Python binary tree traversal

# Python
from itertools import chain
from collections import deque

class n(object):
    left = None
    right = None
    value = None
    
    def __init__(self, value=None, left=None, right=None):
        self.value = value
        self.left = left
        self.right = right
        
    def __call__(self, left=None, right=None):
        self.left = left
        self.right = right
        return self.value

def preorder(node):
    lefts = []
    rights = []
    
    if node.left != None:
        lefts = preorder(node.left)
        
    if node.right != None:
        rights = preorder(node.right)
        
    return chain([node], lefts, rights)
        
def inorder(node):
    lefts = []
    rights = []
    
    if node.left != None:
        lefts = inorder(node.left)
    
    if node.right != None:
        rights = inorder(node.right)
        
    return chain(lefts, [node], rights)


def postorder(node):
    lefts = []
    rights = []
    
    if node.left != None:
        lefts = postorder(node.left)
    
    if node.right != None:
        rights = postorder(node.right)
        
    return chain(lefts, rights, [node])
    
    
def levelorder(node):
    queue = deque([node])
    
    while len(queue) > 0:
        node = queue.pop() # Remove from right
        yield node
        if node.left:
            queue.appendleft(node.left)
        if node.right:
            queue.appendleft(node.right)

# Create nodes
_ = None
A = n('A')
B = n('B')
C = n('C')
D = n('D')
E = n('E')
F = n('F')
G = n('G')
H = n('H')
I = n('I')

# Link the nodes
F(B,G)
B(A,D)
D(C,E)

G(_,I)
I(H,_)

print "Pre order:"
print list(node.value for node in preorder(F))
    
print "In order:"
print list(node.value for node in inorder(F))

print "Post order:"
print list(node.value for node in postorder(F))
    
print "Level order (queue):"
print list(node.value for node in levelorder(F))