code: def _set_pk(self, value):
          
          if self._pk:
              raise ImplementationError('Something wrong happened, the PK was already set !')
          self._pk = value
          self._connected = True
          # Default must be set only at first initialization
          self._set_defaults()
docs: Use the given value as the instance's primary key, if it doesn't have
          one yet (it must be used only for new instances). Then save default values.