A caller placing an order should not have to know that three subsystems exist. That is the whole point of the Facade pattern: one method, place_order, sits in front of Inventory, Payment, and Shipping and coordinates them, so the outside world sees a single simple entry point.
The three subsystems are already written for you and injected into the facade. You never rewrite them, you only call them. Inventory.reserve(item, qty) returns True and decrements stock when there is enough on hand, and returns False otherwise.
Start small. In this step, place_order only talks to Inventory:
class Inventory:
def __init__(self, stock):
self.stock = dict(stock)
def reserve(self, item, qty):
if self.stock.get(item, 0) >= qty:
self.stock[item] -= qty
return True
return False
def release(self, item, qty):
self.stock[item] = self.stock.get(item, 0) + qty
class Payment:
def __init__(self, balance):
self.balance = balance
def charge(self, amount):
if amount <= self.balance:
self.balance -= amount
return True
return False
class Shipping:
def schedule(self, item):
return "trk-" + item
class CheckoutFacade:
def __init__(self, inventory, payment, shipping):
self.inventory = inventory
self.payment = payment
self.shipping = shipping
self.orders = []
def place_order(self, item, qty, amount):
"""Reserve inventory, then confirm the order."""
# TODO: reserve qty of item via self.inventory.reserve(item, qty).
# If the reservation fails, return {"ok": False, "reason": "out_of_stock"}.
# On a successful reservation, return {"ok": True} for now.
pass
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A caller placing an order should not have to know that three subsystems exist. That is the whole point of the Facade pattern: one method, place_order, sits in front of Inventory, Payment, and Shipping and coordinates them, so the outside world sees a single simple entry point.
The three subsystems are already written for you and injected into the facade. You never rewrite them, you only call them. Inventory.reserve(item, qty) returns True and decrements stock when there is enough on hand, and returns False otherwise.
Start small. In this step, place_order only talks to Inventory:
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