def unpersistable(self, reason, sxp=None):
        """
        (internal) Returns an sexp: (unpersistable "reason").  Utility method
        for making note that a particular object could not be serialized.
        """
        if sxp is None:
            sxp = []
        sxp.append(unpersistable_atom)
        if isinstance(reason, unicode):
            reason = reason.encode("utf-8")
        sxp.append(reason)
        return sxp

        (internal) Returns an sexp: (unpersistable "reason").  Utility method
        for making note that a particular object could not be serialized.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/spread/jelly.py