code: def update(self):
      if len(self.__ds1) >= self.__windowSize:
          values1 = np.asarray(self.__ds1[-1 * self.__windowSize :])
          values2 = np.asarray(self.__ds2[-1 * self.__windowSize :])
          self.__updateHedgeRatio(values1, values2)
          self.__updateSpread()
          self.__updateSpreadMeanAndStd(values1, values2)
          self.__updateZScore()
  
imports: from __future__ import print_function
  from pyalgotrade import strategy
  from pyalgotrade import dataseries
  from pyalgotrade.dataseries import aligned
  from pyalgotrade import plotter
  from pyalgotrade.barfeed import yahoofeed
  from pyalgotrade.stratanalyzer import sharpe
  import numpy
  import statsmodels.api
label_desc: Use image node_id 11 for calling the StatArbHelper obj's underlying member method code with example usage: obj.update() without return types