code: def activate_classification(self, exposure, classification=None):
          
          if self.layer_mode == layer_mode_continuous:
              selected_unit = self.parent.step_kw_unit.selected_unit()['key']
              target = self.thresholds.get(exposure['key'])
              if target is None:
                  self.thresholds[exposure['key']] = {}
              target = self.thresholds.get(exposure['key'])
          else:
              selected_unit = None
              target = self.value_maps.get(exposure['key'])
              if target is None:
                  self.value_maps[exposure['key']] = {}
              target = self.value_maps.get(exposure['key'])
  
          if classification is not None:
              if classification['key'] not in target:
                  if self.layer_mode == layer_mode_continuous:
                      default_classes = default_classification_thresholds(
                          classification, selected_unit)
                      target[classification['key']] = {
                          'classes': default_classes,
                          'active': True
                      }
                  else:
                      # Set classes to empty, since we haven't mapped anything
                      target[classification['key']] = {
                          'classes': {},
                          'active': True
                      }
                  return
  
          for classification_key, value in list(target.items()):
              if classification is None:
                  value['active'] = False
                  continue
  
              if classification_key == classification['key']:
                  value['active'] = True
              else:
                  value['active'] = False
docs: Set active to True for classification for the exposure.
  
          If classification = None, all classification set active = False.
  
          :param exposure: Exposure definition.
          :type exposure: dict
  
          :param classification: Classification definition.
          :type classification: dict