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main.py

from typing import Callable

type ValueConverter[T] = Callable[[str], T]

class EnvVar[T]:
    def __init__(self, key: str, convert: ValueConverter[T] = str):
        self.key = key
        self.convert = convert

class DatasetReference:
    def __init__(self, project_id: str | None, dataset_id: str):
        pass
    
    @classmethod
    def from_string(cls, dataset_id: str, default_project_id: str | None = None) -> DatasetReference:
        return cls(default_project_id, dataset_id)

FOO = EnvVar("FOO")
BAR = EnvVar("BAR", int)
BAZ = EnvVar("BAZ", dataset_id)