code: def test_invalidate_pages_cache(self):
      source_util.invalidate_pages_cache()
  
      assert source_util._cached_pages is None
      source_util._on_pages_changed.send.assert_called_once()
  
file_path: streamlit/lib/tests/streamlit/source_util_test.py
imports: import unittest
  from pathlib import Path
  from unittest.mock import MagicMock, patch
  import pytest
  from parameterized import parameterized
  import streamlit.source_util
  from streamlit.util import calc_md5
label_desc: Use image node_id 4 for calling the PageHelperFunctionTests obj's underlying member method code with example usage: obj.test_invalidate_pages_cache() without return types