@abstractproperty
  def optional(self):
Find Trello API credentials (`api_token` and `token`) from config.
dataset/ETHPy150Open pantsbuild/pants/src/python/pants/engine/exp/selectors.py/Selector.optional--crossover with--dataset/ETHPy150Open StackStorm/st2contrib/packs/trello/sensors/list_actions_sensor.py/TrelloListSensor._update_credentials_by_precedence