main.py

# coding: utf-8

from itertools import dropwhile

input = 'nodes.txt'
findA = 'Node3'
findB = 'Node4'


def read_source():
    "Read text file."
    with open(input) as f:
        rows = f.read().split('\n')

    data = {}
    for row in filter(None, rows):
        pair = row.split(' ', 1)
        data[pair[0]] = pair[1] if len(pair) == 2 else None

    return data


def path_for(tree, node):
    "Get path for given node and tree."
    path, item = [], node
    while item is not None:
        path.append(item)
        item = tree[item]
    return path

if __name__ == '__main__':
    tree = read_source()
    pathA, pathB = path_for(tree, findA), path_for(tree, findB)

    try:
        result = dropwhile(lambda x: x not in pathB, pathA).next()
    except StopIteration:
        print 'No common ancestors'