107 lines
3.2 KiB
Python
107 lines
3.2 KiB
Python
# classic_strategy.py
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# Strategy pattern -- classic implementation
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"""
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# tag::CLASSIC_STRATEGY_TESTS[]
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>>> joe = Customer('John Doe', 0) # <1>
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>>> ann = Customer('Ann Smith', 1100)
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>>> cart = (LineItem('banana', 4, Decimal('.5')), # <2>
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... LineItem('apple', 10, Decimal('1.5')),
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... LineItem('watermelon', 5, Decimal(5)))
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>>> Order(joe, cart, FidelityPromo()) # <3>
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<Order total: 42.00 due: 42.00>
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>>> Order(ann, cart, FidelityPromo()) # <4>
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<Order total: 42.00 due: 39.90>
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>>> banana_cart = (LineItem('banana', 30, Decimal('.5')), # <5>
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... LineItem('apple', 10, Decimal('1.5')))
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>>> Order(joe, banana_cart, BulkItemPromo()) # <6>
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<Order total: 30.00 due: 28.50>
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>>> long_cart = tuple(LineItem(str(sku), 1, Decimal(1)) # <7>
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... for sku in range(10))
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>>> Order(joe, long_cart, LargeOrderPromo()) # <8>
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<Order total: 10.00 due: 9.30>
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>>> Order(joe, cart, LargeOrderPromo())
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<Order total: 42.00 due: 42.00>
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# end::CLASSIC_STRATEGY_TESTS[]
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"""
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# tag::CLASSIC_STRATEGY[]
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from abc import ABC, abstractmethod
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from collections.abc import Sequence
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from decimal import Decimal
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from typing import NamedTuple, Optional
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class Customer(NamedTuple):
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name: str
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fidelity: int
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class LineItem(NamedTuple):
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product: str
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quantity: int
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price: Decimal
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def total(self) -> Decimal:
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return self.price * self.quantity
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class Order(NamedTuple): # the Context
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customer: Customer
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cart: Sequence[LineItem]
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promotion: Optional['Promotion'] = None
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def total(self) -> Decimal:
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totals = (item.total() for item in self.cart)
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return sum(totals, start=Decimal(0))
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def due(self) -> Decimal:
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if self.promotion is None:
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discount = Decimal(0)
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else:
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discount = self.promotion.discount(self)
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return self.total() - discount
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def __repr__(self):
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return f'<Order total: {self.total():.2f} due: {self.due():.2f}>'
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class Promotion(ABC): # the Strategy: an abstract base class
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@abstractmethod
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def discount(self, order: Order) -> Decimal:
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"""Return discount as a positive dollar amount"""
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class FidelityPromo(Promotion): # first Concrete Strategy
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"""5% discount for customers with 1000 or more fidelity points"""
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def discount(self, order: Order) -> Decimal:
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rate = Decimal('0.05')
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if order.customer.fidelity >= 1000:
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return order.total() * rate
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return Decimal(0)
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class BulkItemPromo(Promotion): # second Concrete Strategy
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"""10% discount for each LineItem with 20 or more units"""
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def discount(self, order: Order) -> Decimal:
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discount = Decimal(0)
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for item in order.cart:
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if item.quantity >= 20:
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discount += item.total() * Decimal('0.1')
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return discount
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class LargeOrderPromo(Promotion): # third Concrete Strategy
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"""7% discount for orders with 10 or more distinct items"""
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def discount(self, order: Order) -> Decimal:
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distinct_items = {item.product for item in order.cart}
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if len(distinct_items) >= 10:
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return order.total() * Decimal('0.07')
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return Decimal(0)
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# end::CLASSIC_STRATEGY[]
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