Attempted implicit sequence conversion while serving PIL image using Flask uwsgi

Question

I am serving a PNG or SVG image through Flask. Locally it works fine but when I run the application inside docker and send request (POST) I get following error:

RuntimeError: Attempted implicit sequence conversion but the response object is in direct passthrough mode.

Bellow code for serving PIL image through flask:

def serve_image(image: Image, mime_type: FileFormat, download: bool):
    suffix = mime_type.value.split('/')[-1]
    temp_file = tempfile.TemporaryFile(mode='w+b', suffix=suffix)
    if suffix == 'png':
        image.save(temp_file, suffix)
    else:
        # we cant force svg extension in PIL
        image.save(temp_file)
    temp_file.seek(0, 0)
    return send_file(temp_file, mimetype=mime_type.value, as_attachment=download,
                    attachment_filename='img.' + suffix)

I have tried using BytesIO no luck there either. Setting

Response.implicit_sequence_conversion = False
Response.direct_passthrough = False

or

@app.after_request
def after_request_func(r):
    r.direct_passthrough = False
    r.implicit_sequence_conversion = False
    return r

Did not help either.

Answer 1

The problem was in openapi-core Flask validation, and it was solved by creating werkzeug Response with direct passtrough set to False. Other headers like attachment cache-timeout had to be set manually.

res = Response(temp_file, direct_passthrough=False, mimetype=mime_type.value)