preorder.py

import collections

Node = collections.namedtuple('Node', 'value left right')

def make_node(value, left=None, right=None):
    return Node(value, left, right)

def preorder(tree):
    stack = [tree]
    while stack:
        current = stack.pop()
        yield current.value
        if current.right:
            stack.append(current.right)
        if current.left:
            stack.append(current.left)

example = make_node(0,
               left=make_node(6,
                              left=make_node(3),
                              right=make_node(1, right=make_node(4))),
               right=make_node(8, left=make_node(7)))

for node in preorder(example):
    print node