Lesson 3 of 13

Observer: Notify Subscribers

A stock change should update a display and a log. Instead of listing every display type inside the stock object, let interested callbacks register. This is the core idea of Observer.

Subscribe, notify, unsubscribe

class Stock:
    def __init__(self):
        self.quantity = 0
        self.listeners = []
    def subscribe(self, listener):
        self.listeners.append(listener)
    def unsubscribe(self, listener):
        self.listeners.remove(listener)
    def set_quantity(self, quantity):
        self.quantity = quantity
        for listener in list(self.listeners):
            listener(quantity)

def show(quantity):
    print("stock:", quantity)

stock = Stock()
stock.subscribe(show)
stock.set_quantity(3)
stock.unsubscribe(show)
stock.set_quantity(2)

The program prints stock: 3 once. Updating stock to 2 after unsubscription prints nothing. Iterating over a copy keeps the current notification’s recipients stable if a callback changes subscriptions.

Notification policy matters

This example calls listeners synchronously in registration order. Observer does not automatically imply asynchronous execution. A slow or failing listener can affect others; decide whether to propagate errors or handle them individually and continue.

Failing to unsubscribe a closed display can retain unnecessary objects or duplicate notifications. A listener that changes the observed value can also trigger repeated notification loops.

Check your understanding

Every time a display opens, a new callback is registered, but closing it never unsubscribes. What might happen?