def identity(seq1_s, seq2_s):
    """
    function to calculate identity between string; only considering substitution
    :param seq1_s: string, seq1
    :param seq2_s: string, seq2
    :return: integer, identity
    """
    ident_i = 0
    assert len(seq1_s) == len(seq2_s)
    seq1_s = seq1_s.upper()
    seq2_s = seq2_s.upper()
    for pair_t in zip(seq1_s, seq2_s):
        if pair_t[0] == pair_t[1]:
            ident_i += 1
    return ident_i