code: def _recalc_idfs(self, session, grams=None):
          
  
          if not grams:
              grams = session.query(Gram)
          for gram in grams:
              orig_idf = gram.idf
              gram.idf = self._get_idf(session, gram)
              logger.debug('Recalculating {} {} -> {}'.format(
                  gram.gram, orig_idf, gram.idf))
docs: Re-calculate idfs for database.
  
          calculating idfs for gram is taking long time.
          So I made it calculates idfs for some grams.
          If you want make accuracy higher, use this with grams=None.
  
          :param session: DB session
          :type session: :class:`sqlalchemt.orm.Session`
  
          :param grams: grams that you want to re-calculating idfs
          :type grams: A set of :class:`Gram`