code: def mahalanobis(self):
          
          Mahalanobis distance of measurement. E.g. 3 means measurement
          was 3 standard deviations away from the predicted value.
  
          Returns
          -------
          mahalanobis : float
          """
          if self._mahalanobis is None:
              self._mahalanobis = sqrt(float(dot(dot(self.y.T, self.SI), self.y)))
          return self._mahalanobis
docs: Mahalanobis distance of measurement. E.g. 3 means measurement
          was 3 standard deviations away from the predicted value.
  
          Returns
          -------
          mahalanobis : float