Exercise 1: Adding a Fourth Subscriber, InventoryService — Possible Solution
====================================================================

THE NEW SUBSCRIBER
------------------------------
    class InventoryService:
        def __init__(self):
            self.reservations = []
        def reserve_stock(self, event):
            self.reservations.append(f"Reserving stock for order {event['order_id']}")

Follows this chapter's own subscriber shape exactly - a single method
taking the event payload, matching UserService.award_loyalty_points
and EmailService.send_confirmation's own signature.

WIRING IT UP
------------------------------
    bus.subscribe('OrderPlaced', inventory_service.reserve_stock)

One line, added alongside the three existing subscriptions - no
changes anywhere else.

VERIFYING ALL FOUR SUBSCRIBERS FIRE CORRECTLY FROM ONE place_order() CALL
------------------------------
    user_service.users: {'USER-1': {'points': 10}}
    email_service.sent_emails: ['Emailing user USER-1: order total 100']
    analytics_service.logged_events: [{'order_id': 'ORD-1', 'items_total': 100, 'user_id': 'USER-1'}]
    inventory_service.reservations: ['Reserving stock for order ORD-1']

All four subscribers - the two from this chapter's original example,
plus AnalyticsService (already established) and the new
InventoryService - correctly reacted to the single
order_service.place_order('ORD-1', 100, 'USER-1') call.

VERIFYING OrderService STILL HAS ZERO KNOWLEDGE OF THE NEW SUBSCRIBER
------------------------------
    OrderService source references InventoryService: False

Confirmed via inspect.getsource(), exactly matching this chapter's
own verification method for UserService and EmailService.

WHY THIS CONFIRMS THE PATTERN SCALES
------------------------------
This chapter's own text already showed a third subscriber (Analytics)
requiring zero OrderService changes. This exercise confirms a FOURTH
subscriber requires the same zero changes - the event bus's own
decoupling doesn't degrade as more listeners are added; OrderService's
own responsibility stays exactly "publish what happened," regardless
of how many things end up listening for it.

WHY THIS WORKS AS AN ANSWER
------------------------------
The new subscriber follows this chapter's own established interface
exactly, all four subscribers' correct behavior from one publish call
is verified directly, and OrderService's own continued ignorance of
the new subscriber is confirmed via the same source-inspection
technique this chapter already used, not just assumed to still hold.