40. Observer Pattern

The Observer pattern allows objects to be notified of state changes.

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Example

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

class Observer:
    def update(self, msg):
        print("Received:", msg)

subj = Subject()
obs1 = Observer()
subj.attach(obs1)
subj.notify("Hello Observers!")

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Wrap-Up

Observer decouples subjects from observers, useful in GUIs and event systems.