unit-test-generating-unit-tests

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

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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

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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)

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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

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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);
}

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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);
});

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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);
});

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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"));
}

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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