Union of dataclasses not discarding parameters on class change (#833)

Author: mauvilsa

CHANGELOG.rst

@@ -24,6 +24,8 @@ Fixed
 ^^^^^
 - Choices with an int type not working correctly (`#827
   <https://github.com/omni-us/jsonargparse/pull/827>`__).
+- Union of dataclasses not discarding parameters on class change (`#833
+  <https://github.com/omni-us/jsonargparse/pull/833>`__).
 
 
 v4.45.0 (2025-12-26)

jsonargparse/_typehints.py

@@ -1063,11 +1063,15 @@ def adapt_typehints(
                     prev_implicit_defaults = True
 
         if isinstance(prev_val, (dict, Namespace)) and "class_path" not in prev_val:
-            # implicit prev_val class_path and init_args
-            prev_val = Namespace(class_path=get_import_path(typehint), init_args=Namespace(prev_val))
+            # implicit prev_val init_args
+            prev_val = Namespace(class_path=None, init_args=Namespace(prev_val))
 
         val_input = val
-        val = subclass_spec_as_namespace(val, prev_val)
+        if isinstance(prev_val, (dict, Namespace)) and prev_val["class_path"] is None:
+            type_class_path = Namespace(class_path=get_import_path(typehint))
+            val = subclass_spec_as_namespace(val, type_class_path)
+        else:
+            val = subclass_spec_as_namespace(val, prev_val)
         if val and not is_subclass_spec(val) and "init_args" not in val:
             # implicit val class_path
             val = Namespace(class_path=get_import_path(typehint), init_args=val)

jsonargparse_tests/test_dataclasses.py

@@ -686,6 +686,36 @@ def test_class_path_union_dataclasses(parser):
     assert json_or_yaml_load(parser.dump(cfg))["union"] == {"p1": "x", "p2": 0}
 
 
+@dataclasses.dataclass
+class SubA:
+    a: int = 2
+
+
+@dataclasses.dataclass
+class SubB:
+    b: float = 3.4
+
+
+@dataclasses.dataclass
+class SubAorB:
+    a_or_b: Union[SubA, SubB] = dataclasses.field(default_factory=SubA)
+
+
+def test_union_dataclasses(parser):
+    parser.add_class_arguments(SubAorB, "data")
+
+    cfg = parser.parse_args([])
+    init = parser.instantiate_classes(cfg)
+    assert isinstance(init.data, SubAorB)
+    assert isinstance(init.data.a_or_b, SubA)
+
+    cfg = parser.parse_args(["--data.a_or_b.b=4"])
+    assert cfg.data.a_or_b == Namespace(b=4.0)
+    init = parser.instantiate_classes(cfg)
+    assert isinstance(init.data, SubAorB)
+    assert isinstance(init.data.a_or_b, SubB)
+
+
 if type_alias_type:
     IntOrString = type_alias_type("IntOrString", Union[int, str])