Examine this Observer pattern snippet. What is the likely cause if observers are never notified?

Examine this Observer pattern snippet. What is the likely cause if observers are never notified?

class Subject:
    def __init__(self):
        self.observers = []
    def attach(self, observer):
        self.observers.append(observer)
    def notify(self, msg):
        for obs in self.observers:
            obs.update(msg)