code: def __updateZScore(self):
      if (
          self.__spread is not None
          and self.__spreadMean is not None
          and self.__spreadStd is not None
      ):
          self.__zScore = (self.__spread - self.__spreadMean) / float(
              self.__spreadStd
          )
  
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 10 for calling the StatArbHelper obj's underlying member method code with example usage: obj.__updateZScore() without return types