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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 _find_display(self):
 """ Find a usable display, which doesn't have an existing Xvfb file """

        self.display_num = 2
        while os.path.isdir(XVFB_PATH % (self.display_num,)):
            self.display_num += 1