From test-sender@example.com
From: test-sender@example.com
To: test-recipient@example.com
Subject: test simple message

It's a test.

-- Best regards.

text = ''
for attachment in self.attachments_pool:
    text+='Removed ' + attachment.getFilename() +' with hash: ' + attachment.getHash() + 'n'
text = MIMEText(text, 'plain')
self.msg.attach(text)

Traceback (most recent call last):
  File "/usr/lib/python3.5/email/message.py", line 209, in attach
    self._payload.append(payload)
AttributeError: 'str' object has no attribute 'append'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "msg_parser.py", line 386, in <module>
    raise e
  File "msg_parser.py", line 376, in <module>
    main()
  File "msg_parser.py", line 369, in main
    msg_handler.handle()
  File "msg_parser.py", line 329, in handle
    self.msg.attach(text)
  File "/usr/lib/python3.5/email/message.py", line 211, in attach
    raise TypeError("Attach is not valid on a message with a"
TypeError: Attach is not valid on a message with a non-multipart payload

#!/usr/bin/env python3
import email.policy

msg = email.message_from_string("""From test-sender@example.com
From: test-sender@example.com
To: test-recipient@example.com
Subject: test simple message

It's a test.

-- Best regards.""", policy=email.policy.default)

msg.set_content("Заменили текст.")

print(msg)

From: test-sender@example.com
To: test-recipient@example.com
Subject: test simple message
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
MIME-Version: 1.0

0JfQsNC80LXQvdC40LvQuCDRgtC10LrRgdGCLgo=