def average_bqs(bqs):
    """
    Calculate the mean BQS of a given string of quality scores.
    Assumes BQS are in Sanger format, encoded as Phred +33.

    Args:
        bqs (str): String of base quality scores.

    Returns:
        Mean BQS of that string.
    """
    return sum([ord(x) - 33 for x in bqs]) / len(bqs)