classic_strategy.py

# 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, Decimal('.5')),  # <2>
    ...         LineItem('apple', 10, Decimal('1.5')),
    ...         LineItem('watermelon', 5, Decimal(5)))
    >>> 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, Decimal('.5')),  # <5>
    ...                LineItem('apple', 10, Decimal('1.5')))
    >>> Order(joe, banana_cart, BulkItemPromo())  # <6>
    <Order total: 30.00 due: 28.50>
    >>> long_cart = tuple(LineItem(str(sku), 1, Decimal(1)) # <7>
    ...                  for sku in range(10))
    >>> Order(joe, long_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
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)
# end::CLASSIC_STRATEGY[]