code: def filter_line(self, route):
      
      if self._route_type is not None and self._route_type != route.route_type:
        self.info('Skipping route %s due to different route_type value (%s)' %
                  (route['route_id'], route['route_type']))
        return
      self.info('Filtering infrequent trips for route %s.' % route.route_id)
      trip_count = len(route.trips)
      for pattern_id, pattern in route.GetPatternIdTripDict().items():
        ratio = float(1.0 * len(pattern) / trip_count)
        if not self._force:
          if (ratio < self._threshold):
            self.info("\t%d trips on route %s with headsign '%s' recognized "
                      "as unusual (ratio %f)" %
                      (len(pattern),
                      route['route_short_name'],
                      pattern[0]['trip_headsign'],
                      ratio))
            for trip in pattern:
              trip.trip_type = 1 # special
              self.info("\t\tsetting trip_type of trip %s as special" %
                        trip.trip_id)
        else:
          self.info("\t%d trips on route %s with headsign '%s' recognized "
                    "as %s (ratio %f)" %
                    (len(pattern),
                     route['route_short_name'],
                     pattern[0]['trip_headsign'],
                     ('regular', 'unusual')[ratio < self._threshold],
                     ratio))
          for trip in pattern:
            trip.trip_type = ('0','1')[ratio < self._threshold]
            self.info("\t\tsetting trip_type of trip %s as %s" %
                      (trip.trip_id,
                       ('regular', 'unusual')[ratio < self._threshold]))
docs: Mark unusual trips for the given route.