name__}.ReduceLROnPlateau" == cfg.scheduler.class_path
-        assert Namespace(monitor="loss") == cfg.scheduler.init_args
+        assert Namespace(monitor="loss", factor=0.1) == cfg.scheduler.init_args
         init = parser.instantiate_classes(cfg)
         scheduler = init.scheduler(optimizer)
         assert isinstance(scheduler, ReduceLROnPlateau)
@@ -948,6 +949,40 @@ def test_callable_zero_args_return_type_class(parser):
     assert activation.negative_slope == 0.05
 
 
+class ModelRequiredCallableArg:
+    def __init__(
+        self,
+        scheduler: Callable[[Optimizer], ReduceLROnPlateau] = lambda o: ReduceLROnPlateau(o, monitor="acc"),
+    ):
+        self.scheduler = scheduler
+
+
+def test_callable_return_class_required_arg_from_default(parser):
+    parser.add_argument("--cfg", action="config")
+    parser.add_argument("--model", type=ModelRequiredCallableArg)
+
+    cfg = parser.parse_args(["--model=ModelRequiredCallableArg"])
+    assert cfg.model.init_args.scheduler.class_path == f"{__name__}.ReduceLROnPlateau"
+    assert cfg.model.init_args.scheduler.init_args == Namespace(monitor="acc", factor=0.1)
+
+    config = {
+        "model": {
+            "class_path": f"{__name__}.ModelRequiredCallableArg",
+            "init_args": {
+                "scheduler": {
+                    "class_path": f"{__name__}.ReduceLROnPlateau",
+                    "init_args": {
+                        "factor": 0.5,
+                    },
+                },
+            },
+        }
+    }
+    cfg = parser.parse_args([f"--cfg={config}"])
+    assert cfg.model.init_args.scheduler.class_path == f"{__name__}.ReduceLROnPlateau"
+    assert cfg.model.init_args.scheduler.init_args == Namespace(monitor="acc", factor=0.5)
+
+
 # lazy_instance tests