docs: def _union_copy(dict1, dict2):
      """
      Internal wrapper to keep one level of copying out of play, for efficiency.
  
      Only copies data on dict2, but will alter dict1.
      """
  
      for key, value in dict2.items():
          if key in dict1 and isinstance(value, dict):
              dict1[key] = _union_copy(dict1[key], value)
          else:
              dict1[key] = copy.deepcopy(value)
      return dict1