Summing up the good and the bad of dynamic typing and a programmer's mental model.

Tree.py

class Tree():
    pass

class Node(Tree):
    def __init__(self,l,r,value):
        self.l = l
        self.r = r
        self.value = value

class Leaf(Tree):
    def __init__(self):
        pass

def toString(t):
    if (isinstance(t,Node)):
        return "({0},{2},{1})".format(toString(t.l),toString(t.r),str(t.value))
    elif (isinstance(t,Leaf)):
        return "()"
    else:
        return "Neither a Node or a Leaf"


myTree = Node(Leaf(),Leaf(),1)
print toString(myTree)

myTree = Node(Leaf(),"ss",1)
print toString(myTree)

myTree = Node("AAA",Leaf(),1)
print toString(myTree)