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main.py

from typing import TypeVar, Generic
from abc import abstractmethod, ABC

F = TypeVar("F")
class DatumGS(Generic[F]):
    def __init__(self, key: str):
        self.key = key

    @abstractmethod
    def get(self) -> F:
        raise NotImplementedError

class DatumStr(DatumGS[str | None]):

    def get(self) -> str | None:
        return "hello"
    
D = TypeVar("D", bound="DatumGS")
class Wrapper(ABC):
    datum: D[F]

    @classmethod
    @abstractmethod
    def collect(cls) -> F:
        raise NotImplementedError

    @classmethod
    def get(cls) -> F:
        return cls.datum.get()