Interface Segregation Principle

包括的なインターフェースがあるより、いくつかの特定のインターフェースに分割すべき。

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 PaymentProcessor(ABC):

    @abstractmethod
    def auth_sms(self, code):
        pass

    @abstractmethod
    def pay(self, order):
        pass

class DebitPaymentProcessor(PaymentProcessor):

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

    def auth_sms(self, code):
        print(f'Verifying SMS code {code}')
        self.verified = True
    
    def pay(self, order):
        if not self.verified:
            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 auth_sms(self, code):
        raise Exception('Credit card payments don't support SMS code authorization.')

    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):
        self.email_address = email_address
        self.verified = False

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

    def pay(self, order):
        if not self.verified:
            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())
processor = DebitPaymentProcessor('2349875')
processor.auth_sms(465839)
processor.pay(order)

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 PaymentProcessor(ABC):

    @abstractmethod
    def auth_sms(self, code):
        pass

    @abstractmethod
    def pay(self, order):
        pass

class DebitPaymentProcessor(PaymentProcessor):

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

    def auth_sms(self, code):
        print(f'Verifying SMS code {code}')
        self.verified = True
    
    def pay(self, order):
        if not self.verified:
            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 auth_sms(self, code):
        raise Exception('Credit card payments don't support SMS code authorization.')

    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):
        self.email_address = email_address
        self.verified = False

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

    def pay(self, order):
        if not self.verified:
            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())
processor = DebitPaymentProcessor('2349875')
processor.auth_sms(465839)
processor.pay(order)

上記の場合、auth_sms を汎用的な
PaymentProcessor に設定しているが、
CreditPaymentProcessor では本来必要ない。

そのため、PaymentProcessor の
サブクラス PaymentProcessor_SMS を作成し、PaymentProcessor を継承させる。 DebitPaymentProcessor, PaypalPaymentProcessor は auth_sms が必要なため、 PaymentProcessor_SMS を継承させる。

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 PaymentProcessor(ABC):

    @abstractmethod
    def pay(self, order):
        pass

class PaymentProcessor_SMS(PaymentProcessor):

    @abstractmethod
    def auth_sms(self, code):
        pass


class DebitPaymentProcessor(PaymentProcessor_SMS):

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

    def auth_sms(self, code):
        print(f'Verifying SMS code {code}')
        self.verified = True
    
    def pay(self, order):
        if not self.verified:
            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_SMS):

    def __init__(self, email_address):
        self.email_address = email_address
        self.verified = False

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

    def pay(self, order):
        if not self.verified:
            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())
processor = DebitPaymentProcessor('2349875')
processor.auth_sms(465839)
processor.pay(order)

Composision

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 SMSAuth:

    authorized = False
    
    def verify_code(self, code):
        print(f'Verifying code {code}')
        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: SMSAuth):
        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, security_code, authorizer: SMSAuth):
        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 = SMSAuth()
processor = DebitPaymentProcessor('2349875', authorizer)
authorizer.verify_code(465839)
processor.pay(order)

Before#

Before#

After#

Composision#

Before

Before

After

Composision