new_contents: from longclaw.longclawsettings.models import LongclawSettings
  from longclaw.longclawshipping.models import ShippingRate
  from django_countries import countries, fields
  
  class CountryChoices(object):
      '''
      Helper class which returns a list of available countries based on
      the selected shipping options.
  
      If default_shipping_enabled is ``True`` in the longclaw settings, then
      all possible countries are returned. Otherwise only countries for
      which a ``ShippingRate`` has been declared are returned.
      '''
      def __init__(self, **kwargs):
          request = kwargs.get('request', None)
          self._all_countries = True
          if request:
              settings = LongclawSettings.for_site(request.site)
              self._all_countries = settings.default_shipping_enabled
  
      def __call__(self, *args, **kwargs):
          if self._all_countries:
              return countries
          else:
              return ShippingRate.objects.values_list('countries').distinct()
  
  
  class ShippingCountryField(fields.CountryField):
      '''
      Country choice field whose choices are constrained by the
      configured shipping options.
      '''
      def __init__(self, *args, **kwargs):
          kwargs.update({
              'countries': CountryChoices(**kwargs)
          })
          super(ShippingCountryField, self).__init__(*args, **kwargs)
  
  
subject: Add a country field for available shipping countries