Python Add Inline Images to multipart/alternative emails

Question

When adding inline attachments to the email body they are not displayed with the code below.

from email.message import EmailMessage
from email.mime.image import MIMEImage
msg = EmailMessage()
msg["From"] = "[email protected]"
msg["To"] = "[email protected]"
msg["Subject"] = "Test Email"
plain_text, html_message =  # Plain Text and HTML email content created
msg.set_content(plain_text)
msg.add_alternative(html_message, subtype='html')

# adding the inline image to the email
with open('logo.png', 'rb') as img:
    logo = MIMEImage(img.read())
    logo.add_header('Content-ID', f'<connect_logo_purple>')
    logo.add_header('X-Attachment-Id', 'connect_logo_purple.png')
    logo['Content-Disposition'] = f'inline; filename=connect_logo_purple.png'
    msg.attach(logo)

BUT when the inline attachment is added after add_attachment it is displayed.

# ... Create message like above
msg.set_content(plain_text)
msg.add_alternative(html_message, subtype='html')

# Only when an attachment is added does the inline images show up
msg.add_attachment(
    #..details filled in
)

# .. continue with inline image
with open('logo.png', 'rb') as img:
    logo = MIMEImage(img.read())
    logo.add_header('Content-ID', f'<connect_logo_purple>')
    logo.add_header('X-Attachment-Id', 'connect_logo_purple.png')
    logo['Content-Disposition'] = f'inline; filename=connect_logo_purple.png'
    msg.attach(logo)

Answer 1

What I found is when the add_attachment command is called it updates the email's payload to be multipart/mixed so to fix it...

# ... Create message like above
msg.set_content(plain_text)
msg.add_alternative(html_message, subtype='html')

# When there isn't a call to add_attachment... 
with open('logo.png', 'rb') as img:
    logo = MIMEImage(img.read())
    logo.add_header('Content-ID', f'<connect_logo_purple>')
    logo.add_header('X-Attachment-Id', 'connect_logo_purple.png')
    logo['Content-Disposition'] = f'inline; filename=connect_logo_purple.png'

    # Make the EmailMessage's payload `mixed`
    msg.get_payload()[1].make_mixed()   # getting the second part because it is the HTML alternative
    msg.get_payload()[1].attach(logo)