Maybe it's me but I still fail to understand what's the issue here. From what I understand from the last message(s) it seems there is complaint about not being able to use ftplib with threads.
FTP is a "command - response" protocol which is designed to handle one request at a time, meaning you send a command then you are supposed to *wait* to get a response back. 
If you want to do multiple transfers in parallel then you need a separate connection, not a separate thread using the same connection.
ftplib should *not* be thread-safe because of how FTP works.