Displaying a file (image) from S3 via Flask & BytesIO
Rewinding your BytesIO object should do the trick, with file.seek(0) just before send_file(...).
For the record I'm not sure your boto3/botocore calls are "best practices", to try your usecase I ended up with:
from boto3.session import Sessionsession = Session( aws_access_key_id=KEY_ID, aws_secret_access_key=ACCESS_KEY, region_name=REGION_NAME)s3 = session.resource("s3")@base_bp.route("/test-stuff")def test_stuff(): a_file = io.BytesIO() s3_object = s3.Object(BUCKET, PATH) s3_object.download_fileobj(a_file) a_file.seek(0) return send_file(a_file, mimetype=s3_object.content_type)It works on when reading the file from disk because you instanciate your BytesIO with the full content of the file, so it's properly fulfilled and still at "position 0".