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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:
 async def on_shutdown(self, app):
 """ Graceful shutdown handler See https://docs.aiohttp.org/en/stable/web.html#graceful-shutdown """

        for ws in self.clients.copy():
            await ws.close(code=WSCloseCode.GOING_AWAY,
                           message='Server shutdown')
        self.shutdown()