from dataclasses import dataclass
from os import environ


@dataclass
class EnvVar:
    """
    Environment Variable fetcher for use in dataclass ``field(default_factory=...)``

    >>> @dataclass
    >>> class SomeDataclass:
    ...     field_name: str = field(default_factory=EnvVar("SOME_VAR_NAME", "err msg"))

    """

    var_name: str
    err_msg: str = ""

    def __call__(self) -> str:
        try:
            return environ[self.var_name]
        except KeyError as e:
            e.add_note(self.err_msg)
            raise