import concurrent.futures
import contextlib
from typing import Callable
from typing import Generator
from typing import Iterable
from typing import TypeVar

TArg = TypeVar('TArg')
TRet = TypeVar('TRet')

@contextlib.contextmanager
def _thread_mapper(maxsize: int) -> Generator[
    Callable[[Callable[[TArg], TRet], Iterable[TArg]], Iterable[TRet]],
    None, None,
]:
    if maxsize == 1:
        yield map
    else:
        with concurrent.futures.ThreadPoolExecutor(maxsize) as ex:
            yield ex.map


def double(x: int) -> int: return x * 2

with _thread_mapper(1) as m:
    print(list(m(double, [2, 3])))