Using the python module fastAPI, I can't figure out how to return an image. In flask I would do something like this:

@app.route("/vector_image", methods=["POST"])
def image_endpoint():
    # img = ... # Create the image here
    return Response(img, mimetype="image/png")

what's the corresponding call in this module?

I had a similar issue but with a cv2 image. This may be useful for others. Uses the StreamingResponse.

import io
from starlette.responses import StreamingResponse

app = FastAPI()

@app.post("/vector_image")
def image_endpoint(*, vector):
    # Returns a cv2 image array from the document vector
    cv2img = my_function(vector)
    res, im_png = cv2.imencode(".png", cv2img)
    return StreamingResponse(io.BytesIO(im_png.tobytes()), media_type="image/png")

If you already have the bytes of the image in memory

Return a fastapi.responses.Response with your custom content and media_type.

You'll also need to muck with the endpoint decorator to get FastAPI to put the correct media type in the OpenAPI specification.

@app.get(
    "/image",

    # Set what the media type will be in the autogenerated OpenAPI specification.
    # fastapi.tiangolo.com/advanced/additional-responses/#additional-media-types-for-the-main-response
    responses = {
        200: {
            "content": {"image/png": {}}
        }
    }

    # Prevent FastAPI from adding "application/json" as an additional
    # response media type in the autogenerated OpenAPI specification.
    # https://github.com/tiangolo/fastapi/issues/3258
    response_class=Response,
)
def get_image()
    image_bytes: bytes = generate_cat_picture()
    # media_type here sets the media type of the actual response sent to the client.
    return Response(content=image_bytes, media_type="image/png")

See the Response documentation.

If your image exists only on the filesystem

Return a fastapi.responses.FileResponse.

See the FileResponse documentation.

Be careful with StreamingResponse

Other answers suggest StreamingResponse. StreamingResponse is harder to use correctly, so I don't recommend it unless you're sure you can't use Response or FileResponse.

In particular, code like this is pointless. It will not "stream" the image in any useful way.

@app.get("/image")
def get_image()
    image_bytes: bytes = generate_cat_picture()
    # ❌ Don't do this.
    image_stream = io.BytesIO(image_bytes)
    return StreamingResponse(content=image_stream, media_type="image/png")

First of all, StreamingResponse(content=my_iterable) streams by iterating over the chunks provided by my_iterable. But when that iterable is a BytesIO, the chunks will be \n-terminated lines, which won't make sense for a binary image.

And even if the chunk divisions made sense, chunking is pointless here because we had the whole image_bytes bytes object available from the start. We may as well have just passed the whole thing into a Response from the beginning. We don't gain anything by holding data back from FastAPI.

Second, StreamingResponse corresponds to HTTP chunked transfer encoding. (This might depend on your ASGI server, but it's the case for Uvicorn, at least.) And this isn't a good use case for chunked transfer encoding.

Chunked transfer encoding makes sense when you don't know the size of your output ahead of time, and you don't want to wait to collect it all to find out before you start sending it to the client. That can apply to stuff like serving the results of slow database queries, but it doesn't generally apply to serving images.

Unnecessary chunked transfer encoding can be harmful. For example, it means clients can't show progress bars when they're downloading the file. See:

• Content-Length header versus chunked encoding
• Is it a good idea to use Transfer-Encoding: chunked on static files?

It's not properly documented yet, but you can use anything from Starlette.

So, you can use a FileResponse if it's a file in disk with a path: https://www.starlette.io/responses/#fileresponse

If it's a file-like object created in your path operation, in the next stable release of Starlette (used internally by FastAPI) you will also be able to return it in a StreamingResponse.

All the other answer(s) is on point, but now it's so easy to return an image

from fastapi.responses import FileResponse

@app.get("/")
async def main():
    return FileResponse("your_image.jpeg")