def _dr_dx(self, x, y):
        """
        derivative of dr/dx, dr/dy
        :param x:
        :param y:
        :return:
        """

        r = np.sqrt(x**2 + y**2)
        if isinstance(r, int) or isinstance(r, float):
            if r == 0:
                r = 1
        else:
             r[r == 0] = 1
        return x/r, y/r
derivative of dr/dx, dr/dy
        :param x:
        :param y:
        :return: