When to Apply This Skill

• Writing unit tests for any function, method, or class

• Generating tests from a requirement or user story before code exists

• Producing tests for an API endpoint from an OpenAPI spec

Usage

Apply this skill when:

• Writing unit tests for any function, method, or class

• Generating tests from a requirement or user story before code exists

• Producing tests for an API endpoint from an OpenAPI spec

Read references/index.md before executing any step.

Step 1 — Understand the Unit

Read the provided source code or requirement carefully:

• What does this unit DO? (single responsibility check)

• What are its inputs and their types?

• What does it return or mutate?

• What can go wrong? (exceptions, error states)

• Does it call external systems? (DB, API, file, time, random)

Step 2 — Build the Test Case Matrix

Before writing a single line of test code, map out every case in a table:

Unit: calculate_discount(price: float, tier: str) -> float

Condition Input Expected Category

1 Standard tier, valid 100.0, "standard" 90.0 Happy

2 Premium tier 100.0, "premium" 80.0 Happy

3 Unknown tier 100.0, "vip" ValueError Error

4 Price is zero 0.0, "standard" 0.0 Boundary

5 Price is negative -10.0, "standard" ValueError Boundary

6 Price is None None, "standard" TypeError Boundary

7 Tier is None 100.0, None TypeError Boundary

Step 3 — Write Each Test (AAA Pattern)

Python (pytest):

def test_calculate_discount_when_standard_tier_expects_ten_percent_off(): # ARRANGE price = 100.0 tier = "standard" # ACT result = calculate_discount(price, tier) # ASSERT assert result == 90.0

JavaScript (Jest):

test("returns 10% discount for standard tier", () => { // ARRANGE const price = 100.0; const tier = "standard"; // ACT const result = calculateDiscount(price, tier); // ASSERT expect(result).toBe(90.0); });

Java (JUnit 5):

@Test void calculateDiscount_whenStandardTier_returnsTenPercentOff() { double result = calculator.calculateDiscount(100.0, "standard"); assertEquals(90.0, result, 0.001); }

Step 4 — Mock External Dependencies

Python:

@patch("module.external_api.call") def test_fn_when_api_success_expects_result(mock_api): mock_api.return_value = {"status": "ok", "data": 42} result = my_function() assert result == 42 mock_api.assert_called_once()

JavaScript (Jest):

jest.mock("./externalService"); test("should return result when service succeeds", async () => { externalService.fetch.mockResolvedValue({ data: 42 }); const result = await myFunction(); expect(result).toBe(42); expect(externalService.fetch).toHaveBeenCalledTimes(1); });

Step 5 — Parametrize for Multiple Cases

pytest:

@pytest.mark.parametrize("price,tier,expected", [ (100.0, "standard", 90.0), (100.0, "premium", 80.0), (200.0, "standard", 180.0), ]) def test_calculate_discount_parametrized(price, tier, expected): assert calculate_discount(price, tier) == expected

Jest:

test.each([ [100.0, "standard", 90.0], [100.0, "premium", 80.0], ])("discount(%f, %s) should be %f", (price, tier, expected) => { expect(calculateDiscount(price, tier)).toBe(expected); });

Step 6 — Exception Testing

pytest:

def test_calculate_discount_when_negative_price_expects_value_error(): with pytest.raises(ValueError, match="Price cannot be negative"): calculate_discount(-10.0, "standard")

JUnit 5:

@Test void calculateDiscount_whenNegativePrice_throwsIllegalArgumentException() { assertThrows(IllegalArgumentException.class, () -> calculator.calculateDiscount(-10.0, "standard")); }

Quality Checklist

Before emitting the final test file:

• Test matrix was built first (not ad-hoc)

• Every row in the matrix has a corresponding test

• All mocks have assertions (not just setup)

• Tests are fully isolated (no shared mutable state)

• Naming follows: test_unit_when_condition_expects_outcome

• File runs with a single command, zero additional config