Unit Testing Principles & Test-Driven Development (TDD)

A Unit Test is an automated test that exercises the smallest testable unit of software—typically an individual function, method, or class—in complete isolation from external infrastructure, databases, network connections, and third-party dependencies.

graph TD
    E2E[End-to-End Tests<br/>Slow, Expensive, Fragile]
    Integration[Integration Tests<br/>Database, Message Brokers, HTTP Contracts]
    Unit[Unit Tests<br/>Fast, Deterministic, Highly Isolated, Millisecond Execution]

    E2E --> Integration --> Unit

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1. The F.I.R.S.T. Principles of Unit Testing

Robust test suites adhere to the F.I.R.S.T. acronym:

• Fast: A suite of 5,000 unit tests must execute in under 10 seconds. If tests are slow, developers skip running them locally.
• Independent / Isolated: Tests must never depend on the outcome or execution order of other tests. Zero shared mutable state.
• Repeatable: Tests must produce the exact same result whether run on a local Mac, Linux CI runner, or in an airplane without Wi-Fi.
• Self-Validating: Tests must report a binary pass/fail without requiring manual inspection of logs or print statements.
• Timely: Written alongside or immediately before production code (TDD).

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2. The AAA (Arrange-Act-Assert) Pattern

Structure every unit test into three distinct phases:

def test_calculate_order_discount():
    # 1. Arrange (Setup test inputs and preconditions)
    cart = ShoppingCart()
    cart.add_item(price=100.0, quantity=2)
    coupon = DiscountCoupon(percentage=15)
 
    # 2. Act (Invoke the unit under test)
    final_price = cart.apply_discount(coupon)
 
    # 3. Assert (Verify expected outcome)
    assert final_price == 170.0

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3. Table-Driven Tests (Go Idiom)

In Go, table-driven tests eliminate duplicate test functions by looping over test cases:

func TestValidateEmail(t *testing.T) {
	cases := []struct {
		name     string
		email    string
		expected bool
	}{
		{"valid standard", "user@example.com", true},
		{"valid subdomain", "user@mail.co.uk", true},
		{"missing domain", "user@", false},
		{"missing at symbol", "userexample.com", false},
		{"empty string", "", false},
	}
 
	for _, tc := range cases {
		t.Run(tc.name, func(t *testing.T) {
			actual := ValidateEmail(tc.email)
			if actual != tc.expected {
				t.Errorf("ValidateEmail(%q) = %v; want %v", tc.email, actual, tc.expected)
			}
		})
	}
}

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4. Next Step

To test units in complete isolation, external dependencies (databases, payment gateways, file systems) must be replaced with Test Doubles.

→ Test Doubles: Mocks, Stubs, Fakes, Spies, and Dummies

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