config.py

from pathlib import Path
from typing import Any, Dict, Optional, Union

from hydra import compose, initialize_config_dir
from omegaconf import DictConfig
from pydantic import BaseModel, BaseSettings
from pydantic.env_settings import SettingsSourceCallable

# {python project}/app/config.py
project_dir = Path(__file__).parent.parent

def dictconfig_to_pydict(cfg) -> dict:
    """convert omegaconf.DictConfig to python build-in dict

    If initialized Settings with hydra and
    BaseSetting#changing-priority(env_settings > init_settings ...),
    there is bug when partial override config by environ variable.
    Convert dictconfig to python build-in dict to fix it.
    """
    if not isinstance(cfg, DictConfig):
        return cfg
    return {k: dictconfig_to_pydict(v) for k, v in cfg.items()}

def hydra_config_settings_source(settings: BaseSettings) -> Dict[str, Any]:
    # https://docs.pydantic.dev/usage/settings/#adding-sources
    config_dir, config_name = str(project_dir), 'config.yaml'
    try:
        initialize_config_dir(version_base=None, config_dir=config_dir)
        _cfg = compose(config_name)
        return dictconfig_to_pydict(_cfg)
    except Exception as exc:
        raise ValueError(f"initialize config fail. {config_dir = }; {config_name = }") from exc

class Mongo(BaseModel):
    HOST: str
    PORT: int
    DBNAME: str
    USER: Optional[str] = None
    PASSWORD: Optional[str] = None

class Stuff(BaseModel):
    URL: str
    TIMEOUT: int

class Settings(BaseSettings):
    MONGO: Mongo
    STUFF: Stuff

    class Config:
        case_sensitive = True
        frozen = True
        env_nested_delimiter = "__"

        env_file = ".env"
        env_file_encoding = "utf-8"

        # https://pydantic-docs.helpmanual.io/usage/settings/#changing-priority
        @classmethod
        def customise_sources(
            cls,
            init_settings: SettingsSourceCallable,
            env_settings: SettingsSourceCallable,
            file_secret_settings: SettingsSourceCallable,
        ) -> tuple[SettingsSourceCallable, ...]:
            return env_settings, init_settings, hydra_config_settings_source, file_secret_settings


settings = Settings()

if __name__ == '__main__':
    print(settings)