code: def sd(self):
          '''Calculate standard deviation of timeseries'''
          v = self.var()
          if len(v):
              return np.sqrt(v)
          else:
              return Nonf sd(self):
          '''Calculate standard deviation of timeseries'''
          v = self.var()
          if len(v):
              return np.sqrt(v)
          else:
              return None
docs: Calculate standard deviation of timeseries