tdd-python skill

Overview

This skill provides practical Python Test-Driven Development (TDD) expertise focused on pytest, strict red-green-refactor cycles, FastAPI endpoint testing, and Pydantic model validation. It guides test-first design choices and supplies templates and patterns to maintain high test coverage and clear, maintainable code. Use it to drive feature design from tests and to standardize testing practices across Python projects.

How this skill works

The skill inspects user intent and code context to generate test-first artifacts: failing pytest tests, minimal implementation code, and refactor suggestions to reach green status. It produces pytest-friendly test cases for unit logic, FastAPI endpoints, and Pydantic models, plus fixtures and mocks where appropriate. It also recommends next refactor steps and asserts coverage and edge-case handling during the cycle.

When to use it

• When implementing new Python features and you want a strict TDD workflow
• When writing or improving pytest tests for libraries or applications
• When developing or testing FastAPI endpoints and integrations
• When validating Pydantic models, schemas, and data parsing behavior
• When you want clear red-green-refactor guidance to drive design decisions

Best practices

• Write a single failing test that expresses the smallest new behavior before coding
• Use pytest fixtures and parametrization to reduce duplication and cover edge cases
• Keep implementation minimal to pass tests, then refactor for readability and reusability
• Test FastAPI endpoints with TestClient and include status, body schema, and authentication cases
• Validate Pydantic models with positive and negative input examples and explicit error assertions

Example use cases

• Generate a pytest test that defines desired behavior for a new utility function, then produce the minimal implementation to pass it
• Create end-to-end FastAPI tests for an endpoint including request validation, response schema, and error cases
• Write Pydantic model tests that assert parsing, default values, and validation errors for invalid input
• Convert existing feature requests into an ordered TDD task list: failing test, implementation, refactor, and coverage checks
• Provide concise pytest templates and fixtures to bootstrap a test suite for a new Python package

FAQ