code: def invalidate_pages_cache():
      global _cached_pages
  
      LOGGER.debug("Pages directory changed")
      with _pages_cache_lock:
          _cached_pages = None
  
      _on_pages_changed.send()
  
file_path: streamlit/lib/streamlit/source_util.py
imports: import re
  import threading
  from pathlib import Path
  from typing import Any, Callable, Dict, Optional, Tuple, cast
  from blinker import Signal
  from streamlit.logger import get_logger
  from streamlit.string_util import extract_leading_emoji
  from streamlit.util import calc_md5
label_desc: Use image node_id 4 for calling a global function with example usage: invalidate_pages_cache() without return types