code: def GetFileSystem(self, path_spec):
      
      identifier = self._GetFileSystemCacheIdentifier(path_spec)
      return self._file_system_cache.GetObject(identifier)
docs: Retrieves a file system object defined by path specification.
  
      Args:
        path_spec (PathSpec): path specification.
  
      Returns:
        FileSystem: a file system object or None if not cached.