[jackqu7]

Yes, this is possible as long as the second level nested object has a role to stop infinite recursion from occurring. Cycles are not automatically detected.

    class BaseMapper(Mapper):

        __type__ = TestType

        score = Integer()
        nest = Nested('NestedMapper')

        __roles__ = {'nested': blacklist('nest')}

    class NestedMapper(Mapper):

        __type__ = TestType

        back = Nested('BaseMapper', role='nested')
        name = String()

    obj2 = TestType(name='test')
    obj = TestType(score=5, nest=obj2)
    obj2.back = obj

    >> BaseMapper(obj=obj).serialize()
    {'nest': {'back': {'score': 5}, 'name': 'test'}, 'score': 5}

[arnarbi]

That's not a solution as it will not restore the same object graph, it will just repeat values.

One way is to to store a table of objects (as identified by id()) encountered during serialization, indexed by the order you encounter them. If you encounter an object you have already serialized, serialize an index into that table. On deserialization, construct the same kind of table, and deserialize an index with a reference to the same object.

See e.g. AMF for an example format that does this: https://en.wikipedia.org/wiki/Action_Message_Format

[amelius]

The output isn't clear to me:

    {'nest': {'back': {'score': 5}, 'name': 'test'}, 'score': 5}

How can this be mapped back unambiguously to a cyclic structure?

(I might be wrong but it seems to me that the act of serialization has simply expanded the cycle one level deep.)