Load Typed Settings from a TOML File

import tempfile
import tomllib
from dataclasses import dataclass
from pathlib import Path


class ConfigError(ValueError):
    pass


@dataclass(frozen=True)
class DatabaseSettings:
    host: str
    port: int
    name: str
    pool_size: int = 5


@dataclass(frozen=True)
class Settings:
    service: str
    debug: bool
    database: DatabaseSettings


def load_settings(path: Path) -> Settings:
    try:
        raw = tomllib.loads(path.read_text(encoding="utf-8"))
    except tomllib.TOMLDecodeError as error:
        raise ConfigError(f"invalid TOML: {error}") from error

    service = raw.get("service")
    if not isinstance(service, dict):
        raise ConfigError("missing [service] table")
    name = service.get("name")
    debug = service.get("debug", False)
    if not isinstance(name, str) or not name.strip():
        raise ConfigError("service.name must be a non-empty string")
    if not isinstance(debug, bool):
        raise ConfigError("service.debug must be a boolean")

    database = raw.get("database")
    if not isinstance(database, dict):
        raise ConfigError("missing [database] table")
    host = database.get("host")
    port = database.get("port")
    db_name = database.get("name")
    pool_size = database.get("pool_size", 5)
    if not isinstance(host, str) or not host:
        raise ConfigError("database.host must be a non-empty string")
    if not isinstance(port, int) or isinstance(port, bool) or not 1 <= port <= 65535:
        raise ConfigError("database.port must be an integer between 1 and 65535")
    if not isinstance(db_name, str) or not db_name:
        raise ConfigError("database.name must be a non-empty string")
    if not isinstance(pool_size, int) or isinstance(pool_size, bool) or pool_size < 1:
        raise ConfigError("database.pool_size must be a positive integer")

    return Settings(
        service=name.strip(),
        debug=debug,
        database=DatabaseSettings(
            host=host,
            port=port,
            name=db_name,
            pool_size=pool_size,
        ),
    )


def main() -> None:
    with tempfile.TemporaryDirectory() as tmp:
        path = Path(tmp) / "config.toml"
        path.write_text(
            """
[service]
name = "orders"
debug = true

[database]
host = "db.internal"
port = 5432
name = "orders"
""",
            encoding="utf-8",
        )
        settings = load_settings(path)
        print(settings)
        print("pool size:", settings.database.pool_size)

        broken = Path(tmp) / "broken.toml"
        broken.write_text(
            """
[service
name = 'x'
""",
            encoding="utf-8",
        )
        try:
            load_settings(broken)
        except ConfigError as error:
            print("error:", error)


if __name__ == "__main__":
    main()