docs: def reduce_opacity(img, opacity):
      """
      Returns an image with reduced opacity.
      """
      assert opacity >= 0 and opacity <= 1
  
      if img.mode != 'RGBA':
          img = img.convert('RGBA')
      else:
          img = img.copy()
  
      alpha = img.split()[3]
      alpha = ImageEnhance.Brightness(alpha).enhance(opacity)
      img.putalpha(alpha)
  
      return img