Fix default help text for union of submodels.

pydantic_settings/sources.py

@@ -1563,6 +1563,15 @@ def _add_parser_args(
     ) -> ArgumentParser:
         subparsers: Any = None
         alias_path_args: dict[str, str] = {}
+        # Ignore model default if the default is a model and not a subclass of the current model.
+        model_default = (
+            None
+            if (
+                (is_model_class(type(model_default)) or is_pydantic_dataclass(type(model_default)))
+                and not issubclass(type(model_default), model)
+            )
+            else model_default
+        )
         for field_name, field_info in self._sort_arg_fields(model):
             sub_models: list[type[BaseModel]] = self._get_sub_models(model, field_name, field_info)
             alias_names, is_alias_path_only = _get_alias_names(

tests/test_source_cli.py

@@ -447,6 +447,46 @@ class MultilineDoc(BaseSettings, cli_parse_args=True):
         )
 
 
+def test_cli_help_union_of_models(capsys, monkeypatch):
+    class Cat(BaseModel):
+        meow: str = 'meow'
+
+    class Dog(BaseModel):
+        bark: str = 'bark'
+
+    class Bird(BaseModel):
+        caww: str = 'caww'
+        tweet: str
+
+    class Tiger(Cat):
+        roar: str = 'roar'
+
+    class Car(BaseSettings, cli_parse_args=True):
+        driver: Union[Cat, Dog, Bird] = Tiger(meow='purr')
+
+    with monkeypatch.context() as m:
+        m.setattr(sys, 'argv', ['example.py', '--help'])
+
+        with pytest.raises(SystemExit):
+            Car()
+        assert (
+            capsys.readouterr().out
+            == f"""usage: example.py [-h] [--driver JSON] [--driver.meow str] [--driver.bark str]
+                  [--driver.caww str] [--driver.tweet str]
+
+{ARGPARSE_OPTIONS_TEXT}:
+  -h, --help          show this help message and exit
+
+driver options:
+  --driver JSON       set driver from JSON string
+  --driver.meow str   (default: purr)
+  --driver.bark str   (default: bark)
+  --driver.caww str   (default: caww)
+  --driver.tweet str  (ifdef: required)
+"""
+        )
+
+
 def test_cli_help_default_or_none_model(capsys, monkeypatch):
     class DeeperSubModel(BaseModel):
         flag: bool