def post_execute(self):
        """
        Gets executed after the actual execution.

        :return: None if successful, otherwise error message
        :rtype: str
        """
        result = super(Sink, self).post_execute()
        if result is None:
            self._input = None
        return result
Gets executed after the actual execution.

        :return: None if successful, otherwise error message
        :rtype: str