Support loading a specific nested key from YAML in YamlConfigSettingsSource

pydantic_settings/main.py

@@ -62,6 +62,14 @@ class SettingsConfigDict(ConfigDict, total=False):
     json_file_encoding: str | None
     yaml_file: PathType | None
     yaml_file_encoding: str | None
+    yaml_config_section: str | None
+    """
+    Specifies the top-level key in a YAML file from which to load the settings.
+    If provided, the settings will be loaded from the nested section under this key.
+    This is useful when the YAML file contains multiple configuration sections
+    and you only want to load a specific subset into your settings model.
+    """
+
     pyproject_toml_depth: int
     """
     Number of levels **up** from the current working directory to attempt to find a pyproject.toml
@@ -446,6 +454,7 @@ def _settings_build_values(
         json_file_encoding=None,
         yaml_file=None,
         yaml_file_encoding=None,
+        yaml_config_section=None,
         toml_file=None,
         secrets_dir=None,
         protected_namespaces=('model_validate', 'model_dump', 'settings_customise_sources'),

pydantic_settings/sources/providers/yaml.py

@@ -39,14 +39,28 @@ def __init__(
         settings_cls: type[BaseSettings],
         yaml_file: PathType | None = DEFAULT_PATH,
         yaml_file_encoding: str | None = None,
+        yaml_config_section: str | None = None,
     ):
         self.yaml_file_path = yaml_file if yaml_file != DEFAULT_PATH else settings_cls.model_config.get('yaml_file')
         self.yaml_file_encoding = (
             yaml_file_encoding
             if yaml_file_encoding is not None
             else settings_cls.model_config.get('yaml_file_encoding')
         )
+        self.yaml_config_section = (
+            yaml_config_section
+            if yaml_config_section is not None
+            else settings_cls.model_config.get('yaml_config_section')
+        )
         self.yaml_data = self._read_files(self.yaml_file_path)
+
+        if self.yaml_config_section:
+            try:
+                self.yaml_data = self.yaml_data[self.yaml_config_section]
+            except KeyError:
+                raise KeyError(
+                    f'yaml_config_section key "{self.yaml_config_section}" not found in {self.yaml_file_path}'
+                )
         super().__init__(settings_cls, self.yaml_data)
 
     def _read_file(self, file_path: Path) -> dict[str, Any]:

tests/test_source_yaml.py

@@ -166,3 +166,65 @@ def settings_customise_sources(
 
     s = Settings()
     assert s.model_dump() == {'yaml3': 3, 'yaml4': 4}
+
+
+@pytest.mark.skipif(yaml is None, reason='pyYAML is not installed')
+def test_yaml_config_section(tmp_path):
+    p = tmp_path / '.env'
+    p.write_text(
+        """
+    foobar: "Hello"
+    nested:
+        nested_field: "world!"
+    """
+    )
+
+    class Settings(BaseSettings):
+        nested_field: str
+
+        model_config = SettingsConfigDict(yaml_file=p, yaml_config_section='nested')
+
+        @classmethod
+        def settings_customise_sources(
+            cls,
+            settings_cls: type[BaseSettings],
+            init_settings: PydanticBaseSettingsSource,
+            env_settings: PydanticBaseSettingsSource,
+            dotenv_settings: PydanticBaseSettingsSource,
+            file_secret_settings: PydanticBaseSettingsSource,
+        ) -> tuple[PydanticBaseSettingsSource, ...]:
+            return (YamlConfigSettingsSource(settings_cls),)
+
+    s = Settings()
+    assert s.nested_field == 'world!'
+
+
+@pytest.mark.skipif(yaml is None, reason='pyYAML is not installed')
+def test_invalid_yaml_config_section(tmp_path):
+    p = tmp_path / '.env'
+    p.write_text(
+        """
+    foobar: "Hello"
+    nested:
+        nested_field: "world!"
+    """
+    )
+
+    class Settings(BaseSettings):
+        nested_field: str
+
+        model_config = SettingsConfigDict(yaml_file=p, yaml_config_section='invalid_key')
+
+        @classmethod
+        def settings_customise_sources(
+            cls,
+            settings_cls: type[BaseSettings],
+            init_settings: PydanticBaseSettingsSource,
+            env_settings: PydanticBaseSettingsSource,
+            dotenv_settings: PydanticBaseSettingsSource,
+            file_secret_settings: PydanticBaseSettingsSource,
+        ) -> tuple[PydanticBaseSettingsSource, ...]:
+            return (YamlConfigSettingsSource(settings_cls),)
+
+    with pytest.raises(KeyError, match=r'yaml_config_section key ".*" not found in .+'):
+        Settings()