Config example with custom toml loaders and the builtin env loader

.env

MY_APP_APP__PASSWORD="secret_foo"
MY_APP_DB__PASSWORD="secret_bar"

config.py

import importlib.resources
from pathlib import Path

from pydantic import BaseModel, BaseSettings, validator
import toml


def default_settings(*_):
    with importlib.resources.path("my_app", "default.toml") as path:
        with path.open() as f:
            return toml.load(f)

def user_settings(*_):
    user_config = Path("user.toml")  # expected to be in cwd
    try:
        with open(user_config) as f:
            return toml.load(f)
    except FileNotFoundError:
        print(f"User config expected at '{user_config}', but not found.")
        return {}

class App(BaseModel):
    loglevel: str
    user: str
    password: str
    version: tuple[int, int, int]  # really shouldn't be in a config, but good example for a complex setting

    @validator("version", pre=True)
    def parse_version(cls, v):
        if isinstance(v, str):
            raw_version = v.split(".")
            if len(raw_version) != 3:
                  raise ValueError(f"Expected simple semver format, got {v} instead.")
            v = tuple(int(x) for x in raw_version)
        return v

class Model(BaseModel):
    C: float
    gamma: float

class Database(BaseModel):
    backend: str
    host: str
    port: int
    user: str
    password: str

class Settings(BaseSettings):
    app: App
    model: Model
    db: Database

    class Config:
        @classmethod
        def customise_sources(cls, env_settings, **kwargs):
            # the order in which these functions are returned decides the
            # override order, which (apparently) is from right to left
            return env_settings, user_settings, default_settings

        env_prefix = "MY_APP_"
        env_nested_delimiter = "__"


cfg = Settings()

default.toml

# part of the source code, provide default values for all settings
[app]
loglevel = "DEBUG"
user = ""
password = ""
version = "0.10.2"

[model]
C = 5
gamma = 0.01

[db]
backend = "sqlite"
host = ""
port = 0
user = ""
password = ""

user.toml

# in cwd when code is run
[app]
user = "foo"

[db]
backend = "postgresql"
user = "bar"
host = "127.0.23.2"
port = 3099

[model]
C = 7
gamma = 0.3

Given an installed package my_app and after exporting the .env file, the config would look like this:

>>> from my_app.config import cfg
>>> cfg.json()  # indented for readability
{
  "app": {
    "loglevel": "DEBUG",
    "user": "foo",
    "password": "secret_foo",
    "version": [0, 10, 2]
  },
  "model": {
    "C": 7.0,
    "gamma": 0.3
  },
  "db": {
    "backend": "postgresql",
    "host": "127.0.23.2",
    "port": 3099,
    "user": "bar",
    "password": "secret_bar"
  }
}