#!/usr/bin/env python
     # encoding: utf-8
     #
     # Copyright (c) 2008 Doug Hellmann All rights reserved.
     #
     """Tools for demonstrating garbage collection
     """
     #end_pymotw_header
     
     import gc
     from pprint import pprint
     import weakref
     
     class Graph(object):
         def __init__(self, name):
             self.name = name
             self.other = None
         def set_next(self, other):
             print '%s.set_next(%s (%s))' % (self.name, other, type(other))
             self.other = other
         def all_nodes(self):
             "Generate the nodes in the graph sequence."
             yield self
             n = self.other
             while n and n.name != self.name:
                 yield n
                 n = n.other
             if n is self:
                 yield n
             return
         def __str__(self):
             return '->'.join([n.name for n in self.all_nodes()])
         def __repr__(self):
             return '%s(%s)' % (self.__class__.__name__, self.name)
         def __del__(self):
             print '(Deleting %s)' % self.name
             self.set_next(None)
     
     class WeakGraph(Graph):
         def set_next(self, other):
             if other is not None:
                 # See if we should replace the reference
                 # to other with a weakref.
                 if self in other.all_nodes():
                     other = weakref.proxy(other)
             super(WeakGraph, self).set_next(other)
             return
     
     def collect_and_show_garbage():
         "Show what garbage is present."
         print 'Collecting...'
         n = gc.collect()
         print 'Unreachable objects:', n
         print 'Garbage:', 
         pprint(gc.garbage)
     
     def demo(graph_factory):
         print 'Set up graph:'
         one = graph_factory('one')
         two = graph_factory('two')
         three = graph_factory('three')
         one.set_next(two)
         two.set_next(three)
         three.set_next(one)
         
         print
         print 'Graphs:'
         print str(one)
         print str(two)
         print str(three)
         collect_and_show_garbage()
     
         print
         three = None
         two = None
         print 'After 2 references removed:'
         print str(one)
         collect_and_show_garbage()
     
         print
         print 'Removing last reference:'
         one = None
         collect_and_show_garbage()