code: def __updateSpread(self):
      if self.__hedgeRatio is not None:
          self.__spread = (
              self.__ds1[-1] - self.__hedgeRatio * self.__ds2[-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 9 for calling the StatArbHelper obj's underlying member method code with example usage: obj.__updateSpread() without return types