> "you can do immutable/functional programming with regular ol' Python"

One cannot easily and safely do functional programming in Python due it's imperative semantics. For example variables are captured as mutable references and even a list comprehension will mutate its bindings.

If you want to do serious functional programming, use Clojure, Haskell and/or OCaml.

Edit: to the downvoters, it's about using the right tool for the job.