Well, probably the best thing to do to understand the issue is to look at the implementation of Message.walk and is_multipart, and to notice that "is_multipart()" can return a different answer than "msg.get_content_maintype() == 'multipart'" if the Message content type is "message/rfc822".  You may want to do some reading about how to construct Messages and experiment by creating such a message/rfc822 Message to really understand it.