test-driven-development

The Two Gates

Before writing any code, pass it through two gates:

1. The Logic Gate — Triage. Does this piece contain logic?
2. The Iron Rule — For everything that passes the Logic Gate, strict TDD applies.

flowchart TD
    A[New code to write] --> B[Decompose into pieces]
    B --> C{{"Logic Gate: Does this piece contain logic?"}}
    C -->|"Conditionals, transformations, validation, state changes, error handling"| D["Iron Rule: TDD"]
    C -->|"Dataclasses, config, thin wrappers, type aliases, formatting"| E[Write directly]
    D --> F["RED: Write failing test"]
    F --> G["GREEN: Minimal implementation"]
    G --> H["REFACTOR: Clean up"]
    H --> I{More pieces?}
    E --> I
    I -->|Yes| C
    I -->|No| J[Done]

Gate 1: The Logic Gate

Decompose the work into its constituent pieces. For each piece, ask: does this contain logic?

Passes the Logic Gate (requires TDD):

• Conditionals and branching
• Data transformations and computations
• Validation and parsing
• State management and transitions
• Error handling with recovery logic
• Business rules and domain logic
• Algorithm implementations

Does NOT pass the Logic Gate (write directly):

• Dataclasses, NamedTuples, TypedDicts (pure data declarations)
• Type aliases and protocol definitions
• Configuration and constants
• Thin wrappers that only delegate (no branching, no transformation)
• Plot formatting and visual styling
• Boilerplate (__init__.py, module-level setup)
• String templates and static content

When in doubt: If you can't point to a conditional, transformation, or state change, it probably doesn't pass the Logic Gate.

Triage Output

Before writing code, briefly identify what passes and what doesn't:

• "These pieces contain logic and need TDD: X, Y"
• "These are declarations/glue and can be written directly: Z"

Gate 2: The Iron Rule

For code that passes the Logic Gate: never write logic without a preceding failing test.

If logic is written before its test:

1. Stop — the test you write now can only describe what you already wrote, not verify intended behavior
2. Delete the logic and restart test-first
3. This applies to logic specifically, not declarations

Red-Green-Refactor Cycle

RED: Write a Failing Test

Write a single minimal test demonstrating intended behavior:

• Clear, descriptive name
• Test actual functionality (not mocks unless necessary)
• Focus on one behavior
• If test name contains "and", split into separate tests

Verify RED: Confirm Failure

Run tests and confirm:

• Test fails for the expected reason (missing feature)
• Not failing due to syntax errors
• This verification is non-negotiable

GREEN: Minimal Implementation

Write the simplest possible code satisfying the test:

• Avoid feature creep
• Don't refactor unrelated code
• Don't over-engineer

Verify GREEN: Confirm Pass

Confirm:

• The new test passes
• No existing tests break

REFACTOR: Clean Up

Improve code quality while maintaining green status:

• Eliminate duplication
• Improve naming
• Simplify logic
• Run tests after each change

Quality Test Attributes

Minimal: Single responsibility per test Clear: Descriptive name revealing behavior Realistic: Demonstrate intended API usage Focused: One assertion per test (generally)

Common Rationalizations to Reject

"I'll test afterward"

• Tests written post-implementation pass immediately
• Proves nothing about validation capability

"Manual testing covered edge cases"

• Lacks systematic documentation
• Not reproducible

"Tests-after achieve identical goals"

• Tests-first answer: "What should happen?"
• Tests-after answer: "What does this do?"
• Fundamentally different approaches

Verification Checklist

Before considering work complete:

• [ ] Logic Gate triage was performed
• [ ] Every piece containing logic has corresponding tests
• [ ] Each test for logic failed before implementation
• [ ] Failures occurred for expected reasons
• [ ] Code written is minimal
• [ ] All tests pass without errors or warnings
• [ ] Real code tested (mocks only when necessary)
• [ ] Edge cases and error conditions covered for logic

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.