Python Immutable Trees

When I was building immutable python lists interface, I realized, than in the same way an immutable binary tree (or ternary and other) can be created - using python Abstract Base Class. Code looks like:

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class Nil_tree: """class Nil_tree, the empty tree""" def is_empty(self): return True def left(self): return Exception("Empty") def right(self): return Exception("Empty") def __str__(self): return "()" class Binary_tree: """Class Binary_tree, the non empty tree: ( val, left, right)""" def __init__(self, _item, _left, _right): self.item = _item self.left = _left self.right = _right def is_empty(self): return False class Tree(metaclass=ABCMeta): @abstractmethod def is_empty(): pass def item(): pass def left(): pass def right(): pass List.register(Nil_tree); List.register(Binary_tree)

I would call it purely functional, can it be used in practice?  Maybe to do some functional stuff in python. This code plus some basic tree functions freely available on github.
PS I, also added lots of methods to cons lists, code here.