weakref_graph.py

#!/usr/bin/env python


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(%r)' % (self.name, 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 at 0x%x name=%s>' % (self.__class__.__name__,
                                         id(self), self.name)

    def __del__(self):
        print '(Deleting %s)' % self.name
        self.set_next(None)


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 'Graph:'
    print str(one)
    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()