Estoy teniendo un problema muy similar aesta pregunta tan, pero mis intentos de aplicar estas respuestas previas no se han desarrollado y se sugirió que comience como una nueva pregunta:

en el código a continuación, defino un par de funciones getChoices () que pensé que diferirían las referencias circulares, ¿pero no? que pasa aqui por favor

# ns.content/ns/content/foo.py
from zope import schema
from plone.directives import form
from z3c.relationfield.schema import Relation, RelationChoice
from plone.formwidget.contenttree import ObjPathSourceBinder

class IFoo(form.Schema):

    def getBarChoices():
        # avoiding circular refs...
        from bar import IBar
        return ObjPathSourceBinder(object_provides=IBar.__identifier__)

    barChoices = getBarChoices()
    form.widget(bar=AutocompleteFieldWidget)
    bar = Relation(source= barChoices,required=False)

# ns.content/ns/content/bar.py
from zope import schema
from plone.directives import form
from z3c.relationfield.schema import Relation, RelationChoice
from plone.formwidget.contenttree import ObjPathSourceBinder

class IBar(form.Schema):

    def getFooChoices():
        # avoiding circular refs...
        from foo import IFoo
        return ObjPathSourceBinder(object_provides=IFoo.__identifier__)

    fooChoices = getFooChoices()
    form.widget(foo=AutocompleteFieldWidget)
    foo = Relation(source= fooChoices,required=False)

resultingError = """
  File ".../buildout-cache/eggs/martian-0.11.3-py2.7.egg/martian/scan.py", line 217, in resolve
    __import__(used)
  File ".../zeocluster/src/ns.content/ns/content/bar.py", line 32, in <module>
    class IBar(form.Schema):
  File ".../zeocluster/src/ns.content/ns/content/bar.py", line 48, in IBar
    fooChoices = getFooChoices()
  File ".../zeocluster/src/ns.content/ns/content/bar.py", line 38, in getFooChoices
    from ns.content.foo import IFoo
  File ".../zeocluster/src/ns.content/ns/content/foo.py", line 33, in <module>
    class IFoo(form.Schema):
  File ".../zeocluster/src/ns.content/ns/content/foo.py", line 73, in IFoo
    barChoices = getBarChoices()
  File ".../zeocluster/src/ns.content/ns/content/foo.py", line 39, in getBarChoices
    from ns.content.bar import IBar
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File ".../zeocluster/parts/client1/etc/site.zcml", line 16.2-16.23
    ZopeXMLConfigurationError: File ".../buildout-cache/eggs/Products.CMFPlone-4.2.0.1-py2.7.egg/Products/CMFPlone/configure.zcml", line 102.4-106.10
    ZopeXMLConfigurationError: File ".../zeocluster/src/ns.content/ns/content/configure.zcml", line 18.2-18.27
    ImportError: cannot import name IBar
"""