code: def pack_str(cls, value):
          
          assert isinstance(value, str)
          value_len_packed = cls.pack_int_base128(len(value))
          return struct.pack("<%ds%ds" % (len(value_len_packed), len(value)), value_len_packed,  value)
docs: Pack string field
          <field> ::= <int32_varint><data>
  
          :param value: string to be packed
          :type value: bytes or str
  
          :return: packed value
          :rtype: bytes