Ignore propagation of model default if not of same type.

Author: kschwab

pydantic_settings/sources.py

@@ -1615,6 +1615,17 @@ def _add_parser_args(
                         model_default=PydanticUndefined,
                     )
             else:
+                # For union of models, ignore model default if not of same type as current model.
+                model_default = (
+                    None
+                    if (
+                        model_default is not PydanticUndefined
+                        and (is_model_class(type(model_default)) or is_pydantic_dataclass(type(model_default)))
+                        and type(model_default) is not model
+                    )
+                    else model_default
+                )
+
                 flag_prefix: str = self._cli_flag_prefix
                 is_append_action = _annotation_contains_types(
                     field_info.annotation, (list, set, dict, Sequence, Mapping), is_strip_annotated=True
@@ -1866,9 +1877,7 @@ def _help_format(self, field_name: str, field_info: FieldInfo, model_default: An
                 _help += f' ({ifdef}required)' if _help else f'({ifdef}required)'
         else:
             default = f'(default: {self.cli_parse_none_str})'
-            if (is_model_class(type(model_default)) or is_pydantic_dataclass(type(model_default))) and hasattr(
-                model_default, field_name
-            ):
+            if is_model_class(type(model_default)) or is_pydantic_dataclass(type(model_default)):
                 default = f'(default: {getattr(model_default, field_name)})'
             elif model_default not in (PydanticUndefined, None) and _is_function(model_default):
                 default = f'(default factory: {self._metavar_format(model_default)})'

tests/test_source_cli.py

@@ -447,6 +447,42 @@ 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 Car(BaseSettings, cli_parse_args=True):
+        driver: Cat | Dog | Bird = Cat(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]
+
+{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