code: def __onOrderBookUpdate(self, orderBookUpdate):
      bid = orderBookUpdate.getBidPrices()[0]
      ask = orderBookUpdate.getAskPrices()[0]
  
      if bid != self.__bid or ask != self.__ask:
          self.__bid = bid
          self.__ask = ask
          self.info(
              "Order book updated. Best bid: %s. Best ask: %s"
              % (self.__bid, self.__ask)
          )
  
file_path: pyalgotrade/samples/tutorial_bitstamp_1.py
imports: from pyalgotrade.bitstamp import barfeed
  from pyalgotrade.bitstamp import broker
  from pyalgotrade import strategy
  from pyalgotrade.technical import ma
  from pyalgotrade.technical import cross
label_desc: Use image node_id 2 for calling the Strategy obj's underlying member method code with example usage: obj.__onOrderBookUpdate(orderBookUpdate) without return types