Fake only what you do not own

How to do it

Put each outside service behind a small interface. In tests, pass a fake that records calls and returns set answers. Do not mock your own classes inside one feature, because the test then checks the mocks instead of the code.

Example

class FakePayments:
    def __init__(self):
        self.charges = []

    def charge(self, amount, card):
        self.charges.append(amount)
        return "ch_test_1"

def test_checkout_charges_the_cart_total(db):
    payments = FakePayments()
    checkout(cart_with_total(42_00), payments=payments, db=db)
    assert payments.charges == [42_00]

class FakePayments {
  charges: number[] = [];
  charge(amount: number) { this.charges.push(amount); return 'ch_test_1'; }
}

test('checkout charges the cart total', async () => {
  const payments = new FakePayments();
  await checkout(cartWithTotal(4200), { payments, db });
  expect(payments.charges).toEqual([4200]);
});

How to check

Search the integration tests for mocks of your own modules, such as vi.mock('../, jest.mock('./ or mock.patch("shop.. There are none. Doubles exist only for outside services.