Python Strategy

---
title: Strategy Promotion Example (Ramalho, 2022, p343)
---
classDiagram
    Promotion <|-- LargeOrderPromo
    Promotion <|-- FidelityPromo
    Promotion <|-- BulkItemPromo
	Promotion <--o  Order: Aggregation
    Promotion: discount()
	class Order {
	   total()
	   due()
	}
    class LargeOrderPromo{
       discount()
    }
    class FidelityPromo{
       discount()
    }
    class BulkItemPromo{
		discount()
    }

from abc import ABC, abstractmethod
from collections.abc import Sequence
from decimal import Decimal
from typing import NamedTuple, Optional

class Customer(NamedTuple):
    name: str
    fidelity: int

class LineItem(NamedTuple):
    product: str
    quantity: int
    price: Decimal
    def total(self) -> Decimal:
        return self.price * self.quantity

class Order(NamedTuple):  # the Context
    customer: Customer
    cart: Sequence[LineItem]
    promotion: Optional['Promotion'] = None
    def total(self) -> Decimal:
        totals = (item.total() for item in self.cart)
		return sum(totals, start=Decimal(0))
    def due(self) -> Decimal:
        if self.promotion is None:
            discount = Decimal(0)
        else:
            discount = self.promotion.discount(self)
        return self.total() - discount
    def __repr__(self):
        return f'<Order total: {self.total():.2f} due: {self.due():.2f}>'

class Promotion(ABC):  # the Strategy: an abstract base class
    @abstractmethod
    def discount(self, order: Order) -> Decimal:
        """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) -> Decimal:
        rate = Decimal('0.05')
        if order.customer.fidelity >= 1000:
            return order.total() * rate
        return Decimal(0)

class BulkItemPromo(Promotion):  # second Concrete Strategy
    """10% discount for each LineItem with 20 or more units"""
    def discount(self, order: Order) -> Decimal:
        discount = Decimal(0)
        for item in order.cart:
            if item.quantity >= 20:
                discount += item.total() * Decimal('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) -> Decimal:
        distinct_items = {item.product for item in order.cart}
        if len(distinct_items) >= 10:
            return order.total() * Decimal('0.07')
        return Decimal(0)

>>> joe = Customer('John Doe', 0)
>>> ann = Customer('Ann Smith', 1100)
>>> cart = (LineItem('banana', 4, Decimal('.5')),
...         LineItem('apple', 10, Decimal('1.5')),
...         LineItem('watermelon', 5, Decimal(5)))
>>> Order(joe, cart, FidelityPromo())
<Order total: 42.00 due: 42.00>
>>> Order(ann, cart, FidelityPromo())
<Order total: 42.00 due: 39.90>
>>> banana_cart = (LineItem('banana', 30, Decimal('.5')),
...                LineItem('apple', 10, Decimal('1.5')))
>>> Order(joe, banana_cart, BulkItemPromo())
<Order total: 30.00 due: 28.50>
>>> long_cart = tuple(LineItem(str(sku), 1, Decimal(1))
...                  for sku in range(10))
>>> Order(joe, long_cart, LargeOrderPromo())
<Order total: 10.00 due: 9.30>
>>> Order(joe, cart, LargeOrderPromo())
<Order total: 42.00 due: 42.00>