Doesn't have to throw an error, could just return an empty list, doesn't matter. All I am saying is that the problem the article describes, where lack of algebr

Doesn't have to throw an error, could just return an empty list, doesn't matter. All I am saying is that the problem the article describes, where lack of algebraic data types in python makes it impossible to describe the tree structure doesn't exist. There is no need to explicitly represent an empty tree. It is enough to know where your tree ends. Yes, it is more elegant with the ADTs, but it is not impossible without them. Also, if I really wanted to implement Leaf type, I would probably do it via __new__ and a bit of metaprogramming.