Fix: Failing to parse list of dataclasses with nested optional dataclass

CHANGELOG.rst

@@ -32,6 +32,8 @@ Fixed
   <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>`__).
+- Failing to parse list of dataclasses with nested optional dataclass (`#527
+  <https://github.com/omni-us/jsonargparse/pull/527>`__).
 
 
 v4.29.0 (2024-05-24)

jsonargparse/_signatures.py

@@ -103,7 +103,7 @@ def add_class_arguments(
             prefix = nested_key + "." if nested_key else ""
             defaults = default.lazy_get_init_args()
             if defaults:
-                defaults = {prefix + k: v for k, v in defaults.items() if k not in skip}
+                defaults = {prefix + k: v for k, v in defaults.__dict__.items() if k not in skip}
                 self.set_defaults(**defaults)  # type: ignore[attr-defined]
 
         return added_args

jsonargparse_tests/test_dataclass_like.py

@@ -186,6 +186,22 @@ def test_add_class_optional_without_default(parser):
     assert parser.parse_args(["--param=null"]) == Namespace(param=None)
 
 
+@dataclasses.dataclass
+class ListOptionalA:
+    x: int
+
+
+@dataclasses.dataclass
+class ListOptionalB:
+    a: Optional[ListOptionalA] = None
+
+
+def test_list_nested_optional_dataclass(parser):
+    parser.add_argument("--b", type=List[ListOptionalB])
+    cfg = parser.parse_args(['--b=[{"a":{"x":1}}]'])
+    assert cfg.b == [Namespace(a=Namespace(x=1))]
+
+
 def test_add_argument_dataclass_type(parser):
     parser.add_argument("--b", type=DataClassB, default=DataClassB(b1=7.0))
     cfg = parser.get_defaults()