Why can't I specify 2 join subject and To?

Question

Python

You need to parse the gmail box to the account of sent letters, you need to parse the fields to whom. topic, well, and a request for a specific date.

r = {"To": None, "Subject": None}
        for i in response["payload"]["headers"]:
            if i["name"] in r.keys():
                r[i["name"]] = i["value"]

here we actually parse the email, the subject.
then I generate the result, here is the heading in it, for some reason I can’t specify 2 join
subject and To

user_id='me'
    req_date = ("%s-%s-%s" % (flags.year,flags.month,flags.day))
    date = datetime.datetime.strptime(req_date , '%Y-%m-%d').date()
    before_date = return_tomorrows_date(date)
    credentials = get_credentials()
    http = credentials.authorize(httplib2.Http())
    service = discovery.build('gmail', 'v1', http=http)
    query = ('from:%s after:%s before:%s' % (flags.sender, date, before_date))
    messages = ListMessagesMatchingQuery(service,user_id,query)
    generate_results(req_date,flags.sender,",".join([x["Subject"] for x in messages])) или  generate_results(req_date,flags.sender,",".join([x["To"] for x in messages]))

if you specify it separately, everything works, but I need to join both to and subject
and output it.