code: def sequence_molecular_weight(seq):
      
      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
docs: Returns the molecular weight of the polypeptide sequence.
  
      Notes
      -----
      Units = Daltons
  
      Parameters
      ----------
      seq : str
          Sequence of amino acids.