How to use the pyteomics.cmass.std_aa_mass function in pyteomics

def reset_modifications() -> None:
    """
    Undo all static modifications and reset to the standard amino acid
    monoisotopic masses.
    """
    global _aa_mass
    _aa_mass = mass.std_aa_mass.copy()

import operator
import warnings
from typing import Any, Dict, Iterable, List, Optional, Tuple, Union

import numba as nb
import numpy as np
from rdkit import Chem
try:
    from pyteomics import cmass as mass
except ImportError:
    from pyteomics import mass

from spectrum_utils import utils


_aa_mass = mass.std_aa_mass.copy()


def static_modification(amino_acid: str, mass_diff: float) -> None:
    """
    Globally modify the monoisotopic mass of an amino acid to set a static
    modification.

    Parameters
    ----------
    amino_acid : str
        The amino acid whose monoisotopic mass is modified.
    mass_diff : float
        The *mass difference* to be added to the amino acid's original
        monoisotopic mass.
    """
    global _aa_mass