Dependency Inversion Principle

ソースコードの依存関係が（具象ではなく）抽象だけを参照しているシステムが最も柔軟である。

Before

from abc import ABC, abstractmethod

class Order:

    def __init__(self):
        self.items = []
        self.quantities = []
        self.prices = []
        self.status = 'open'

    def add_item(self, name, quantity, price):
        self.items.append(name)
        self.quantities.append(quantity)
        self.prices.append(price)

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

class Authorizer(ABC):
    @abstractmethod
    def is_authorized(self) -> bool:
        pass


class Authorizer_SMS(Authorizer):

    def __init__(self):
        self.authorized = False

    def verify_code(self, code):
        print(f'Verifying SMS code {code}')
        self.authorized = True

    def is_authorized(self) -> bool:
        return self.authorized

class Authorizer_Google(Authorizer):

    def __init__(self):
        self.authorized = False

    def verify_code(self, code):
        print(f'Verifying Google auth code {code}')
        self.authorized = True

    def is_authorized(self) -> bool:
        return self.authorized

class Authorizer_Robot(Authorizer):

    def __init__(self):
        self.authorized = False

    def not_a_robot(self):
        self.authorized = True

    def is_authorized(self) -> bool:
        return self.authorized


class PaymentProcessor(ABC):

    @abstractmethod
    def pay(self, order):
        pass


class DebitPaymentProcessor(PaymentProcessor):

    def __init__(self, security_code, authorizer: Authorizer):
        self.security_code = security_code
        self.authorizer = authorizer
    
    def pay(self, order):
        if not self.authorizer.is_authorized():
            raise Exception('Not authorized')
        print('Processing debit payment type')
        print(f'Verifying security code: {self.security_code}')
        order.status = 'paid'

class CreditPaymentProcessor(PaymentProcessor):

    def __init__(self, security_code):
        self.security_code = security_code

    def pay(self, order):
        print('Processing credit payment type')
        print(f'Verifying security code: {self.security_code}')
        order.status = 'paid'

class PaypalPaymentProcessor(PaymentProcessor):

    def __init__(self, email_address, authorizer: Authorizer):
        self.email_address = email_address
        self.authorizer = authorizer

    def pay(self, order):
        if not self.authorizer.is_authorized():
            raise Exception('Not authorized')
        print('Processing paypal payment type')
        print(f'Using email address: {self.email_address}')
        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())
authorizer = Authorizer_Robot()
# authorizer.verify_code(465839)
authorizer.not_a_robot()
processor = PaypalPaymentProcessor('hi@arjancodes.com', authorizer)
processor.pay(order)

After

from abc import ABC, abstractmethod

class Order:

    def __init__(self):
        self.items = []
        self.quantities = []
        self.prices = []
        self.status = 'open'

    def add_item(self, name, quantity, price):
        self.items.append(name)
        self.quantities.append(quantity)
        self.prices.append(price)

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

class Authorizer(ABC):
    @abstractmethod
    def is_authorized(self) -> bool:
        pass

class SMSAuth(Authorizer):

    authorized = False
    
    def verify_code(self, code):
        print(f'Verifying code {code}')
        self.authorized = True

    def is_authorized(self) -> bool:
        return self.authorized

class NotARobot(Authorizer):
    
    authorized = False

    def not_a_robot(self):
        print(f'Are you a robot? Nope.')
        self.authorized = True

    def is_authorized(self) -> bool:
        return self.authorized

class PaymentProcessor(ABC):

    @abstractmethod
    def pay(self, order):
        pass


class DebitPaymentProcessor(PaymentProcessor):

    def __init__(self, security_code, authorizer: Authorizer):
        self.authorizer = authorizer
        self.security_code = security_code

    def pay(self, order):
        if not self.authorizer.is_authorized():
            raise Exception('Not authorized')
        print('Processing debit payment type')
        print(f'Verifying security code: {self.security_code}')
        order.status = 'paid'

class CreditPaymentProcessor(PaymentProcessor):

    def __init__(self, security_code):
        self.security_code = security_code

    def pay(self, order):
        print('Processing credit payment type')
        print(f'Verifying security code: {self.security_code}')
        order.status = 'paid'

class PaypalPaymentProcessor(PaymentProcessor):

    def __init__(self, email_address, authorizer: Authorizer):
        self.authorizer = authorizer
        self.email_address = email_address

    def pay(self, order):
        if not self.authorizer.is_authorized():
            raise Exception('Not authorized')
        print('Processing paypal payment type')
        print(f'Using email address: {self.email_address}')
        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())
authorizer = NotARobot()
processor = DebitPaymentProcessor('2349875', authorizer)
authorizer.not_a_robot()
processor.pay(order)

Before#

After#

Before

After