Python Flask send_file StringIO blank files

The issue here is that in Python 3 you need to use StringIO with csv.write and send_file requires BytesIO, so you have to do both.

@app.route('/test_download')
def test_download():
    row = ['hello', 'world']
    proxy = io.StringIO()
    
    writer = csv.writer(proxy)
    writer.writerow(row)
    
    # Creating the byteIO object from the StringIO Object
    mem = io.BytesIO()
    mem.write(proxy.getvalue().encode())
    # seeking was necessary. Python 3.5.2, Flask 0.12.2
    mem.seek(0)
    proxy.close()

    return send_file(
        mem,
        as_attachment=True,
        download_name="test.csv",
        mimetype="text/csv"
    )

_{Prior to Flask 2.0, download_name was called attachment_filename.}