a half-baked `gather` for `concurrent.futures`

concurrent_gather.py

import threading
from concurrent.futures import Future
from typing import TypeVar

T = TypeVar("T")


def gather(*futures: Future[T], return_exceptions=False) -> Future[list[T]]:
    if return_exceptions:
        raise NotImplementedError("concurrent.futures.gather: can't yet return_exceptions")
    all_done: Future[list[T]] = Future()
    counter = threading.Semaphore(len(futures))

    def check_all_done(future):
        del future  # unused
        if not counter.acquire(blocking=False):
            try:
                all_done.set_result([future.result() for future in futures])
            except Exception as e:
                all_done.set_exception(e)

    check_all_done(None)
    for future in futures:
        future.add_done_callback(check_all_done)

    return all_done