Testing

Testing Strategy

Decide what to test and how to test it. Write tests that catch real regressions.

"Tests should be coupled to the behavior of code and decoupled from the structure of code." — Kent Beck

The Self-Test

Ask these 5 questions about EVERY test you write. If any answer is "no", rewrite the test.

The Refactoring Litmus Test: After writing a test, imagine completely rewriting the internals while keeping the same public behavior. Would the test still pass? If not, it's coupled to structure and will become a maintenance burden.

Test Double Decision Tree

Choose the simplest test double that gives confidence. Work top to bottom — stop at the first "yes".

Can I use the REAL implementation?
├─ Yes → Use it (always the first choice)
└─ No → Is it slow, non-deterministic, or expensive?
         ├─ Yes → Is a FAKE available? (in-memory DB, fake server)
         │        ├─ Yes → Use the fake
         │        └─ No → STUB specific return values (keep count low)
         └─ No → Do I need to verify external SIDE EFFECTS?
                  (email sent, record saved, event published)
                  ├─ Yes → Interaction test with verify (LAST RESORT)
                  └─ No → Re-examine — you probably CAN use the real thing

When to Mock

Mock AT boundaries (external edges of your system):

• External HTTP APIs → fake server or stub responses
• Databases → in-memory DB, fake repository, or testcontainers
• File system → in-memory FS or temp directories
• System clock → inject controllable clock
• Non-deterministic sources → seeded random, fixed UUIDs
• Expensive third-party calls → stub at the adapter layer

Never mock these:

• Internal collaborator classes
• Value objects or data structures
• Pure functions or utilities
• Anything you can instantiate cheaply

"Don't Mock What You Don't Own": If you must mock a third-party API, wrap it in your own adapter and mock the adapter.

What to Test

Think in behaviors, not methods. Each test covers one behavior: "Given X, when Y, then Z."

Identify Behaviors

Don't write one test per method. Write one test per behavior:

# BAD — one method, one test (grows unwieldy)
def test_process_transaction():
    # tests display, validation, AND balance check in one test
    ...

# GOOD — one behavior, one test
def test_process_transaction_displays_item_name(): ...
def test_process_transaction_rejects_negative_amount(): ...
def test_process_transaction_warns_on_low_balance(): ...

Priority Order

1. Edge cases and error conditions — these catch real bugs
2. Business rules and invariants — the core logic
3. Integration boundaries — where systems meet
4. Happy path — last, not first (it's usually the most obvious)

Usually Skip

Third-party library internals, simple getters/setters, framework boilerplate, and implementation details that may change.

Bug Fixes

Always write a failing test FIRST that reproduces the bug. Then fix it. Never fix a bug without a regression test.

A failure you found by exercising the code by hand is a bug like any other — reproduce the observed failure in a failing test before fixing it, then re-exercise by hand to confirm.

Anti-Patterns: Before and After

1. Over-Mocking → Use Real Implementations

# BEFORE — testing mocks, not code
@patch("myapp.cache.get")
@patch("myapp.db.query")
@patch("myapp.validator.check")
def test_process(mock_check, mock_cache, mock_db):
    mock_db.return_value = {"id": 1}
    mock_cache.return_value = None
    mock_check.return_value = True
    result = process(1)  # What are we even testing?
    assert result == {"id": 1}

# AFTER — test with real collaborators
def test_process():
    db = InMemoryDatabase({"users": [{"id": 1, "name": "Alice"}]})
    service = ProcessingService(db=db, cache=MemoryCache())
    result = service.process(1)
    assert result.name == "Alice"

2. Mirror Tests → Test Outcomes, Not Steps

# BEFORE — mirrors implementation step by step
def test_register_user():
    service.register("alice@test.com", "pass123")
    mock_validator.validate.assert_called_once_with("alice@test.com")
    mock_db.insert.assert_called_once()
    mock_email.send.assert_called_once_with(
        to="alice@test.com", template="welcome"
    )

# AFTER — asserts observable outcomes
def test_register_user():
    service.register("alice@test.com", "pass123")
    assert service.get_user("alice@test.com") is not None  # user exists
    assert len(email_server.sent) == 1                      # email sent
    assert email_server.sent[0].to == "alice@test.com"

Writing Good Tests

Naming Conventions

Describe what scenario is tested and what outcome is expected:

# Good — scenario + expected outcome
def test_expired_token_returns_401(): ...
def test_checkout_with_empty_cart_raises_error(): ...
def test_transfer_insufficient_balance_raises_error(): ...
def test_login_wrong_password_locks_after_3_attempts(): ...
def test_search_returns_empty_list_when_no_matches(): ...

# Bad — vague, no outcome
def test_token_expiry_check(): ...
def test_transfer(): ...
def test_login_error(): ...
def test_search_works(): ...

Test Structure: AAA / GWT

Structure each test as Arrange → Act → Assert (or Given → When → Then). Keep sections visually distinct — whitespace or comments between the three phases help readability.

DAMP, Not DRY

Tests should be Descriptive And Meaningful Phrases. Duplicate for clarity:

• Use helper methods for constructing test objects (factories with sensible defaults)
• Use factory helpers like make_user(**overrides) to build test objects with sensible defaults — override only fields relevant to the test
• Avoid helpers that hide what's being asserted
• Each test should be readable without scrolling to shared setup
• Prefer explicit inline values over shared constants with ambiguous names

No Logic in Test Bodies

Tests should be trivially correct on inspection. Straight-line code only:

• ❌ No loops, conditionals, or string concatenation in assertions
• ❌ No computed expected values
• ✅ Hardcode every expected value
• ✅ One clear path from setup → action → assertion

Test Quality Checklist

Before committing tests, verify each one:

- [ ] Passes the 5-question Self-Test (above)
- [ ] Name describes scenario + expected outcome
- [ ] No logic (loops, conditionals) in test body
- [ ] Each test is self-contained (DAMP: readable without shared context)
- [ ] Failure message tells you what's wrong without reading the test
- [ ] Uses real implementations where possible (mocks only at boundaries)
- [ ] Tests behavior through public API, not internal methods

Rationalization Prevention