I don't follow. In your example, if x is a string, there is no problem since '' + x is just x. If x is not a string, the new path would not trigger, so the behavior would be as before.

EDIT: OK, I think if the example is sum([some_string, some_not_string], '') then there is a possibility that some_string + some_not_string might be a valid expression computing a value while ''.join([some_string, some_not_string]) would complain that some_not_string is not a string.