def do_execute(self):
        """
        The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str
        """
        if self.storagehandler is None:
            return "No storage handler available!"
        expr = str(self.resolve_option("expression")).replace(
            "{X}", str(self.storagehandler.storage[str(self.resolve_option("storage_name"))]))
        expr = self.storagehandler.expand(expr)
        self.storagehandler.storage[self.resolve_option("storage_name")] = eval(expr)
        self._output.append(self.input)
        return None
The actual execution of the actor.

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