code: def _set_defaults(self):
          
          for field_name in self._fields:
              if field_name in self._init_fields:
                  continue
              field = self.get_field(field_name)
              if hasattr(field, "default"):
                  field.proxy_set(field.default)
docs: Set default values to fields. We assume that they are not yet populated
          as this method is called just after creation of a new pk.