#!/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
     
     flags = (gc.DEBUG_COLLECTABLE |
              gc.DEBUG_UNCOLLECTABLE |
              gc.DEBUG_INSTANCES
              )
     gc.set_debug(flags)
     
     class Graph:
         def __init__(self, name):
             self.name = name
             self.next = None
             print 'Creating %s 0x%x (%s)' % (self.__class__.__name__, id(self), name)
         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)
     
     class CleanupGraph(Graph):
         def __del__(self):
             print '%s.__del__()' % self
     
     # Construct a graph cycle
     one = Graph('one')
     two = Graph('two')
     one.set_next(two)
     two.set_next(one)
     
     # Construct another node that stands on its own
     three = CleanupGraph('three')
     
     # Construct a graph cycle with a finalizer
     four = CleanupGraph('four')
     five = CleanupGraph('five')
     four.set_next(five)
     five.set_next(four)
     
     # Remove references to the graph nodes in this module's namespace
     one = two = three = four = five = None
     
     print
     
     # Force a sweep
     print 'Collecting'
     gc.collect()
     print 'Done'