Open Closed Principle

ソフトウェアの構成要素は拡張に対しては開いていて、修正に対しては閉じていなければならない。

ソフトウェアの振る舞いは、既存の成果物を変更せず拡張できるようにすべき。

Before

class Order:

    def __init__(self) -> None:
        self.items = []
        self.quantities = []
        self.prices = []
        self.status = "open"

    def add_item(self, name: str, quantity: int, price: int) -> None:
        self.items.append(name)
        self.quantities.append(quantity)
        self.prices.append(price)

    def total_price(self) -> int:
        total = 0
        for i in range(len(self.prices)):
            total += self.quantities[i] * self.prices[i]
        return total


class PaymentProcessor:
    def pay_debit(self, order: Order, security_code: str) -> None:
        print("Processing debit payment type")
        print(f"Verifying security code: {security_code}")
        order.status = "paid"

    def pay_credit(self, order: Order, security_code: str) -> None:
        print("Processing credit payment type")
        print(f"Verifying security code: {security_code}")
        order.status = "paid"


order = Order()
order.add_item("Keyboard", 1, 50)
order.add_item("SSD", 1, 150)
order.add_item("USB cable", 2, 5)

print(order.total_price())
processor = PaymentProcessor()
processor.pay_debit(order, "0372846")

After

from abc import ABC, abstractmethod

class Order:

    def __init__(self) -> None:
        self.items = []
        self.quantities = []
        self.prices = []
        self.status = "open"

    def add_item(self, name: str, quantity: int, price: int) -> None:
        self.items.append(name)
        self.quantities.append(quantity)
        self.prices.append(price)

    def total_price(self) -> int:
        total = 0
        for i in range(len(self.prices)):
            total += self.quantities[i] * self.prices[i]
        return total


class PaymentProcessor(ABC):
    
    @abstractmethod
    def pay(self, order: Order, security_code: str) -> None:
        pass

class DebitPaymentProcessor(PaymentProcessor):
    def pay(self, order: Order, security_code: str) -> None:
        print('Processing debit payment type')
        print(f'Verifying security code: {security_code}')
        order.status = 'paid'

class CreditPaymentProcessor(PaymentProcessor):
    def pay(self, order: Order, security_code: str) -> None:
        print('Processing debit payment type')
        print(f"Verifying security code: {security_code}")
        order.status = 'paid'

order = Order()
order.add_item("Keyboard", 1, 50)
order.add_item("SSD", 1, 150)
order.add_item("USB cable", 2, 5)

print(order.total_price())
processor = CreditPaymentProcessor()
processor.pay(order, "0372846")

Before#

After#

Before

After