Support pickleability of classes with callable args

CHANGELOG.rst

@@ -11,14 +11,16 @@ 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.
 
-
 v4.40.1 (2025-05-??)
 --------------------
 
 Fixed
 ^^^^^
 - ``print_shtab`` incorrectly parsed from environment variable (`#725
   <https://github.com/omni-us/jsonargparse/pull/725>`__).
+- ``adapt_class_type`` used a locally defined `partial_instance` wrapper
+  function that is not pickleable (`#728
+  <https://github.com/omni-us/jsonargparse/pull/728>`__).
 
 
 v4.40.0 (2025-05-16)

jsonargparse/_typehints.py

@@ -1424,11 +1424,11 @@ def adapt_class_type(
         instantiator_fn = get_class_instantiator()
 
         if partial_classes:
-
-            def partial_instance(*args):
-                return instantiator_fn(val_class, *args, **{**init_args, **dict_kwargs})
-
-            return partial_instance
+            return partial(
+                instantiator_fn,
+                val_class,
+                **{**init_args, **dict_kwargs},
+            )
         return instantiator_fn(val_class, **{**init_args, **dict_kwargs})
 
     prev_init_args = prev_val.get("init_args") if isinstance(prev_val, Namespace) else None

jsonargparse_tests/test_typehints.py

@@ -1239,6 +1239,21 @@ def test_callable_args_return_type_class_subconfig(parser, tmp_cwd):
     assert optimizer.momentum == 0.8
 
 
+def test_callable_args_pickleable(parser, tmp_cwd):
+    config = {
+        "class_path": "Adam",
+        "init_args": {"momentum": 0.8},
+    }
+    Path("optimizer.yaml").write_text(json_or_yaml_dump(config))
+    parser.add_class_arguments(CallableSubconfig, "m", sub_configs=True)
+    cfg = parser.parse_args(["--m.o=optimizer.yaml"])
+    init = parser.instantiate_classes(cfg)
+
+    filepath = str(tmp_cwd) + "/pickled.pkl"
+    with open(filepath, "wb") as f:
+        pickle.dump(init, f)
+
+
 class Module:
     pass