code: def __updateSpreadMeanAndStd(self, values1, values2):
      if self.__hedgeRatio is not None:
          spread = values1 - values2 * self.__hedgeRatio
          self.__spreadMean = spread.mean()
          self.__spreadStd = spread.std(ddof=1)
  
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 8 for calling the StatArbHelper obj's underlying member method code with example usage: obj.__updateSpreadMeanAndStd(values1, values2) without return types