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