code: def value_to_message(self, value):
          
          if not isinstance(value, self.type):
              raise EncodeError('Expected type %s, got %s: %r' %
                                (self.type.__name__,
                                 type(value).__name__,
                                 value))
          return value
docs: Convert a value instance to a message.
  
          Used by serializers to convert Python user types to underlying
          messages for transmission.
  
          Args:
            value: A value of type self.type.
  
          Returns:
            An instance of type self.message_type.