For example, if we call the greet user function right here, see we immediately get this red underline which says unresolved reference, greet user which basically means python doesn't know what is this greet user, it's not defined anywhere.

例如，如果我们在这里调用 greet user 函数，我们会立即看到这个红色下划线，上面写着未解析的引用，greet user 这基本上意味着 python 不知道这个 greet user 是什么，它没有在任何地方定义。