How to send an existing message in a new email without losing formatting

Context: I'm attempting to send an existing email in my inbox to a new thread.

Problem: I've successfully sent the email body using this function however the body loses the formatting of the original email and only sends text.

I think it makes more sense to add the entire payload to the request body, as documented on the gmail API page "Try this API" section:

However when I add payload to the request body:

def create_message(sender, to, subject, thread_id, message_id, payload, service):
  """Create a message for an email.

  Args:
    sender: Email address of the sender.
    to: Email address of the receiver.
    subject: The subject of the email message.
    message_text: The text of the email message.

  Returns:
    An object containing a base64url encoded email object.
  """

  message = MIMEMultipart('alternative')
  message['to'] = to
  message['from'] = sender
  message['subject'] = 'Re: %s' %subject

  return {'raw': raw, 'threadId': thread_id, 'payload': payload}

The emails are sent with no content. How can I add an existing email to a new thread without having to decode and encode and lose the email's formatting?

Solution

After messing around I've made two functions that can pass the plain and html content types to a new email for anyone else who might be struggling:

def get_all_parts(service, user_id, msg_id):
  message = service.users().messages().get(userId=user_id, id=msg_id, format='raw').execute()
  msg_bytes = base64.urlsafe_b64decode(message['raw'].encode('ASCII'))
  b = email.message_from_bytes(msg_bytes)
  body = []

  if b.is_multipart():
    for part in b.walk():
      if part.get_content_maintype() != 'multipart':
        this_part = []
        this_part.append(part.get_payload(decode=True))
        this_part.append(part.get_content_type())
        body.append(this_part)

  return body

def create_message(sender, to, subject, thread_id, message_id, message_text, service):
  message = MIMEMultipart()
  message['to'] = to
  message['from'] = sender
  message['subject'] = 'Re: %s' %subject
  for part in message_text:
    text = part[1].split('/') # 'text/plain' -> ['text', 'plain']
    new_part = MIMEText(str(part[0]), text[1])
    print(part[1])
    message.attach(new_part)

  raw = base64.urlsafe_b64encode(message.as_string().encode('UTF-8')).decode('ascii')   
  body = {'raw': raw, 'threadId': thread_id}

enter code here

This is definitely not an exhaustive function for all emails but works for alternative content types.