def getLog(self, identifier):
        """
        Given an integer, return a LogReader for an old log file.
        """
        filename = "%s.%d" % (self.path, identifier)
        if not os.path.exists(filename):
            raise ValueError("no such logfile exists")
        return LogReader(filename)

        Given an integer, return a LogReader for an old log file.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/logfile.py