Fix List type incorrectly using defaults from previous item (#560)

Author: mauvilsa

CHANGELOG.rst

@@ -19,6 +19,8 @@ Fixed
 ^^^^^
 - ``dict`` types not correctly forwarding previous nested values when parsing
   (`#559 <https://github.com/omni-us/jsonargparse/pull/559>`__).
+- ``List`` type incorrectly using defaults from previous item (`#560
+  <https://github.com/omni-us/jsonargparse/pull/560>`__).
 
 
 v4.32.0 (2024-07-19)

jsonargparse/_typehints.py

@@ -871,10 +871,11 @@ def adapt_typehints(
         elif not isinstance(val, list):
             raise_unexpected_value(f"Expected a {typehint_origin}", val)
         if subtypehints is not None:
-            adapt_kwargs_n = adapt_kwargs
             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]}
+                    adapt_kwargs_n = {**deepcopy(adapt_kwargs), "prev_val": prev_val[n]}
+                else:
+                    adapt_kwargs_n = deepcopy(adapt_kwargs)
                 val[n] = adapt_typehints(v, subtypehints[0], list_item=True, **adapt_kwargs_n)
 
     # Dict, Mapping

jsonargparse_tests/test_dataclass_like.py

@@ -203,6 +203,18 @@ def test_list_nested_optional_dataclass(parser):
     assert cfg.b == [Namespace(a=Namespace(x=1))]
 
 
+@dataclasses.dataclass
+class ItemData:
+    x: int = 1
+    y: str = "one"
+
+
+def test_list_append_defaults(parser):
+    parser.add_argument("--list", type=List[ItemData])
+    cfg = parser.parse_args(["--list+={}", '--list+={"x":2,"y":"two"}', '--list+={"x":3}'])
+    assert cfg.list == [Namespace(x=1, y="one"), Namespace(x=2, y="two"), Namespace(x=3, y="one")]
+
+
 def test_add_argument_dataclass_type(parser):
     parser.add_argument("--b", type=DataClassB, default=DataClassB(b1=7.0))
     cfg = parser.get_defaults()