code: def _close_open_file(self, file_des):
          
          self.open_files[file_des] = None
          heapq.heappush(self._free_fd_heap, file_des)
docs: Remove file object with given descriptor from the list
          of open files.
  
          Sets the entry in open_files to None.
  
          Args:
              file_des: Descriptor of file object to be removed from
              open files list.