how to handle BytesIO in pyvips

pyvips_test.py

import asyncio
from functools import partial
from io import BytesIO
import aiohttp
import pyvips

async def get_image():
    async with aiohttp.ClientSession() as session:
        async with session.get("https://user-images.githubusercontent.com/29148882/43419292-d2ae9d68-9440-11e8-80f5-dcdaaae59e2e.png") as req:
            image_bytes = BytesIO(await req.read())
    return image_bytes

def invert_pyvips(image_bytes):
    image = pyvips.Image.new_from_buffer(image_bytes.read(), "")
    image = image * [-1, -1, -1, 1] + [255, 255, 255, 0]
    return image.write_to_buffer(".png")

async def main():
    image_bytes = await get_image()
    func = partial(invert_pyvips, image_bytes)
    print(type(await loop.run_in_executor(None, func)))

loop = asyncio.get_event_loop()
loop.run_until_complete(main())