code: def _parse_canonical_decimal128(doc):
      
      d_str = doc['$numberDecimal']
      if len(doc) != 1:
          raise TypeError('Bad $numberDecimal, extra field(s): %s' % (doc,))
      if not isinstance(d_str, string_type):
          raise TypeError('$numberDecimal must be string: %s' % (doc,))
      return Decimal128(d_str)
docs: Decode a JSON decimal128 to bson.decimal128.Decimal128.