code: def test_optimize_full_l():
      im = Image.frombytes("L", (16, 16), bytes(range(256)))
      test_file = BytesIO()
      im.save(test_file, "GIF", optimize=True)
      assert im.mode == "L"
  
imports: from io import BytesIO
  import pytest
  from PIL import GifImagePlugin, Image, ImageDraw, ImagePalette, features
  from .helper import assert_image_equal, assert_image_equal_tofile, assert_image_similar, hopper, is_pypy, netpbm_available
label_desc: Use image node_id 8 for calling a global function with example usage: test_optimize_full_l() without return types