pytest-what-to-test-and-mocking

Pytest: What to Test and Mocking

Core Requirements

We mock external API calls (mark live tests with @pytest.mark.flaky()). We reuse fixtures from conftest.py. We always use pytest as testing framework - never import unittest. We use pytest-mock (mocker fixture) and monkeypatch for mocking, not unittest.mock.

What to Test

Test your business logic, not the libraries you depend on.

Test Your Business Logic

def test_calculates_discount_correctly():
    """Test our custom discount calculation"""
    order = Order(subtotal=Decimal("100.00"))
    discount = calculate_discount(order, coupon_code="SAVE20")
    assert discount == Decimal("20.00")

def test_handles_invalid_coupon_gracefully():
    """Test our error handling for invalid coupons"""
    order = Order(subtotal=Decimal("100.00"))
    discount = calculate_discount(order, coupon_code="INVALID")
    assert discount == Decimal("0.00")

def test_order_state_transitions():
    """Test our state machine logic"""
    order = Order(status="pending")
    order.mark_paid()
    assert order.status == "paid"
    assert order.paid_at is not None

Test Integration Points

def test_sends_correct_parameters_to_stripe(mocker):
    """Test how we call Stripe API"""
    mock_stripe = mocker.patch("stripe.Charge.create")
    process_payment(order_id="123", amount=Decimal("50.00"))
    mock_stripe.assert_called_once_with(
        amount=5000,  # cents
        currency="usd",
        source="tok_visa"
    )

def test_processes_stripe_response_correctly():
    """Test how we transform Stripe data"""
    stripe_response = {"id": "ch_123", "status": "succeeded"}
    payment = Payment.from_stripe_response(stripe_response)
    assert payment.external_id == "ch_123"
    assert payment.status == "completed"

Skip Library Testing

We don't test that Pydantic validates, Django filters work, or httpx makes requests. These libraries test themselves. We focus on our logic that uses these tools.