python csv只写某些字段名，而不是全部

I must be missing something, but I don't get it. I have a csv, it has 1200 fields. I'm only interested in 30. How do you get that to work? I can read/write the whole shebang, which is ok, but i'd really like to just write out the 30. I have a list of the fieldnames and I'm kinda hacking the header. I must be missing something, but I don't get it