How to send animated GIF from memory to FastAPI endpoint?

I'm trying to send a GIF from memory to my FastAPI endpoint, which works, but the gif isn't animated. When I save it locally instead the animation works fine.
I don't want to save the image, but instead keep it in memory until it's returned to the endpoint.

I've already checked out this post, but still couldn't get it working: https://stackoverflow.com/questions/67571477/python-fastapi-returned-gif-image-is-not-animating

So how can I return an animated .gif using FastAPI?

This is my attempted solution:

# main.py

@app.get('/youtube/{video_id}.gif')
def youtube_thumbnail (video_id: str, width: int = 320, height: int = 180):
    image = util.create_youtube_gif_thumbnail(video_id)
    return util.gif_to_streaming_response(image)

# util.py

def gif_to_streaming_response(image: Image):
    imgio = BytesIO()
    image.save(imgio, 'GIF')
    imgio.seek(0)
    return StreamingResponse(content=imgio, media_type="image/gif")


def create_gif(images, duration):
    gif = images[0]
    output = BytesIO() # for some reason this doesn't work (it shows a still image), but if I save the image with output = "./file.gif" the animation works!
    # this works
    output = "./file.gif"
    gif.save(output, format='GIF', save_all=True, append_images=images, loop=0, duration=duration)
    return gif


def create_youtube_gif_thumbnail(video_id: str):
    images = [read_img_from_url(f"https://i.ytimg.com/vi/{video_id}/{i}.jpg") for i in range(1,3)]
    return create_gif(images, 150)

Here's the full codebase: https://github.com/Snailedlt/Markdown-Videos/blob/418de75e200bf9d9f4f02e5a667af4c9b226b5d3/util.py#L74

Your code is very difficult to follow, but it looks like you're saving something different to what you're returning from your endpoint.

If I flatten most of your code into a single method it's clearer what's going on:

@app.get('/youtube/{video_id}.gif')
def youtube_thumbnail (video_id: str, width: int = 320, height: int = 180):
    # from util.create_youtube_gif_thumbnail(video_id)
    images = [read_img_from_url(f"https://i.ytimg.com/vi/{video_id}/{i}.jpg") for i in range(1,3)]
    # from create_gif(images, 150)
    image = images[0]
    image.save("./file.gif", format='GIF', save_all=True, append_images=images, loop=0, duration=150)
    # from util.gif_to_streaming_response(image)
    imgio = BytesIO()
    image.save(imgio, 'GIF')
    imgio.seek(0)
    return StreamingResponse(content=imgio, media_type="image/gif")

So you're passing save_all and append_images when writing to file.gif, but not as part of the StreamingResponse.

I.e., it's effectively doing:

@app.get('/youtube/{video_id}.gif')
def youtube_thumbnail (video_id: str):
    images = [read_img_from_url(f"https://i.ytimg.com/vi/{video_id}/{i}.jpg") for i in range(1,3)]
    image = images[0]
    imgio = BytesIO()
    image.save(imgio, 'GIF')
    imgio.seek(0)
    return StreamingResponse(content=imgio, media_type="image/gif")

While, I'd suggest doing:

@app.get('/youtube/{video_id}.gif')
def youtube_thumbnail (video_id: str):
    first, *following = (
        read_img_from_url(f"https://i.ytimg.com/vi/{video_id}/{i+1}.jpg")
        for i in range(3)
    )
    buf = BytesIO()
    first.save(buf, format='GIF', save_all=True, append_images=following, loop=0, duration=150)
    return Response(buf.getvalue(), media_type="image/gif")

Note that I've split the frames up into first and following so that the first doesn't have its duration doubled (as you were implicitly including it twice). Next, your range(1,3) only iterates over [1, 2] rather than [1, 2, 3] which seem to be the valid range for the videos I tried. Finally, there is little value in using a StreamingResponse as the whole response is already in memory (due to the use of BytesIO) hence just passing the bytearray directly to a Response seems acceptable.

I'd also suggest modifying read_img_from_url to check for errors, something like:

def read_img_from_url(url: str) -> Image.Image:
    res = requests.get(url)
    res.raise_for_status()
    return Image.open(BytesIO(res.content))

It's annoying that requests doesn't raise errors like this automatically. You likely need a try/catch block anyway as resolver/socket exceptions might be raised internally.