03·Observer, Pub/Sub, signals, asyncio queues

Introduction

The Observer pattern answers "who should be notified when something changes?". In Python it shows up at many scales: from a single class with a callback list, through Django/Flask signals (blinker), all the way to asyncio.Queue with back-pressure and brokers (Redis Pub/Sub, RabbitMQ, Kafka). This lesson covers the classic Observer, its pitfalls (leaks via strong references, no exception isolation), the difference between Observer and Pub/Sub, asyncio queues as producer/consumer, signals as loose coupling between modules, and when event-driven architecture genuinely pays off.