Parse email and send reply using googleapiclient in Python

I'm currently working on a project and I have chosen to use Gmail for sending and receiving emails. I want to be able to send an email, have a user reply to it, and parse their response. The response can be any number of lines (so something like response.split('\n')[0] won't work). It should then be able to reply directly to that email thread.

I've been following the googleapiclient tutorials, but they leave a lot to be desired. However, I've managed to read email threads using:

service.users.threads().get(userId='me', id=thread_id).execute()

where thread_id is (predictably) the ID of the email thread (which I find elsewhere). In the large dict returned by this, there is a section of base64 data which contains the content of the email. This was the only place I could find the actual data for the response. Unfortunately, I get this when it is decoded:

b'This is my response from my phone\r\n\r\nOn Sat, 28 Nov 2020, 8:40 PM , <myemail@gmail.com>\r\nwrote:\r\n\r\n> This is sent from the python script\r\n>\r\n'

This is all the data in the thread, however, I only want the response as there is clearly no way to split this to get only the data I need. The best I can think of is to parse out anything of the form On <date>, <time>, but that could lead to problems. There must be another way to extract only This is my response from my phone and no other data.

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Once I get the response, I want to parse it and reply with an appropriate response based on the contents of the message. I would prefer to reply directly to the thread, rather than starting a new one. Unfortunately, all the Google documentation says is:

If you're trying to send a reply and want the email to thread, make sure that:

1. The Subject headers match
2. The References and In-Reply-To headers follow the RFC 2822 standard.

The documentation provides this code (with some minor modifications by me) for sending an email:

def create_message(sender, to, subject, message_text):
  message = MIMEText(message_text)
  message['to'] = to
  message['from'] = sender
  message['subject'] = subject
  return {'raw': base64.urlsafe_b64encode(message.as_bytes()).decode()}

Sending a reply with the same subject line is pretty straight forward (message['subject'] = same_subject_as_before), but I don't even know where to start with the References and In-Reply-To headers. How do I set these?

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