how to set the default value of bool type in plone dexterity addform based on parent

Question

I'm trying to control the default for a schema.Bool, but I want it based on a condition. I'm have tried two ways and I can't figure it out.

first option:

def isCreatedInHomePage():

     print "is this pic being created at the site base? if it is, then default true.  if not default false"
     return False


class IPics(form.Schema):
    form.widget(displayOnHomepage=SingleCheckBoxFieldWidget)
    displayOnHomepage = schema.Bool(
            title=_(u"Display On Site Homepage"),
            #default=True,
            defaultFactory=isCreatedInHomePage,
            required=False,
        )

while the defaultFactory does get called and will correctly set true or false (based on what i tell it), i could not figure out how to check what the parent page from where the form was called. any thoughts on what i could use there?

the second option i try to control it in the AddForm itself:

class AddForm(dexterity.AddForm):
    grok.name('pics')
    grok.context(IPics)

    def updateWidgets(self):
        print "in update widgets "

        super(AddForm, self).updateWidgets()
        # this will give an empty list if False (or not selected) and will
        # return "['selected']" if True (check box is checked)
        print 'self.widgets["displayOnHomepage"].value', self.widgets["displayOnHomepage"].value

        self.widgets["displayOnHomepage"].value = ['selected']

        # this will return to whatever is set in the above line.
        print 'self.widgets["displayOnHomepage"].value', self.widgets["displayOnHomepage"].value

    def createAndAdd(self, data):
        ...
        ...
        ...

Any help would be appreciated :)

Answer 1

working with the answer from @Mathias i got other ideas and solved the problem for this problem. i feel this is a workaround and that a better solution exists. i put it into the try to avoid possible attribute error.

@form.default_value(field=IPics['displayOnHomepage'])
def isCreatedInHomePage(data):
    try:
        print data.context.getParentNode().portal_type
        if data.context.getParentNode().portal_type == 'Plone Site':
            return True
        else:
            return False
    except:
        pass