strategy.py

# strategy.py
# Strategy pattern -- function-based implementation

"""
# tag::STRATEGY_TESTS[]

    >>> joe = Customer('John Doe', 0)  # <1>
    >>> 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, fidelity_promo)  # <2>
    <Order total: 42.00 due: 42.00>
    >>> Order(ann, cart, fidelity_promo)
    <Order total: 42.00 due: 39.90>
    >>> banana_cart = [LineItem('banana', 30, Decimal('.5')),
    ...                LineItem('apple', 10, Decimal('1.5'))]
    >>> Order(joe, banana_cart, bulk_item_promo)  # <3>
    <Order total: 30.00 due: 28.50>
    >>> long_cart = [LineItem(str(item_code), 1, Decimal(1))
    ...               for item_code in range(10)]
    >>> Order(joe, long_cart, large_order_promo)
    <Order total: 10.00 due: 9.30>
    >>> Order(joe, cart, large_order_promo)
    <Order total: 42.00 due: 42.00>

# end::STRATEGY_TESTS[]
"""
# tag::STRATEGY[]

from collections.abc import Sequence
from dataclasses import dataclass
from decimal import Decimal
from typing import Optional, Callable, NamedTuple


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


class LineItem(NamedTuple):
    product: str
    quantity: int
    price: Decimal

    def total(self):
        return self.price * self.quantity

@dataclass(frozen=True)
class Order:  # the Context
    customer: Customer
    cart: Sequence[LineItem]
    promotion: Optional[Callable[['Order'], Decimal]] = None  # <1>

    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(self)  # <2>
        return self.total() - discount

    def __repr__(self):
        return f'<Order total: {self.total():.2f} due: {self.due():.2f}>'


# <3>


def fidelity_promo(order: Order) -> Decimal:  # <4>
    """5% discount for customers with 1000 or more fidelity points"""
    if order.customer.fidelity >= 1000:
        return order.total() * Decimal('0.05')
    return Decimal(0)


def bulk_item_promo(order: Order) -> Decimal:
    """10% discount for each LineItem with 20 or more units"""
    discount = Decimal(0)
    for item in order.cart:
        if item.quantity >= 20:
            discount += item.total() * Decimal('0.1')
    return discount


def large_order_promo(order: Order) -> Decimal:
    """7% discount for orders with 10 or more distinct items"""
    distinct_items = {item.product for item in order.cart}
    if len(distinct_items) >= 10:
        return order.total() * Decimal('0.07')
    return Decimal(0)


# end::STRATEGY[]