test-unit-write.md

Unit Test Writer

This skill generates complete unit tests from scratch or extends coverage for functions, classes, and modules.

Quick Start

Ask the agent to write tests:

• "Write unit tests for this function"
• "Add tests covering the failure paths"
• "Generate test cases for this class"
• "Help me test this service"

Provide the source code, and the agent will:

1. Analyze the public API and I/O boundaries
2. Select appropriate mock strategies (stub/patch/inject)
3. Generate test cases covering happy paths, failures, and edge cases
4. Output tests in language-idiomatic format (pytest, Jest, MUnit)

What This Skill Does

test-unit-write follows a 7-step workflow:

1. Load language reference — Detect language/framework and load conventions
2. Analyse source — Identify public API, dependencies, I/O boundaries, failure conditions
3. Choose mock strategy — Decide how to isolate HTTP, DB, files, clock, randomness
4. Scaffold test file — Setup imports, fixtures, mocking infrastructure
5. Write test cases — Happy path, failure paths (≥1 per behaviour), edge/boundary cases
6. Assert correctly — Use framework-specific matchers, not generic truthy checks
7. Run and validate — Execute tests; fix any failures before returning

Generated Test Structure

# pytest example
import pytest
from unittest.mock import MagicMock
from order_service import OrderService

@pytest.fixture
def mock_payment_gateway():
    """Mock payment gateway for order processing."""
    return MagicMock()

def test_order_succeeds_with_valid_input(mock_payment_gateway):
    """Happy path: order placed with valid amount and payment success."""
    service = OrderService(mock_payment_gateway)
    service.process_order(customer_id=123, amount=50.00)
    assert mock_payment_gateway.charge.called

def test_order_fails_on_insufficient_funds(mock_payment_gateway):
    """Failure path: payment gateway returns insufficient funds."""
    mock_payment_gateway.charge.side_effect = ValueError("Insufficient funds")
    service = OrderService(mock_payment_gateway)
    with pytest.raises(ValueError, match="Insufficient funds"):
        service.process_order(customer_id=123, amount=50.00)

Coverage Requirements

Each test addresses:

• Success path — main behaviour under ideal conditions
• ≥1 failure path per behaviour — exceptions, error conditions, guards
• Boundary values — empty, zero, None/null, max/min values

Language Support

• Python — pytest with fixtures, parametrization, monkeypatch
• JavaScript / TypeScript — Jest with mocks, spies, async/await
• Scala — MUnit with scopes, fixtures, assertions

When To Use

• Writing tests for new functions or classes
• Extending coverage for existing code
• Scaffolding test structure and mocking strategy
• Learning what good test coverage looks like

When NOT To Use

• Reviewing existing tests → use test-unit-review
• Abstract standards questions → use test-unit-standards
• Test doubles / mocking patterns → separate skill (test-mocking-patterns, future)
• Integration tests → use integration test skill (future)

Related Skills

• test-unit-standards — Reference for naming, isolation, assertions
• test-unit-review — Audit tests after writing