code: def diff(self, i, order=1):
          
  
          old_val = copy(self[i])
          d = derivative(
              func=self.func_for_diff,
              x0=old_val,
              dx=self.eps[i],
              n=order,
              args=[i],
              order=self.diff_order)
          self[i] = old_val
          return d
docs: N.diff(i, order=1)
  
          Derivative wrt index i to given order.