tree.py

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):
      return self.total(self.left) + self.total(self.right) + self.cargo

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