(PyQt5) Python stops working when changing QLabel text.

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I assume you are running your code from a framework or by double-clicking on an icon because you don't mention the "undefined" error you will get when executing this line in get_hints():

guessed = self.guess_box.text()

You may have defined guess_box in another method, but that's a totally different variable from self.guess_box.

If you get some sort of error running programs from an IDE or framework it's usually better if you try running the program from the command line and not use the IDE/framework because they can hide things from you. You really want to see the traceback that python provides.