How to use the aiofiles.threadpool function in aiofiles

def serve_file(reader, writer):
        full_filename = str(file)
        f = yield from threadpool.open(full_filename, mode='rb')
        writer.write((yield from f.read()))
        yield from f.close()
        writer.close()

def serve_file(_, writer):
        file = yield from aiofiles.threadpool.open(filename, mode='rb')
        try:
            while True:
                data = yield from file.read(1)
                if not data:
                    break
                writer.write(data)
                yield from writer.drain()
            yield from writer.drain()
        finally:
            writer.close()
            yield from file.close()

def test_slow_file(monkeypatch, unused_tcp_port):
    """Monkey patch open and file.read(), and assert the loop still works."""
    filename = join(dirname(__file__), '..', 'resources', 'multiline_file.txt')

    with open(filename, mode='rb') as f:
        contents = f.read()

    def new_open(*args, **kwargs):
        time.sleep(1)
        return open(*args, **kwargs)

    monkeypatch.setattr(aiofiles.threadpool, 'sync_open', value=new_open)

    @asyncio.coroutine
    def serve_file(_, writer):
        file = yield from aiofiles.threadpool.open(filename, mode='rb')
        try:
            while True:
                data = yield from file.read(1)
                if not data:
                    break
                writer.write(data)
                yield from writer.drain()
            yield from writer.drain()
        finally:
            writer.close()
            yield from file.close()