@abc.abstractmethod<EOL><INDENT>def get_time_periods(self):<DEDENT>
return<EOL>
Gets all ``TimePeriods``.

        In plenary mode, the returned list contains all known time
        periods or an error results. Otherwise, the returned list may
        contain only those time periods that are accessible through this
        session.

        :return: a ``TimePeriodList``
        :rtype: ``osid.calendaring.TimePeriodList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*