code: def select_by_key(self, key):
          
          for item in self.children.values():
              if 'selected' in item.attributes:
                  del item.attributes['selected']
          self.children[key].attributes['selected'] = 'selected'
          self._selected_key = key
          self._selected_item = self.children[key]
docs: Selects an item by its unique string identifier.
  
          Args:
              key (str): Unique string identifier of the DropDownItem that have to be selected.