ChoiceField - dynamic attribute value assignment

gistfile1.txt

I need to assign the value to the 'initial' attributes for the images and flavors fields.
Here is how I do it currently.
No errors, the print shows that assignment took, but in the actual html form, I still have my default choice list, with no item selected, as desired

forms.py:

class UpdateWorkload(forms.SelfHandlingForm):
    name = forms.CharField(max_length="255", label=_("Workload Name"))
    description = forms.CharField(widget=forms.Textarea,
                                  label=_("Description"), required=False)
    image_choices = []
    images = forms.ChoiceField(label=_("Images"), choices=image_choices, initial="")
    flavor_choices = []
    flavors = forms.ChoiceField(label=_("Flavors"), choices=flavor_choices, initial="")

    def __init__(self, request, image_choices=image_choices, flavor_choices=flavor_choices,
                 *args, **kwargs):
        super(UpdateWorkload, self).__init__(request, *args, **kwargs)

        images, self._more, self._prev = api.glance.image_list_detailed(
            self.request)
        flavors = api.nova.flavor_list(request, True)

        for image in images:
                image_choices.append((image.id, image.name))
        if len(image_choices) > 1:
            image_choices.insert(0, ('', _("Select an Image")))
        self.fields['images'].choices = image_choices

        for flavor in flavors:
                flavor_choices.append((flavor.id, flavor.name))
        if len(flavor_choices) > 1:
            flavor_choices.insert(0, ('', _("Select an Flavor")))
        self.fields['flavors'].choices = flavor_choices
        selected_workload = kwargs['initial']
        selected_image = selected_workload['image']
        selected_flavor = selected_workload['flavor']

        print "initial: " + self.fields['images'].initial

        self.fields['images'].initial = str(selected_image)
        self.fields['images'].initial = str(selected_flavor)

        print "initial after assignment: " + self.fields['images'].initial