How to use the persistent.Persistent._p_invalidate function in persistent

def loadblkdata(self):
        # ensure ._v_blkdata is loaded
        # (it could be not, if e.g. loadblk is called second time for the same
        #  blk - we loaded it first time and thrown ._v_blkdata away as unneeded
        #  intermediate copy not to waste memory - see below)
        if self._v_blkdata is None:
            # ZODB says: further accesses will cause object data to be reloaded.
            # NOTE directly Persistent._p_invalidate() to avoid
            # virtmem->loadblk->invalidate callback
            Persistent._p_invalidate(self)

        blkdata = self._v_blkdata
        assert blkdata is not None
        # do not waste memory - ._v_blkdata is used only as intermediate copy on path
        # from DB to fileh mapping. See also counterpart action in __getstate__().
        self._v_blkdata = None

        return blkdata

def _p_invalidate(self):
        # do real invalidation only once - else we already lost ._v_zfile last time
        if self._p_state is GHOST:
            return
        # on invalidation we must be already bound
        # (to know which ZBigFileH to propagate invalidation to)
        assert self._v_zfile    is not None
        assert self._v_blk      is not None
        self._v_zfile.invalidateblk(self._v_blk)
        Persistent._p_invalidate(self)