Flask export/upload database table in cvs/xlsx format

[DeaD_EyE]

Use cw.writerows on the query.
If you iterate manually with a for-loop, you must add row by row with cw.writerow (without s at the end).

@admin_role.route('/download')
@login_required
def post():
    si = StringIO()
    cw = csv.writer(si)
    cw.writerows(Book.query.all())
    output = make_response(si.getvalue())
    output.headers["Content-Disposition"] = "attachment; filename=export.csv"
    output.headers["Content-type"] = "text/csv"
    return output

For safety reasons you should limit the query and use a paginate function.
Use a cache if you want to speed up requests.