Wait for subscriber callback to receive message - best practice

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Hello, I am wondering what is the best practice in python to wait for a subscribed topic to start publishing, before using its messages in the spin method. An simplified example of what I am doing right now is provided below, but I was thinking if there was a more elegant solution.

If remove the is_subbed flag I get the AttributeError, that the class has no attribute 'sub_msg'. I guess this could be fixed by initializing self.sub_msg in the __init__ method with some arbitrary value, which will then be used for the first few runs of the spin method. But I need to get the correct messages right from the start. I hope this hasn't been asked yet, at least I haven't found a good answer yet.

class SomeClass(object):
    def __init__(self):
        rospy.init_node('node_name', anonymous=True)
        self.sub = rospy.Subscriber('sub_name', sub_type, self.sub_callback, queue_size=10)
        self.rate = 100       
        self.is_subbed = False

    def sub_callback(self, msg):
        self.sub_msg = msg
        self.is_subbed = True

    def spin(self):
        r = rospy.Rate(self.rate)
        while not rospy.is_shutdown():
            # Skip method if callback has not been called yet
            if not self.is_subbed:
                continue
            print(self.sub_msg) 
            r.sleep()

if __name__ == "__main__":
    SC = SomeClass()
    SC.spin()