How to Use Stubs, Fakes, Spies, and Mocks in Python Testing

class PaymentGateway:
    def charge(self, amount):
        raise NotImplementedError


class StubPaymentGateway(PaymentGateway):
    """Returns a fixed response without any logic."""
    def charge(self, amount):
        return {"success": True, "transaction_id": "stub-12345"}


class FakePaymentGateway(PaymentGateway):
    """Simulates the real system with in-memory storage."""
    def __init__(self):
        self.transactions = []

    def charge(self, amount):
        self.transactions.append(amount)
        return {"success": True, "transaction_id": f"fake-{len(self.transactions)}"}


class SpyPaymentGateway(PaymentGateway):
    """Records calls for later assertions."""
    def __init__(self):
        self.called_with = []

    def charge(self, amount):
        self.called_with.append(amount)
        return {"success": True, "transaction_id": "spy-0001"}


class MockPaymentGateway(PaymentGateway):
    """Pre-programmed with expectations and verifies them."""
    def __init__(self, expected_amount):
        self.expected_amount = expected_amount
        self.called = False

    def charge(self, amount):
        self.called = True
        assert amount == self.expected_amount, f"Expected {self.expected_amount}, got {amount}"
        return {"success": True, "transaction_id": "mock-0001"}

    def verify(self):
        assert self.called, "charge() was never called"


def process_order(gateway, amount):
    result = gateway.charge(amount)
    if result["success"]:
        print(f"Order of ${amount} processed successfully")


if __name__ == "__main__":
    # Stub - just returns fixed data
    stub = StubPaymentGateway()
    process_order(stub, 100)

    # Fake - real in-memory behavior
    fake = FakePaymentGateway()
    process_order(fake, 50)
    process_order(fake, 75)
    print(f"Fake recorded transactions: {fake.transactions}")

    # Spy - records calls
    spy = SpyPaymentGateway()
    process_order(spy, 30)
    print(f"Spy was called with: {spy.called_with}")

    # Mock - verifies specific expected behavior
    mock = MockPaymentGateway(expected_amount=200)
    process_order(mock, 200)
    mock.verify()
    print("Mock verified successfully")