The big difference between Python and modern functional languages is that Python defaults to mutability. Clojure is immutable by default, and that makes it much

The big difference between Python and modern functional languages is that Python defaults to mutability. Clojure is immutable by default, and that makes it much easier to reason about code in isolation. When you're working on a large project, it's really valuable to be able to make a change in one place and know that you're not affecting something in another. When things are passed around by reference, it becomes very difficult to guarantee the scope of any change. Even when using immutable data structure libraries in a mutable by default language doesn't really give you many guarantees. As soon as somebody puts a mutable object into your immutable data structure all bets are off. So, you really want a language that defaults to immutability and has different semantics for working with mutable data.