code: def _init_idxs_strpat(self, usr_hdrs):
          
          strpat = self.strpat_hdrs.keys()
          self.idxs_strpat = [
              Idx for Hdr, Idx in self.hdr2idx.items() if Hdr in usr_hdrs and Hdr in strpat]
docs: List of indexes whose values will be strings.