#!/usr/bin/env python
     # encoding: utf-8
     #
     # Copyright (c) 2010 Doug Hellmann.  All rights reserved.
     #
     """Show the objects with references to a given object.
     """
     #end_pymotw_header
     
     import gc
     import pprint
     import Queue
     
     class Graph(object):
         def __init__(self, name):
             self.name = name
             self.next = None
         def set_next(self, next):
             print 'Linking nodes %s.next = %s' % (self, next)
             self.next = next
         def __repr__(self):
             return '%s(%s)' % (self.__class__.__name__, self.name)
     
     # Construct a graph cycle
     one = Graph('one')
     two = Graph('two')
     three = Graph('three')
     one.set_next(two)
     two.set_next(three)
     three.set_next(one)
     
     print
     
     seen = set()
     to_process = Queue.Queue()
     
     # Start with an empty object chain and Graph three.
     to_process.put( ([], three) )
     
     # Look for cycles, building the object chain for each object we find
     # in the queue so we can print the full cycle when we're done.
     while not to_process.empty():
         chain, next = to_process.get()
         chain = chain[:]
         chain.append(next)
         print 'Examining:', repr(next)
         seen.add(id(next))
         for r in gc.get_referents(next):
             if isinstance(r, basestring) or isinstance(r, type):
                 # Ignore strings and classes
                 pass
             elif id(r) in seen:
                 print
                 print 'Found a cycle to %s:' % r
                 for i, link in enumerate(chain):
                     print '  %d: ' % i,
                     pprint.pprint(link)
             else:
                 to_process.put( (chain, r) )