circular import references from zope schemas

i'm having a problem very similar to this SO question, but my attempts to apply these previous answers isn't going thru and it was suggested i start it as a new question:

in the code below i define a couple of getChoices() functions that i thought would defer the circular refs, but no!? what's wrong here, please?

# 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
"""

You're calling getBarChoices() at definition time, when defining the class IFoo. So from bar import IBar will be executed while parsing foo.py leading to the circular import.

As far as I see it, you have basically two choices:

1) Use a string as identifier for object_provides.

You're doing that already anyway by using IFoo.__identifier__, but if you make it static instead of dynamic that will eliminate your circular dependencies:

source = ObjPathSourceBinder(object_provides='ns.content.bar.IBar')
bar = Relation(source=source,required=False)

No need to import IBar in foo.py. This has the obvious disadvantage that the location of IBar is now hardcoded in your code, so whenever you change the name or location of IBar you'll need to update its dotted name in foo.py.

2) Marker interfaces

The other alternative would be to let IFoo and IBar implement marker interfaces that you keep in a third file, ns/content/interfaces.py for example. That way you could do something along the lines of

interfaces.py

from zope.interface import Interface

class IBarMarker(Interface):
    """Marker interface for IBar objects.
    """

class IFooMarker(Interface):
    """Marker interface for IFoo objects.
    """

foo.py

from zope.interface import directlyProvides
from plone.directives import form
from plone.formwidget.contenttree import ObjPathSourceBinder
from plone.formwidget.autocomplete import AutocompleteFieldWidget
from z3c.relationfield.schema import RelationChoice

from ns.content.interfaces import IBarMarker
from ns.content.interfaces import IFooMarker


class IFoo(form.Schema):
    directlyProvides(IFooMarker)

    form.widget(bar=AutocompleteFieldWidget)
    bar = RelationChoice(source=ObjPathSourceBinder(
                            object_provides=IBarMarker.__identifier__),
                         required=False)

bar.py

class IBar(form.Schema):
    directlyProvides(IBarMarker)

    form.widget(foo=AutocompleteFieldWidget)
    foo = RelationChoice(source=ObjPathSourceBinder(
                            object_provides=IFooMarker.__identifier__),
                         required=False)