<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def running_state(self, running_state):
 """Sets the running_state of this MaintenanceWindow. :param running_state: The running_state of this MaintenanceWindow. # noqa: E501 :type: str """

        allowed_values = ["ONGOING", "PENDING", "ENDED"]  # noqa: E501
        if running_state not in allowed_values:
            raise ValueError(
                "Invalid value for `running_state` ({0}), must be one of {1}"  # noqa: E501
                .format(running_state, allowed_values)
            )

        self._running_state = running_state