Python Tkinter error object has no attribute
Your initial call to guess_number in your initializer method is probably being invoked before you press the button and trigger the new_window event callback. In guess_number you're trying to pass self.window as an argument to Label() but it would be undefined at that time.
First at all, you should never create a new attribute out of the __init__ method.
That said, Mike pointed the trouble’s reason: you created the window object inside the new_window method, but did not called it.
You must call new_window before call guess_number – or call one inside other.
I suggest that you set window to None and call new_window inside __init__ method, then (after that) call guess_number.