code: def update_source_hashes(self, instance):
          
          self.init_instance(instance)
          instance._ik['source_hashes'] = dict(
              (attname, hash(getattr(instance, attname)))
              for attname in self.get_source_fields(instance))
          return instance._ik['source_hashes']
docs: Stores hashes of the source image files so that they can be compared
          later to see whether the source image has changed (and therefore whether
          the spec file needs to be regenerated).