code: def checkcache(filename=None):
      
  
      if filename is None:
          filenames = cache.keys()
      else:
          if filename in cache:
              filenames = [filename]
          else:
              return
  
      for filename in filenames:
          size, mtime, lines, fullname = cache[filename]
          if mtime is None:
              continue   # no-op for files loaded via a __loader__
          try:
              stat = os.stat(fullname)
          except os.error:
              del cache[filename]
              continue
          if size != stat.st_size or mtime != stat.st_mtime:
              del cache[filename]
docs: Discard cache entries that are out of date.
      (This is not checked upon each call!)