from __future__ import annotations

import os
from dataclasses import dataclass


@dataclass(frozen=True, slots=True)
class AppConfig:
    app_name: str
    debug: bool
    port: int
    database_url: str

    @classmethod
    def from_env(cls) -> AppConfig:
        port_raw = os.environ.get("PORT", "8000")
        try:
            port = int(port_raw)
        except ValueError as e:
            raise ValueError(f"PORT must be an int, got {port_raw!r}") from e
        if not 1 <= port <= 65535:
            raise ValueError(f"PORT out of range: {port}")

        debug = os.environ.get("DEBUG", "0").lower() in {"1", "true", "yes"}
        database_url = os.environ.get("DATABASE_URL", "sqlite:///./app.db")
        if not database_url:
            raise ValueError("DATABASE_URL must not be empty")

        return cls(
            app_name=os.environ.get("APP_NAME", "demo"),
            debug=debug,
            port=port,
            database_url=database_url,
        )


def main() -> None:
    cfg = AppConfig.from_env()
    print(cfg)


if __name__ == "__main__":
    main()

set PORT=9000
set DEBUG=true
python dataclass_config.py