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Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Instead of hard-wiring concrete classes, use dependency injection to pass abstractions. This decouples components and makes the system easier to test and extend.

Diagram

Violation

In violation.py PaymentProcessor directly creates a CreditCardPayment inside its constructor:

class PaymentProcessor:
    def __init__(self) -> None:
        self.payment = CreditCardPayment()  # hard-wired dependency

    def process(self, amount: float) -> None:
        self.payment.pay(amount)

Switching to cash or cryptocurrency requires modifying the processor itself.

Correct

In correct.py both the processor and payment methods depend on a Payment abstraction:

class Payment(ABC):
    @abstractmethod
    def pay(self, amount: float) -> None: ...

class PaymentProcessor:
    def __init__(self, payment: Payment) -> None:
        self.payment = payment

    def process(self, amount: float) -> None:
        self.payment.pay(amount)

New payment methods are added by implementing Payment — the processor never changes:

for method in (CashPayment(), CreditCardPayment(), CryptoPayment()):
    processor = PaymentProcessor(method)
    processor.process(150.00)