non-working example of implementing `persistent_id` on top of PyYAML

yaml_and_persitent_id.py

#! /usr/bin/python

import yaml

class Persistable(object):
    # simulate a unique id
    _unique = 0

    def __init__(self, *args, **kw):
        Persistable._unique += 1
        self.persistent_id = ("%s.%d" %
                              (self.__class__.__name__, Persistable._unique))


def persistable_representer(dumper, data):
    id = data.persistent_id
    outfile = open(id, 'w')
    outfile.write(yaml.dump(data))
    outfile.close()
    return dumper.represent_scalar(u'!xref', u'%s' % id)
yaml.add_representer(Persistable, persistable_representer)


def persistable_constructor(loader, node):
    xref = loader.construct_scalar(node)
    infile = open(xref, 'r')
    value = yaml.load(infile.read())
    infile.close()
    return value
yaml.add_constructor(u'!xref', persistable_constructor)


class Foo(Persistable):
    def __init__(self):
        self.one = 1
        Persistable.__init__(self)

class Bar(Persistable):
    def __init__(self, foo):
        self.foo = foo
        Persistable.__init__(self)


# this works, but does not consider `foo` and `bar` as `Persistable` objects
foo = Foo()
bar = Bar(foo)
print "=== foo ==="
print yaml.dump(foo)
print "=== bar ==="
print yaml.dump(bar)

# this generates a "recursion depth exceeded" error
baz = Bar(Persistable())
print "=== baz ==="
print yaml.dump(baz)