code: def get_activities_by_ids(self, activity_ids=None):
          
          if activity_ids is None:
              raise NullArgument()
          activities = []
          for i in activity_ids:
              activity = None
              url_path = construct_url('activities',
                                       bank_id=self._catalog_idstr,
                                       act_id=i)
              try:
                  activity = self._get_request(url_path)
              except (NotFound, OperationFailed):
                  if self._activity_view == PLENARY:
                      raise
                  else:
                      pass
              if activity:
                  if not (self._activity_view == COMPARATIVE and
                          activity in activities):
                      activities.append(activity)
          return objects.ActivityList(activities)
docs: Gets an ActivityList corresponding to the given IdList.
          In plenary mode, the returned list contains all of the
          activities specified in the Id list, in the order of the list,
          including duplicates, or an error results if an Id in the
          supplied list is not found or inaccessible. Otherwise,
          inaccessible Activities may be omitted from the list and may
          present the elements in any order including returning a unique
          set.
          arg:    activityIds (osid.id.IdList): the list of Ids to
                  retrieve
          return: (osid.learning.ActivityList) - the returned Activity
                  list
          raise:  NotFound - an Id was not found
          raise:  NullArgument - activityIds is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method must be implemented.