ch08, 09, 10: example files

10-dp-1class-func/classic_strategy.py

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+# classic_strategy.py
+# Strategy pattern -- classic implementation
+
+"""
+# tag::CLASSIC_STRATEGY_TESTS[]
+
+    >>> joe = Customer('John Doe', 0)  # <1>
+    >>> ann = Customer('Ann Smith', 1100)
+    >>> cart = [LineItem('banana', 4, .5),  # <2>
+    ...         LineItem('apple', 10, 1.5),
+    ...         LineItem('watermellon', 5, 5.0)]
+    >>> Order(joe, cart, FidelityPromo())  # <3>
+    <Order total: 42.00 due: 42.00>
+    >>> Order(ann, cart, FidelityPromo())  # <4>
+    <Order total: 42.00 due: 39.90>
+    >>> banana_cart = [LineItem('banana', 30, .5),  # <5>
+    ...                LineItem('apple', 10, 1.5)]
+    >>> Order(joe, banana_cart, BulkItemPromo())  # <6>
+    <Order total: 30.00 due: 28.50>
+    >>> big_cart = [LineItem(str(item_code), 1, 1.0) # <7>
+    ...               for item_code in range(10)]
+    >>> Order(joe, big_cart, LargeOrderPromo())  # <8>
+    <Order total: 10.00 due: 9.30>
+    >>> Order(joe, cart, LargeOrderPromo())
+    <Order total: 42.00 due: 42.00>
+
+# end::CLASSIC_STRATEGY_TESTS[]
+"""
+# tag::CLASSIC_STRATEGY[]
+
+from abc import ABC, abstractmethod
+import typing
+from typing import Sequence, Optional
+
+
+class Customer(typing.NamedTuple):
+    name: str
+    fidelity: int
+
+
+class LineItem:
+    def __init__(self, product: str, quantity: int, price: float):
+        self.product = product
+        self.quantity = quantity
+        self.price = price
+
+    def total(self):
+        return self.price * self.quantity
+
+
+class Order:  # the Context
+    def __init__(
+        self,
+        customer: Customer,
+        cart: Sequence[LineItem],
+        promotion: Optional['Promotion'] = None,
+    ):
+        self.customer = customer
+        self.cart = list(cart)
+        self.promotion = promotion
+
+    def total(self) -> float:
+        if not hasattr(self, '__total'):
+            self.__total = sum(item.total() for item in self.cart)
+        return self.__total
+
+    def due(self) -> float:
+        if self.promotion is None:
+            discount = 0.0
+        else:
+            discount = self.promotion.discount(self)
+        return self.total() - discount
+
+    def __repr__(self):
+        fmt = '<Order total: {:.2f} due: {:.2f}>'
+        return fmt.format(self.total(), self.due())
+
+
+class Promotion(ABC):  # the Strategy: an abstract base class
+    @abstractmethod
+    def discount(self, order: Order) -> float:
+        """Return discount as a positive dollar amount"""
+
+
+class FidelityPromo(Promotion):  # first Concrete Strategy
+    """5% discount for customers with 1000 or more fidelity points"""
+
+    def discount(self, order: Order) -> float:
+        return order.total() * 0.05 if order.customer.fidelity >= 1000 else 0
+
+
+class BulkItemPromo(Promotion):  # second Concrete Strategy
+    """10% discount for each LineItem with 20 or more units"""
+
+    def discount(self, order: Order) -> float:
+        discount = 0
+        for item in order.cart:
+            if item.quantity >= 20:
+                discount += item.total() * 0.1
+        return discount
+
+
+class LargeOrderPromo(Promotion):  # third Concrete Strategy
+    """7% discount for orders with 10 or more distinct items"""
+
+    def discount(self, order: Order) -> float:
+        distinct_items = {item.product for item in order.cart}
+        if len(distinct_items) >= 10:
+            return order.total() * 0.07
+        return 0
+
+
+# end::CLASSIC_STRATEGY[]