binary tree

gistfile1.txt

#!/usr/bin/python

#Building the trees
class Tree(object):
   def __init__(self, cargo, left=None, right=None):
      self.cargo = cargo
      self.left = left
      self.right = right

   def __str__(self):
      return str(self.cargo)

   #Traversing through the tree
   def total(self):
      if self == None: return 0
      return self.total(self.left) + self.total(self.right) + self.cargo

   #Traversing through the tree using preorder
   def print_tree(tree):
      if tree == None: return
      print tree.cargo
      print_tree(tree.left)
      print_tree(tree.right)

#Allocating the child nodes
left = Tree(2)
right = Tree(3)
tree = Tree(1, left, right)
#tree = Tree(1, Tree(2), Tree(3))
print_tree(tree)

Error Messages:
File "./tree.py", line 30, in <module>
    print print_tree(tree)
NameError: name 'print_tree' is not defined