Declarative description of environment variables for environs

env_var.py

from typing import Any, Callable, Dict, Optional, Type, TypeVar

from environs import Env

__all__ = ('EnvVar',)

T = TypeVar('T')


class EnvVar:
    name: Any

    def __init__(
        self,
        method: Callable[[Env, str, Optional[Dict[str, Any]]], Any] = Env.str,
        name: Optional[str] = None,
        kwargs: Optional[Dict[str, Any]] = None,
    ):
        self.name = name
        self.method = method
        self.kwargs = kwargs or {}

    def __set_name__(self, owner: Type[T], name: str) -> None:
        if not self.name:
            self._check_name(owner, name)
            self.name = name

    def __get__(self, instance: T, owner: Type[T]) -> Any:
        if not instance:
            raise AttributeError(
                f"type object '{owner.__name__}' "
                f"has no attribute '{self.name}'"
            )

        return self.method(instance, self.name, **self.kwargs)

    def __set__(self, instance, value):
        raise TypeError("can't set attribute")

    @staticmethod
    def _check_name(owner, name):
        if not name.isupper():
            raise RuntimeError(
                f'{owner.__qualname__}.{name} must be in uppercase'
            )


class Environment(Env):
    PATH = EnvVar()


if __name__ == '__main__':
    env = Environment()
    print(env.PATH)