head_tuple.py

from typing import Generic, overload, Literal, TypeVar, Union


_Head = TypeVar("_Head")
_Tail = TypeVar("_Tail")


class HeadTuple(Generic[_Head, _Tail]):
    def __init__(self, head: _Head, *tail: _Tail) -> None:
        self._head = head
        self._tail = tail

    @overload
    def __getitem__(self, index: Literal[0]) -> _Head:
        ...

    @overload
    def __getitem__(self, index: int) -> _Tail:
        ...

    def __getitem__(self, index: Union[Literal[0], int]) -> Union[_Head, _Tail]:
        if index == 0:
            return self._head
        else:
            return self._tail[index]


f = HeadTuple("Hello", 1, 2, 3)