def buildProtocol(self, addr):
        """
        Return the pre-constructed protocol instance and arrange to fire the
        waiting L{Deferred} to indicate success establishing the connection.
        """
        self.pending = self.reactor.callLater(
            0, self.fire, self.deferred.callback, self.instance)
        self.deferred = None
        return self.instance

        Return the pre-constructed protocol instance and arrange to fire the
        waiting L{Deferred} to indicate success establishing the connection.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/protocol.py