def METHOD_NAME(self, residualFn=None):
    if residualFn is not None:
        return residualFn(self.var, self.matrix, self.RHSvector)
    else:
        comm = self.var.mesh.communicator
        residual, globalMatrix = self._calcResidualVectorNonOverlapping_()
        return comm.Norm2(residual)