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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 shutdown(self):
 """ Disconnect all cached connections. @returns: a deferred that fires once all connection are disconnected. @rtype: L{Deferred} """

        self._shuttingDown = {key: Deferred()
                              for key in self.cachedConnections.keys()}
        return DeferredList(
            [maybeDeferred(p.transport.loseConnection)
             for p in self.cachedConnections.values()]
             + self._shuttingDown.values())