a version of concurrent.futures.as_completed that doesn't consume the whole iterable of futures before producing the first one.

as_completed.py

from concurrent.futures import Future, wait, FIRST_COMPLETED
from typing import Iterable


def as_completed(fs: Iterable[Future], buffer: int = 5):
    """
    Self-throttling version of concurrent.futures.as_completed.
    Contrary to the standard library version, does not consume
    Future iterable immediately.
    """
    done, not_done = set(), set()
    fs = iter(fs)
    pending = True

    while pending or not_done:
        if pending:
            for i in range(buffer - len(not_done)):
                try:
                    not_done.add(next(fs))
                except StopIteration:
                    pending = False
                    break

        done, not_done = wait(
            fs=not_done,
            timeout=None,
            return_when=FIRST_COMPLETED
        )

        yield from done