code: def test_index_trigger(self):
      select = Select(options=[1, 2, 3])
      observations = []
  
      def f(change):
          observations.append(change.new)
  
      select.observe(f, "index")
      assert select.index == 0
      select.options = [4, 5, 6]
      assert select.index == 0
      assert select.value == 4
      assert select.label == "4"
      assert observations == [0]
  
file_path: catboost/contrib/python/ipywidgets/py3/ipywidgets/widgets/tests/test_widget_selection.py
imports: import inspect
  from unittest import TestCase
  from traitlets import TraitError
  from ipywidgets import Dropdown, SelectionSlider, Select
label_desc: Use image node_id 2 for calling the TestSelection obj's underlying member method code with example usage: obj.test_index_trigger() without return types