["python","python-2.7","generator","python-typing"]
What is the return type hint of a generator function?

I'm trying to write a :rtype: type hint for a generator function. What is the type it returns? For example, say I have this functions which yields strings: def read_text_file(fn): """ Yields the lines of the text file one by one. :param fn: Path of text file to read. :type fn: str :rtype: ???????????????? <======================= what goes here? """ with open(fn, 'rt') as text_file: for line in text_file: yield line The return type isn't just a string, it's some kind of iterable of strings? So I can't just write :rtype: str. What's the right hint?
Generator Generator[str, None, None] or Iterator[str]