Python Test-Driven Development expertise with pytest, strict red-green-refactor methodology, domain-first modeling (DDD), principled DRY discipline, FastAPI testing patterns, and Pydantic model testing. Use when implementing Python features with TDD, writing pytest tests, testing FastAPI endpoints, developing with test-first approach, modeling domain concepts, refactoring duplication, or when user mentions 'Python TDD', 'pytest', 'FastAPI testing', 'red-green-refactor', 'Python unit tests', 'te…
Python Test-Driven Development expertise with pytest, strict red-green-refactor methodology, domain-first modeling (DDD), principled DRY discipline, FastAPI testing patterns, and Pydantic model testing.
Use when implementing Python features with TDD, writing pytest tests, testing FastAPI endpoints, developing with test-first approach, modeling domain concepts, refactoring duplication, or when user mentions 'Python TDD', 'pytest', 'FastAPI testing', 'red-green-refactor', 'Python unit tests', 'test-driven Python', 'domain modeling', 'value object', 'ubiquitous language', 'DRY', or 'Python test coverage'.
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SKILL.md
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 (testrejectsrefundwhenamountexceedsdailylimit) — never mechanical (testreturns_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.
