Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask the user first):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor Cycle

RED — Write Failing Test

Write one minimal test showing what should happen.

Good test:

def test_retries_failed_operations_3_times():
    attempts = 0
    def operation():
        nonlocal attempts
        attempts += 1
        if attempts < 3:
            raise Exception('fail')
        return 'success'

    result = retry_operation(operation)

    assert result == 'success'
    assert attempts == 3

Clear name, tests real behavior, one thing.

Bad test:

def test_retry_works():
    mock = MagicMock()
    mock.side_effect = [Exception(), Exception(), 'success']
    result = retry_operation(mock)
    assert result == 'success'  # What about retry count? Timing?

Vague name, tests mock not real code.

Requirements:

• One behavior per test
• Clear descriptive name ("and" in name? Split it)
• Real code, not mocks (unless truly unavoidable)
• Name describes behavior, not implementation

Verify RED — Watch It Fail

MANDATORY. Never skip.

pytest tests/test_feature.py::test_specific_behavior -v

Confirm:

• Test fails (not errors from typos)
• Failure message is expected
• Fails because the feature is missing

Test passes immediately? You're testing existing behavior. Fix the test.

Test errors? Fix the error, re-run until it fails correctly.

GREEN — Minimal Code

Write the simplest code to pass the test. Nothing more.

Good:

def add(a, b):
    return a + b  # Nothing extra

Bad:

def add(a, b):
    result = a + b
    logging.info(f"Adding {a} + {b} = {result}")  # Extra!
    return result

Don't add features, refactor other code, or "improve" beyond the test.

Cheating is OK in GREEN:

• Hardcode return values
• Copy-paste
• Duplicate code
• Skip edge cases

We'll fix it in REFACTOR.

Verify GREEN — Watch It Pass

MANDATORY.

# Run the specific test
pytest tests/test_feature.py::test_specific_behavior -v

# Then run ALL tests to check for regressions
pytest tests/ -q

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix the code, not the test. Other tests fail? Fix regressions now.

REFACTOR — Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers
• Simplify expressions

Keep tests green throughout. Don't add behavior.

If tests fail during refactor: Undo immediately. Take smaller steps.

Repeat

Next failing test for next behavior. One cycle at a time.

Avoid Horizontal Slices

Do not write all tests first and then all implementation. That is horizontal slicing. Use vertical tracer bullets instead:

WRONG:
  RED:   test1, test2, test3, test4
  GREEN: impl1, impl2, impl3, impl4

RIGHT:
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3

A tracer bullet is one end-to-end behavior slice. It proves the path works, teaches you about the interface, and keeps each next test grounded.

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test the wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (high confidence)
• Keep it and add tests after (low confidence, likely bugs)

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Already manually tested"	Ad-hoc ≠ systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Keeping unverified code is debt.
"Keep as reference, write tests first"	You'll adapt it. That's testing after. Delete means delete.
"Need to explore first"	Fine. Throw away exploration, start with TDD.
"TDD will slow me down"	TDD faster than debugging. Pragmatic = test-first.

Red Flags — STOP and Start Over

If you catch yourself doing any of these, delete the code and restart with TDD:

• Code before test
• Test after implementation
• Test passes immediately on first run
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"

All of these mean: Delete code. Start over with TDD.

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Problem	Solution
Don't know how to test	Write the wished-for API. Write the assertion first. Ask the user.
Test too complicated	Design too complicated. Simplify the interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract helpers. Still complex? Simplify the design.

Testing Anti-Patterns

• Testing mock behavior instead of real behavior — mocks should verify interactions, not replace the system under test
• Testing implementation details — test behavior/results, not internal calls
• Happy path only — always test edge cases, errors, and boundaries
• Brittle tests — tests should verify behavior, not structure; refactoring shouldn't break them

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without the user's explicit permission.