code: def test_class_map_no_background(classes):
      class_map = ClassMap(classes, background=None)
      assert class_map.get_by_id(0) == "a"
      assert class_map.get_by_name("a") == 0
  
imports: import pytest
  from icevision.all import *
label_desc: Use image node_id 2 for calling a global function with example usage: test_class_map_no_background(classes) without return types