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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _screen_size_cb(self, w, tf):
 """This function is called when the user clicks the 'Screen size' checkbox. `tf` is True if checked, False otherwise. """

        self._screen_size = tf
        self.w.width.set_enabled(not tf)
        self.w.height.set_enabled(not tf)
        self.w.lock_aspect.set_enabled(not tf)
        if self._screen_size:
            wd, ht = self.fitsimage.get_window_size()
            self._configure_cb(self.fitsimage, wd, ht)