z3c.forms dynamic sources provider returns empty dictionary as a context object

Question

I'm using Plone 4.1.4 and I'm trying to get dynamic sources for a schema.Choice to work, I need to populate country list which in turn depends on the context object.

I'm using this example: http://plone.org/products/dexterity/documentation/manual/developer-manual/advanced/vocabularies

For IContextSourceBinder, example, an empty dictionary instead of actual context object is returned:

from zope import interface
from zope.schema.vocabulary import SimpleVocabulary, SimpleTerm
from zope.schema.interfaces import IContextSourceBinder
import zope.schema
from z3c.form import form

class CountryGenerator(object):
    interface.implements(IContextSourceBinder)
    def __call__(self, context):
        #context is == {}
        import pdb; pdb.set_trace()
        return SimpleVocabulary([
            SimpleTerm(value="not_selected", title=_("Country Not Selected"))
                ])

class IStep(interface.Interface):
    region = schema.Choice(title=_("Select your country"),
                        required=True,
                        source=CountryGenerator,
                        default="not_selected")
class Step(form.Form):
    fields = field.Fields(IStep)
    label = _("Step")
    description = _("Select your country")

When debugging point is hit inside CountryGenerator.__call__() method and I examine context object, the latter turn out to be just an empty dictionary.

When I try to use named utility example in the article mentioned above, and the similar thing happens, context there is also {}.

Could anyone point me to what I might be doing wrong?

UPDATE

ZCML for the form wrapper that calls the form is

<browser:page
  name="view"
  for="Products.oldproduct.MyFolderishClass"
  class=".file.RegionClass"
  permission="zope2.View"
  />

Where RegionClass inherits from Form wrapper, could it be permission or traversal issue?

Answer 1

Because your source is a class, you need to instantiate it:

class IStep(interface.Interface):
    region = schema.Choice(title=_("Select your country"),
                        required=True,
                        source=CountryGenerator(),
                        default="not_selected")

In certain circumstances, such as with using sub forms or complex form widgets (widget within a widget for list selections, etc), you need to follow the __parent__ pointers to a proper outer context for getting back to the Plone context.