code: def _get_sensor_names(self):
          
          # if the user didn't tell us what sensors to work with, let's figure it
          # out
          if not self.attrs.get('sensor'):
              # reader finder could return multiple readers
              return set([sensor for reader_instance in self.readers.values()
                          for sensor in reader_instance.sensor_names])
          elif not isinstance(self.attrs['sensor'], (set, tuple, list)):
              return set([self.attrs['sensor']])
          else:
              return set(self.attrs['sensor'])
docs: Join the sensors from all loaded readers.