Exercise 1: Adding a Third Port, DiscountPort — Possible Solution
====================================================================

THE NEW PORT AND ITS TWO ADAPTERS
------------------------------
    class DiscountPort:
        def get_promo_discount(self, promo_code): raise NotImplementedError

    class FakeDiscountAdapter(DiscountPort):
        def __init__(self, promos): self.promos = promos
        def get_promo_discount(self, promo_code): return self.promos.get(promo_code, 0)

    class RealDiscountAdapter(DiscountPort):
        def __init__(self, promos): self.promos = promos
        def get_promo_discount(self, promo_code):
            time.sleep(0.01)
            return self.promos.get(promo_code, 0)

Follows this chapter's own two-adapters-per-port shape exactly - same
method signature on both, the "real" one adding the identical
time.sleep(0.01) this chapter's own RealInventoryAdapter used.

WIRING IT INTO PricingEngine
------------------------------
    def __init__(self, inventory_port, notification_port, discount_port):
        self.inventory = inventory_port
        self.notifications = notification_port
        self.discounts = discount_port
    def calculate_price(self, product_id, base_price, promo_code=None):
        stock = self.inventory.get_stock_level(product_id)
        price = base_price
        if stock < 10:
            self.notifications.notify_low_stock(product_id)
            price = price * 1.15
        if promo_code:
            discount_pct = self.discounts.get_promo_discount(promo_code)
            price = price * (1 - discount_pct)
        return price

VERIFYING IDENTICAL RESULTS FROM FAKE AND REAL ADAPTERS
------------------------------
Pricing PROD-1 (stock 5, low-stock rule triggers) with promo code
SAVE5 (5% off):

    price via fake adapters (with promo): 109.24999999999999
    price via real adapters (with promo): 109.24999999999999
    manual check: 100*1.15*(1-0.05) = 109.24999999999999
    identical: True

Both adapters, and a manual calculation, all agree exactly.

VERIFYING THE CORE STILL REFERENCES NO CONCRETE ADAPTER
------------------------------
    PricingEngine references FakeDiscountAdapter: False
    PricingEngine references RealDiscountAdapter: False

Confirmed via the same inspect.getsource() technique this chapter
already used for the first two ports - adding a third port didn't
require PricingEngine to know about either of its adapters either.

WHY THIS WORKS AS AN ANSWER
------------------------------
The new port and its two adapters mirror this chapter's own
established shape exactly, the business result is verified identical
between fake and real adapters and cross-checked by hand, and the
dependency-inversion property is re-verified rather than assumed to
still hold once a third port was added.