Have you looked at Clojure?

[Huge ramble to follow]

Clojure is a particularly interesting functional language for a python programmer IMO. It shares a number of asthetic and cultural properties with python that other lisps and other functional languages don't provide.

Syntactically they are approximately distantish cousins. Clojure's syntax is much more restricted that pythons (obviously ;) but more prominent than other the other big lisps. Of note there is literal support for Vectors (much closer to pythons lists than the functional singly linked list), Maps, Sets, and these are used in the special forms too. A comparison (lets use identical implementations of factorial - taking some liberties so that the code is actually semantically identical, There are more idiomatic versions down the thread)

def fac(n):
"A function to calculate factorial in python"
return reduce(lambda x, y: x * y, range(1, n + 1))

(defn fac
"A function to calculate factorial in clojure"
[n]
(reduce #(* %1 %2) (range 1 (+ n 1)))

In particular there is the vector in the form ([n]) used for the arguments list. The other thing to comment on is the funny looking #(* %1 %2) bit, thats an anonymous function/expression. There are two ways to write functions in clojure, that way and using a fn form. That same expression would look like

(fn [x y] (* x y))

Clearly this becomes a matter of readability in most cases. Like python it means you have two ways to express a function. In this case you could actually just drop * in instead of the lambda, because it is just a symbol referencing a function like any other name. Python would require importing the operators library to do the same thing.

Both languages are quite opinionated about the right way to do things. You are probably familiar with pythons. Clojure has a strong philosophy behind it, but its still pragmatic. Clojure promotes pure functional programming wherever its possible, and provides an excellent range of tools to break out of that where needed.

Like Python, Clojure has good package management, good core data structures, lazy sequences (much stronger than pythons generators), quite a few batteries included (in particular if you consider clojure.contrib) though not as many as python.

So if there are so many similarities, why would you switch? A number of reasons have made me do more and more of my development in python rather than clojure.

First up i want to do more functional programming. Like many long time python programmers i started using functools and itertools, generators etc more and more, but functional style is not encouraged by certain parts of the community.

Secondly, concurrency. I've written a bit of concurrent code in python (usually in an actor style using the queue objects) but python is ill suited to this, Clojure's persistent datastructures are much stronger here. In addition there is no GIL so concurrent code is able to run fast, not just asynchronously. On top of this, clojures references types provide _many_ options for concurrency which often simplify your design considerably compared to writing a message passing thing in python.

Packaging and deployment is much better. If you are using a unix system http://github.com/technomancy/leiningen is a must get. This is a build tool that manages dependancies for you automatically. It integrates with the clojars.org repository. This pairing replaces pip/easy_install and the cheeseshop as well as tools like virtualenv.

The community is great.

If you want to get started, the following comment by hga might serve you well http://news.ycombinator.com/item?id=1033503 Rich Hickey's various presentations and lectures make a compelling case for the language, and teach you a lot about how to use it at the same time. Even if you never end up writing code in clojure, these videos will teach you a huge amount, i highly recommend them.

As you can probably tell from the amount of blather here, clojure has impressed me, and i would recommend any python programmer who has become interested in functional stuff check it out.