code: def convert_aa_code(x):
      
      if len(x) == 1:
          return amino_acid_codes[x.upper()]
      elif len(x) == 3:
          return inverse_aa_codes[x.upper()]
      else:
          raise ValueError("Can only convert 1-letter or 3-letter amino acid codes, "
                           "not %r" % x)
docs: Converts between 3-letter and 1-letter amino acid codes.