def gen(start, finish):
    while start < finish:
        yield str(start * 0.33)
        sleep(1)
        start += 1

async def stream_testgen(request):
    loop = asyncio.get_event_loop()
    is_first = True
    with ThreadPoolExecutor() as pool:
        for line in await loop.run_in_executor(pool, gen, 1, 5):
            data, content_type = formatter(line)
            if is_first:
                stream = web.StreamResponse(status=200, reason="OK", headers={"Content-Type": content_type})
                await stream.prepare(request)
                is_first = False
            await stream.write(data)
            await stream.drain()
        return stream