List of indexes whose values will be strings.

def _init_idxs_strpat(self, usr_hdrs):
        """List of indexes whose values will be strings."""
        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]