Identify the bug in this Observer pattern implementation:

Identify the bug in this Observer pattern implementation:

class Subject:
    def __init__(self):
        self.observers = set()
    def attach(self, observer):
        self.observers.add(observer)
    def notify(self):
        for obs in self.observers:
            obs.update()

class Observer:
    def update(self, state):
        print(f"State updated to {state}")

subject = Subject()
obs = Observer()
subject.attach(obs)
subject.notify()