Question

PYTHON: Conan is writing a module to contain different implementations of trees. After his first tree, the BinaryTreeclass, he wrote test code and is having problems understanding the error. Locate his problem and explain how you would fix it.

class Node(object):
def __init__(self, data=None):
self.data = data
def __str__(self):
return "NODE: " + str(self.data)
  
class Tree(object):
def __init__(self):
self.root_node = None
self.size = 0
  
def __len__(self):
return self.size
  
def add(self, data):
raise NotImplementedError("Add method not implemented.")
  
def inorder_traversal(self):
raise NotImplementedError("inorder_traversal not implemented.")
  

class BinaryTreeNode(Node):
def __init__(self, data=None):
Node.__init__(self, data)
self.left_child = None
self.right_child = None
  
class BinaryTree(Tree):
def __init__(self):
super().__init__()
  
def add(self, data):
if self.root_node == None:
self.root_node = BinaryTreeNode(data)
print(self.root_node)
else:
parent = None
current = self.root_node
while current != None:
if data < current.data:
parent = current
current = current.left_child
elif data > current.data:
parent = current
current = current.right_child
else:
raise ValueError("Value already in Tree.")
if data < parent.data:
parent.left_child = BinaryTreeNode(data)
elif data > parent.data:
parent.right_child = BinaryTreeNode(data)
else:
raise ValueError("Reached insert statement but value is already inserted.")
print(parent.left_child, parent.right_child)
Tree.size += 1
  
  
if __name__ == "__main__":
btree = BinaryTree()
btree.add(5)
btree.add(3)
btree.add(7)
btree.add(4)
btree.add(9)
btree.add(2)
print(len(btree))

Answer 1

The error occurs at the line Tree.size += 1. Here the code is referencing to an object known as Tree with attribute size. But we haven't instantiated a Tree object in the class. Because the BinaryTree class is a subclass of Tree class the size attribute is a part of BinaryTree class itself, so we have to refer to the size attribute by using self.size. Replacing Tree.size+=1 with self.size+=1 gives us the desired result.