Returns the molecular weight of the polypeptide sequence.

    Notes
    -----
    Units = Daltons

    Parameters
    ----------
    seq : str
        Sequence of amino acids.

def sequence_molecular_weight(seq):
    """Returns the molecular weight of the polypeptide sequence.

    Notes
    -----
    Units = Daltons

    Parameters
    ----------
    seq : str
        Sequence of amino acids.
    """
    if 'X' in seq:
        warnings.warn(_nc_warning_str, NoncanonicalWarning)
    return sum(
        [residue_mwt[aa] * n for aa, n in Counter(seq).items()]) + water_mass