code: def read_index(fn):
      
      index = None
      with open(fn, "rb") as i_file:
          if i_file.read(len(_CHECK_STRING)) != _CHECK_STRING:
              raise ValueError("{}: not a valid index file".format(fn))
  
          index = pd.read_csv(io.StringIO(
              zlib.decompress(i_file.read()).decode(encoding="utf-8"),
          ))
  
      return index
docs: Reads index from file.
  
      Args:
          fn (str): the name of the file containing the index.
  
      Returns:
          pandas.DataFrame: the index of the file.
  
      Before reading the index, we check the first couple of bytes to see if it
      is a valid index file.