code: def active_classification(keywords, exposure_key):
      
      classifications = None
      if 'classification' in keywords:
          return keywords['classification']
      if 'layer_mode' in keywords and keywords['layer_mode'] == \
              layer_mode_continuous['key']:
          classifications = keywords['thresholds'].get(exposure_key)
      elif 'value_maps' in keywords:
          classifications = keywords['value_maps'].get(exposure_key)
  
      if classifications is None:
          return None
      for classification, value in list(classifications.items()):
          if value['active']:
              return classification
      return None
docs: Helper to retrieve active classification for an exposure.
  
      :param keywords: Hazard layer keywords.
      :type keywords: dict
  
      :param exposure_key: The exposure key.
      :type exposure_key: str
  
      :returns: The active classification key. None if there is no active one.
      :rtype: str