Using python 3.7, how to ensure that a parsed email is re-generated correctly after modifying its body content correctly without losing any content?

I would like to parse (serialize) a multipart email (or any and all types with multiple email attachments having inline images, attachments etc), set its body content-type with some extra information and re-generate the email before sending it out. How can this be achieved with the most simplest python code? Using python version 3.6. Right now I am using email.walk() and getting the first available text/plain, text/html and assume its the body of the covering mail, Further parsing recursively thru' the different parts of the mail by walk()...this seems cumbersome. Any easy method?

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