#!/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)
         def __del__(self):
             print '%s.__del__()' % self
     
     # Construct two graph cycles
     one = Graph('one')
     two = Graph('two')
     three = Graph('three')
     one.set_next(two)
     two.set_next(three)
     three.set_next(one)
     
     # Remove references to the graph nodes in this module's namespace
     one = two = three = None
     
     # Collecting now keeps the objects as uncollectable
     print
     print 'Collecting...'
     n = gc.collect()
     print 'Unreachable objects:', n
     print 'Remaining Garbage:', 
     pprint.pprint(gc.garbage)
     
     REFERRERS_TO_IGNORE = [ locals(), globals(), gc.garbage ]
     
     def find_referring_graphs(obj):
         print 'Looking for references to %s' % repr(obj)
         referrers = (r for r in gc.get_referrers(obj)
                      if r not in REFERRERS_TO_IGNORE)
         for ref in referrers:
             if isinstance(ref, Graph):
                 # A graph node
                 yield ref
             elif isinstance(ref, dict):
                 # An instance or other namespace dictionary
                 for parent in find_referring_graphs(ref):
                     yield parent
     
     # Look for objects that refer to the objects that remain in
     # gc.garbage.
     print
     print 'Clearing referrers:'
     for obj in gc.garbage:
         for ref in find_referring_graphs(obj):
             ref.set_next(None)
             del ref # remove local reference so the node can be deleted
         del obj # remove local reference so the node can be deleted
     
     # Clear references held by gc.garbage
     print
     print 'Clearing gc.garbage:'
     del gc.garbage[:]
             
     # Everything should have been freed this time
     print
     print 'Collecting...'
     n = gc.collect()
     print 'Unreachable objects:', n
     print 'Remaining Garbage:', 
     pprint.pprint(gc.garbage)