code: def fill_gradient(surface, color, gradient,
                    rect=None, vertical=True, forward=True):
  
      
  
      if rect is None:
          rect = surface.get_rect()
  
      x1, x2 = rect.left, rect.right
      y1, y2 = rect.top, rect.bottom
  
      if vertical:
          h = y2 - y1
      else:
          h = x2 - x1
  
      assert h > 0
  
      if forward:
          a, b = color, gradient
      else:
          b, a = color, gradient
  
      rate = (float(b[0] - a[0]) / h,
              float(b[1] - a[1]) / h,
              float(b[2] - a[2]) / h)
  
      fn_line = pygame.draw.line
      if vertical:
          for line in range(y1, y2):
              color = (min(max(a[0] + (rate[0] * (line - y1)), 0), 255),
                       min(max(a[1] + (rate[1] * (line - y1)), 0), 255),
                       min(max(a[2] + (rate[2] * (line - y1)), 0), 255))
              fn_line(surface, color, (x1, line), (x2, line))
      else:
          for col in range(x1, x2):
              color = (min(max(a[0] + (rate[0] * (col - x1)), 0), 255),
                       min(max(a[1] + (rate[1] * (col - x1)), 0), 255),
                       min(max(a[2] + (rate[2] * (col - x1)), 0), 255))
              fn_line(surface, color, (col, y1), (col, y2))
docs: Fill a surface with a linear gradient pattern.
  
      color
  
          starting color
  
      gradient
  
          final color
  
      rect
  
          area to fill; default is surface's rect
  
      vertical
  
          True=vertical; False=horizontal
  
      forward
  
          True=forward; False=reverse
  
      See http://www.pygame.org/wiki/GradientCode