snqb/tdd

TDD Discipline

Write the test. Watch it fail. Write minimal code. Watch it pass. Repeat.

The Cycle

RED → GREEN → REFACTOR → commit

1. RED: Write one test. Run it. Watch it FAIL (not error—fail because feature is missing).
2. GREEN: Write simplest code to pass. Just enough. No more.
3. REFACTOR: Clean up while tests green. Then commit.

Rules

• No production code without a failing test first
• Wrote code before the test? Delete it. Start over.
• If you didn't watch the test fail, you don't know if it tests the right thing.

When to Skip

• Throwaway prototypes (that you'll actually throw away)
• Generated code
• Config files

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

Anti-Patterns

| Don't | Do | |-------|-----| | Test mocks instead of real code | Test real code | | test_retry_works | retries_failed_operations_3_times | | Multiple behaviors in one test | One behavior per test | | Keep "reference code" after deleting untested code | Delete all untested code |

Examples

Good Test Name

def test_transfer_fails_when_insufficient_balance():
    ...

Bad Test Name

def test_transfer():  # What about transfer?
    ...

Minimal Green

# RED: test expects add(2, 3) == 5

# GREEN (wrong - too much):
def add(a, b):
    if not isinstance(a, (int, float)):
        raise TypeError("...")
    return a + b

# GREEN (right - just enough):
def add(a, b):
    return a + b