code: def surface_brightness(self, x, y, kwargs_list, k=None):
          
          x = np.array(x, dtype=float)
          y = np.array(y, dtype=float)
          flux = np.zeros_like(x)
          for i, func in enumerate(self.func_list):
              if k is None or k == i:
                  out = np.array(func.function(x, y, **kwargs_list[i]), dtype=float)
                  flux += out
          return flux
docs: :param x: coordinate in units of arcsec relative to the center of the image
          :type x: set or single 1d numpy array