Binary Tree in Python

Question

This is my code-snippet for a Binary Tree implementation in Python. This WORKS when I run the PreOrder function.

class Node:
 def __init__(self,data):
 self.left = None
 self.right = None
 self.data = data
class BinaryTree(Node):
 def __init__(self):
 self.root = None
 def addNode(self,data):
 return Node(data)
 def insert(self,root,data):
 if(root == None):
 root = self.addNode(data)
 else:
 if(data <= root.data):
 root.left = self.insert(root.left,data)
 else:
 root.right = self.insert(root.right,data)
 return root
 def PreOrder(self,root):
 if root == None:
 pass
 else:
 print(root.data)
 self.PreOrder(root.left)
 self.PreOrder(root.right)
a = BinaryTree()
root = a.addNode(2)
#root = None
a.insert(root,4)
a.insert(root,34)
a.insert(root,45)
a.insert(root,46)
a.insert(root,41)
a.insert(root,48)
a.PreOrder(root)

However changing the 2nd and the 3rd lines in main to

#root = a.addNode(2)
root = None

doesn't print anything. I feel I am missing out on something basic here. Any clarifications will be gratefully appreciated.

• Martijn Pieters: You have a self.root attribute but never assign to it, then keep passing around an explicit root. Why not just do self.root = and use self.root throughout?
• QuestionEverything: Why do people keep telling Binary Tree where they really mean Binary Search Tree?

Answer 1

You're passing None to your function, which you defined with:

if root == None:
 pass

That is why nothing gets printed.

Also, this is just a personal view, I would actually make PreOrder accept only the self argument, and do the PreOrder from there, making it very simple to define recursively.

Basically something like:

 def PreOrder(self):
 print self.data 
 if self.left:
 print self.left.PreOrder()
 if self.right:
 print self.right.PreOrder()

But this is a matter of preference, your solution works just fine.

As blatant promotion, I actually wrote a post about writing a basic BinaryTree in Python recently, if you care to check it out, it can be found here:

http://intothewebs.tumblr.com/post/40256328302/embrace-the-basics-binary-tree

UPDATE:

Ok, after you comment, I understand your doubt.

The root parameter that is passed into your method isn't really changed, because params in Python are passed by value:

How do I pass a variable by reference?

Read the accepted answer in this question, it's awesome and should explain what I mean by that.

Of you have:

root = None
a = a.insert(root,4)
a.insert...

And so forth, your code should work.

• Bayko: Sorry if I wasnt clear in the question. After changing to "root = None" I still have it followed by a.Insert(root,4) and so forth. So how is root none when I enter the PreOrder function?

Answer 2

You have root = None and then in PreOrder your first line is if root == None: pass so it isn't going to do anything for you.

Answer 3

class Node:
def __init__(self,data):
 self.left = None
 self.right = None
 self.data = data
class BinaryTree:
 def __init__(self):
 self.root = None
 def insert_node(self,root,element):
 if self.root is None:
 self.root = Node(element)
 else:
 if root is None:
 root = Node(element)
 elif root.data <= element:
 root.right = self.insert_node(root.right,element)
 elif root.data > element:
 root.left = self.insert_node(root.left,element)
 return root
 def PreOrder(self,root):
 if root is not None:
 print(root.data)
 if root.left is not None:
 self.PreOrder(root.left)
 if root.right is not None:
 self.PreOrder(root.right)
a = BinaryTree()
a.insert_node(a.root,3)
a.insert_node(a.root,4)
a.insert_node(a.root,34)
a.insert_node(a.root,45)
a.insert_node(a.root,46)
a.insert_node(a.root,2)
a.insert_node(a.root,48)
a.PreOrder(a.root)