TDD Python Skill

Python Test-Driven Development following strict red-green-refactor cycle with pytest — sharpened by DDD at Red (model the domain first) and DRY at Refactor (deduplicate same-concept repetition only).

Description

Systematic Python implementation using TDD methodology. Tests are written first and drive design; domain vocabulary drives the tests; duplication is removed only when the same concept repeats, not when syntax coincidentally looks alike.

Discipline Layering

• Before Red — Model the Domain: Name the entity, value object, or aggregate in the business's language. Decide entity vs. value object. Reject primitive obsession — wrap str/int in EmailAddress, Money, NewType, or pydantic.BaseModel when they carry rules. Keep domain logic out of FastAPI routes.
• Red: Write a failing test. Name it in domain terms (test_rejects_refund_when_amount_exceeds_daily_limit) — never mechanical (test_returns_false).
• Green: Minimal code to pass. No speculative abstractions.
• Refactor — DRY with discipline: Deduplicate only when the same domain concept appears ≥3 times with the same meaning. Three similar lines beats a premature abstraction. Promote recurring primitives to value objects. When uncertain, inline.

What's Included

• Examples: Python TDD cycles, FastAPI TDD, Pydantic model TDD, value-object modeling
• Reference: pytest patterns, Python testing best practices, branded-type idioms (NewType, frozen=True)
• Templates: pytest templates, TDD workflows

Use When

• Implementing Python features with TDD
• Modeling domain concepts (entities, value objects, aggregates)
• FastAPI endpoint development (keeping domain rules out of routes)
• Pydantic model development
• Refactoring duplication after a green test

Related Agents

• tdd-python-implementer

Skill Version: 1.1