#!/usr/bin/env python
     # encoding: utf-8
     #
     # Copyright (c) 2009 Doug Hellmann All rights reserved.
     #
     """
     """
     #end_pymotw_header
     
     import pickle
     
     class Node(object):
         """A simple digraph where each node knows about the other nodes
         it leads to.
         """
         def __init__(self, name):
             self.name = name
             self.connections = []
             return
     
         def add_edge(self, node):
             "Create an edge between this node and the other."
             self.connections.append(node)
             return
     
         def __iter__(self):
             return iter(self.connections)
     
     def preorder_traversal(root, seen=None, parent=None):
         """Generator function to yield the edges via a preorder traversal."""
         if seen is None:
             seen = set()
         yield (parent, root)
         if root in seen:
             return
         seen.add(root)
         for node in root:
             for (parent, subnode) in preorder_traversal(node, seen, root):
                 yield (parent, subnode)
         return
         
     def show_edges(root):
         "Print all of the edges in the graph."
         for parent, child in preorder_traversal(root):
             if not parent:
                 continue
             print '%5s -> %2s (%s)' % (parent.name, child.name, id(child))
     
     # Set up the nodes.
     root = Node('root')
     a = Node('a')
     b = Node('b')
     c = Node('c')
     
     # Add edges between them.
     root.add_edge(a)
     root.add_edge(b)
     a.add_edge(b)
     b.add_edge(a)
     b.add_edge(c)
     a.add_edge(a)
     
     print 'ORIGINAL GRAPH:'
     show_edges(root)
     
     # Pickle and unpickle the graph to create
     # a new set of nodes.
     dumped = pickle.dumps(root)
     reloaded = pickle.loads(dumped)
     
     print
     print 'RELOADED GRAPH:'
     show_edges(reloaded)