fields.py

class FieldRoot(object):
    def __init__(self, __id__, __parent_repr__=lambda x: x):
        self.__id__ = __id__
        self.__parent_repr__ = __parent_repr__

    def __repr__(self):
        return self.__parent_repr__(self.__id__)

    def __embed__(self, child, __id__):
        return child(__id__,
                     __parent_repr__=lambda x: self.__parent_repr__(
                         self.__id__ + '{' + x + '}'))


class B(FieldRoot):
    @property
    def a(self):
        return self.__embed__(A, 'a')


class A(FieldRoot):
    @property
    def b(self):
        return self.__embed__(B, 'b')


a = A('a')

# a.b.a.b.a.b.a.b = 'a{b{a{b{a{b{a{b}}}}}}}'