Adapter Example: A Legacy Payment Gateway Behind a New Interface

from abc import ABC, abstractmethod
from decimal import Decimal


class PaymentProcessor(ABC):
    @abstractmethod
    def pay(self, amount: Decimal) -> bool: ...


class LegacyGateway:
    def make_transaction(self, cents: int, currency: str) -> dict:
        print(f"legacy gateway: charging {cents} {currency} cents")
        return {"status": "OK", "reference": "TX-1042"}


class LegacyGatewayAdapter(PaymentProcessor):
    def __init__(self, gateway: LegacyGateway, currency: str) -> None:
        self._gateway = gateway
        self._currency = currency

    def pay(self, amount: Decimal) -> bool:
        cents = int(amount * 100)
        response = self._gateway.make_transaction(cents, self._currency)
        return response["status"] == "OK"


class Checkout:
    def __init__(self, processor: PaymentProcessor) -> None:
        self._processor = processor

    def complete(self, total: Decimal) -> str:
        return "paid" if self._processor.pay(total) else "declined"


checkout = Checkout(LegacyGatewayAdapter(LegacyGateway(), "GBP"))
print(checkout.complete(Decimal("24.99")))