node_test.py

class Node(object):
  def __init__(self, value, connected=None):
    self.value = value
    if connected:
      self.connected = [node for node in connected]
    else:
      self.connected = []
    
  def add_connection(self, node):
    if isinstance(node, Node):
      self.connected.append(node)
      node.connected.append(self)
    else:
      raise Exception('nodes must be Node instances')
      
  def is_connected(self, node):
    return node in self.connected