def serialize(self, node: SchemaNode,
                  value: Any) -> Union[str, ColanderNullType]:
        """
        Serializes Python object to string representation.
        """
        if value is None:
            retval = ''
        else:
            # noinspection PyUnresolvedReferences
            retval = self.type_.serialize(node, value)
        # log.debug("AllowNoneType.serialize: {!r} -> {!r}", value, retval)
        return retval