Another option occurred to me: as_completed could return an object that implements both synchronous and asynchronous iteration protocol:


class as_completed:
    def __init__(fs, *, loop=None, timeout=None):
        self.__fs = fs
        self.__loop = loop
        self.__timeout = timeout

    def __iter__(self):
        # current implementation here
        ...

    async def __aiter__(self):
        # new async implementation here
        ...

    def __next__(self):
        # defined for backward compatibility with code that expects
        # as_completed() to return an iterator rather than an iterable
        if self._iter is None:
            self._iter = iter(self)
        return next(self._iter)

With that design there wouldn't need to be a new function under a different name; instead, as_completed could just be documented as an asynchronous iterable, with the old synchronous iteration supported for backward compatibility.