How to use the pyteomics.mass.Unimod function in pyteomics

def generate_aa_comp():
    """
    >>> aa_comp = generate_aa_comp()
    >>> aa_comp["M"]
    Composition({'H': 9, 'C': 5, 'S': 1, 'O': 1, 'N': 1})
    >>> aa_comp["Z"]
    Composition({'H': 9, 'C': 5, 'S': 1, 'O': 2, 'N': 1})
    """
    db = pyteomics.mass.Unimod()
    aa_comp = dict(pyteomics.mass.std_aa_comp)
    s = db.by_title("Oxidation")["composition"]
    aa_comp["Z"] = aa_comp["M"] + s
    s = db.by_title("Carbamidomethyl")["composition"]
    aa_comp["C"] = aa_comp["C"] + s
    return aa_comp