def subscribe(self, callback):
        """
        Subscribes a client for callback events.

        The first subscription will trigger the connection to the
        pulse server.
        """
        need_configure = not bool(self.callbacks)
        self.callbacks.add(callback)

        if need_configure:
            create_task(self._configure())

        Subscribes a client for callback events.

        The first subscription will trigger the connection to the
        pulse server.
        
libqtile/widget/pulse_volume.py