code: def cache_file(filename, reload_on_change=False, opts=default_opts):
      
      filename = pyc2py(filename)
      if filename in file_cache:
          if reload_on_change: checkcache(filename)
          pass
      else:
          opts['use_linecache_lines'] = True
          update_cache(filename, opts)
          pass
      if filename in file_cache:
          return file_cache[filename].path
      else: return None
      return
docs: Cache filename if it is not already cached.
      Return the expanded filename for it in the cache
      or nil if we can not find the file.