That strikes me really weird, so I've tried a couple of times and it seems that I am not doing any mistake.
I have a torrent file which has more than one file listed. I want only the file with 3 as its id to be downloaded, I pass the argument files_wanted= to add_torrents() yet it downloads all of the files. If I create a list of all file id's then remove 3, it works as it should.
I haven't checked the code yet and using this work-around. When I have time I'll check and post a suggestion hopefully.