In the following code, what is the main issue that prevents observers from receiving notifications?

In the following code, what is the main issue that prevents observers from receiving notifications?

class Subject:
    def __init__(self):
        self.observers = set()
    def subscribe(self, observer):
        self.observers.add(observer)
    def notify(self, message):
        for obs in self.observers:
            obs.receive(message)

class Observer:
    def update(self, message):
        print(f"Got: {message}")

subject = Subject()
obs1 = Observer()
subject.subscribe(obs1)
subject.notify("Update")