i think you got the right answer . 这不是上海吧!! 不是 is more accurate. but the two translations have similar meaning. the first one , 这不是上海吧?the man who spoke it think the place is cant be shanghai. but 这是上海吧?he think it should be shang hai. , it also rests with context. sometimes chinese speak a Affirmative sentence with 不（not) to emphasize， easy to get confused .

In my opinion, it should be translate into: This wouldn't be Shanghai, 这里不是上海， would it? 你以为是吗？/你以为这里是上海吗？ No, it's not......不，你错了/不，不是 看起来是某个人把另外一个地方当成了上海，被人严重地指出他的错误。 例如，一个小孩，晚上九点了，吵着要去东方明珠电视塔玩，被大人狠狠地训了一顿，哈哈哈。

In English, you should say yes when the fact is correct and no when the fact is wrong. e.g. 1. Here is Shanghai. Q: Is this Shanghai? A: Yes, this is Shanghai. Q: This is not Shanghai, isn't it? A: Yes it is Shanghai. 2. Here is not Shanghai. Q: Is this Shanghai? A: No, this is not Shanghai. Q: This is not Shanghai, isn't it? A: No, it is not Shanghai. However in Chinese, you should say yes when the question that someone asked is right and no when the question is wrong. 1. Here is Shanghai. Q: 这是上海吗？ A: 是的（提问者说对了，这确实是上海），这是上海。 Q: 这不是上海，对吗？ A: 不（提问者说错了，这是上海，而他说这不是上海），这是上海。 2. Here is not Shanghai. Q: 这是上海吗？ A: 不（提问者说错了，所以回答不），这不是上海。 Q: 这不是上海，对吗？ A: 是的（提问者说对了，所以回答是的），这不是上海。 所以这句话应该翻译为： This wouldn't be Shanghai, would it? No, it's not... 这不是上海吧？是的，这不是上海。。。

吧 is a modal particle and is similar to adding tag words to questions or indicating we don't know something. It can also be used as a suggestion and to show agreement. It really depends on the context. 我们放学后去游泳吧! [ let's go swimming after we finish school!) 好吧! [*agreeing* Ok!] 你日语说得真好，你在日本住了很久吧？[You speak Japanese really good, you've lived in Japan for a long time right?] For your question, both sentences could work. 吧 in a question implies doubt, like if you were to ask a little kid about something, such as directions: 邮局离这里很附近吧？-You don't know if the post office is close to here, do you? as opposed to : 你知道邮局离这里很附近吗？Do you know if the post office is close to here?[this is a normal question] The two sentences can be translated as the following: 这是上海吧？-This is shanghai, isn't it? 这不是上海吧？-This isn't shanghai, is it? As you can see, there are very small but subtle differences.

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