Fix dataclass nested in a list not setting defaults (#357).

CHANGELOG.rst

@@ -19,6 +19,8 @@ Fixed
 ^^^^^
 - Remove private ``linked_targets`` parameter from API Reference (`#317
   <https://github.com/omni-us/jsonargparse/issues/317>`__).
+- Dataclass nested in list not setting defaults (`#357
+  <https://github.com/omni-us/jsonargparse/issues/357>`__)
 
 
 v4.24.0 (2023-08-23)

jsonargparse/_typehints.py

@@ -576,6 +576,7 @@ def adapt_typehints(
     instantiate_classes=False,
     prev_val=None,
     append=False,
+    list_item=False,
     enable_path=False,
     sub_add_kwargs=None,
 ):
@@ -717,7 +718,7 @@ def adapt_typehints(
             for n, v in enumerate(val):
                 if isinstance(prev_val, list) and len(prev_val) == len(val):
                     adapt_kwargs_n = {**adapt_kwargs, "prev_val": prev_val[n]}
-                val[n] = adapt_typehints(v, subtypehints[0], **adapt_kwargs_n)
+                val[n] = adapt_typehints(v, subtypehints[0], list_item=True, **adapt_kwargs_n)
 
     # Dict, Mapping
     elif typehint_origin in mapping_origin_types:
@@ -802,7 +803,7 @@ def adapt_typehints(
         if serialize:
             val = load_value(parser.dump(val, **dump_kwargs.get()))
         elif isinstance(val, (dict, Namespace)):
-            val = parser.parse_object(val, defaults=sub_defaults.get())
+            val = parser.parse_object(val, defaults=sub_defaults.get() or list_item)
         elif isinstance(val, NestedArg):
             val = parser.parse_args([f"--{val.key}={val.val}"])
         else:

jsonargparse_tests/test_dataclass_like.py

@@ -98,6 +98,23 @@ def test_add_dataclass_arguments(parser, subtests):
             parser.add_dataclass_arguments(MixedClass, "c")
 
 
+@dataclasses.dataclass
+class NestedDefaultsA:
+    x: list = dataclasses.field(default_factory=list)
+    v: int = 1
+
+
+@dataclasses.dataclass
+class NestedDefaultsB:
+    a: List[NestedDefaultsA]
+
+
+def test_add_dataclass_nested_defaults(parser):
+    parser.add_dataclass_arguments(NestedDefaultsB, "data")
+    cfg = parser.parse_args(["--data.a=[{}]"])
+    assert cfg.data == Namespace(a=[Namespace(x=[], v=1)])
+
+
 class ClassDataAttributes:
     def __init__(
         self,