> Observer, the Adapter, or the Visitor

It depends on the programming language one uses, really. You won't find many references to these patterns on Python's mailing list (or at least I didn't when I was new to the language and used to spend a lot of time there, ~6-7 years ago). Also, when I've tried to learn Lisp or Erlang I didn't find any mentions to such particular patterns (or maybe only in passing), the focus was on something else. And I'm sure there are a lot of other languages outhere who don't bother with patterns.