def aa_to_index(aa):
    """
    :param aa: Three character amino acid name.
    :returns: Integer index as per BioPython, unknown/non-standard amino acids return 20.
    """
    if Polypeptide.is_aa(aa, standard=True):
        return Polypeptide.three_to_index(aa)
    else:
        return 20
def aa_to_index(aa):
    if Polypeptide.is_aa(aa, standard=True):
        return Polypeptide.three_to_index(aa)
    else:
        return 20