Python Dev

Iron Law

NO FASTAPI ENDPOINT WITHOUT A PYDANTIC v2 INPUT MODEL — never trust raw request data; validate at the boundary, always

Python 3.14 Development Skill

Code Conventions

• Use Python 3.14 with type hints everywhere
• FastAPI for REST APIs, Pydantic v2 for validation
• Async by default: async def endpoints, asyncpg for PostgreSQL
• Use uv for package management, ruff for linting, mypy for types
• Folder structure: api/routes/ → services/ → repositories/ → models/
• Tests: pytest + pytest-asyncio + httpx

Quick Scaffold — New Python Project

# Using uv (preferred)
uv init my-service
cd my-service
uv add fastapi uvicorn pydantic pydantic-settings
uv add --dev pytest pytest-asyncio httpx ruff mypy

# Or using pip
mkdir my-service && cd my-service
python -m venv .venv
source .venv/bin/activate    # Windows: .venv\Scripts\activate
pip install fastapi uvicorn pydantic pydantic-settings
pip install -e ".[dev]"

Reference Files

Code templates and project config: Read reference/fastapi-templates.md for FastAPI app structure, Pydantic models, route handlers, SQLAlchemy models, pytest fixtures, pyproject.toml template, Docker configuration, and common commands.

Advanced patterns and profiling: Read reference/python-advanced-patterns.md for performance profiling (cProfile, py-spy, memory_profiler), pytest-benchmark, property-based testing with Hypothesis, structural pattern matching, descriptors, and memory optimization techniques.

Auth, security and authorization: Read reference/fastapi-auth-security.md for JWT tokens (python-jose), bcrypt password hashing (cost 12), OAuth2PasswordBearer dependency injection, RBAC role hierarchy, PBAC permission maps, resource ownership checks, auth-specific rate limit configuration, and pytest auth test patterns.

Error handling, retry, and circuit breakers: Read reference/fastapi-error-handling.md for custom exception hierarchy (AppError → NotFoundError/ValidationError/ExternalServiceError), FastAPI exception handlers, async retry with tenacity, async circuit breaker with circuitbreaker, error logging context middleware, and pytest patterns for error paths.

Backend service consistency patterns: Read reference/backend-service-patterns.md for 12 mandatory patterns covering all services/backend/<service>/ code: timing-safe internal secret guard (P1), fail-fast config fields (P2), structlog naming (P3), session ownership (P4), Pub/Sub singleton (P5), fire-and-forget (P6), Redis lifecycle (P7), DB count queries (P8), GCS URI validation (P9), shared auth helpers (P10), no hardcoded config (P11), DRY shared utilities (P12). Includes pre-commit grep gates and new/modified service checklists.

Live E2E integration testing: Read reference/backend-live-e2e-standard.md for why unit tests (ASGITransport + mocks) miss real integration bugs, the per-service test pattern (health, auth gate, fake JWT rejection, internal secret gate, input validation), auth ordering rules (FastAPI validates auth before Pydantic), HTTP method verification from OpenAPI spec, and the feedback loop for fixing live failures.

Pub/Sub and Firestore messaging patterns: Read reference/pubsub-firestore-patterns.md for the three canonical messaging patterns: Pattern A (fire-and-forget asyncio.create_task for informational events), Pattern B (transactional outbox with SELECT FOR UPDATE SKIP LOCKED for load-bearing events), Pattern C (Firestore status bus for real-time mobile UI). Includes decision tree, full implementation templates, subscriber patterns, and hard rules for each.

Process

1. Scaffold project structure using uv or pip commands above
2. Read reference files when you need specific templates or configuration
3. Create folder structure: src/my_service/ with subfolders api/routes/, models/, services/, core/
4. Write main.py using FastAPI app template with lifespan context manager
5. Define Pydantic models for request/response validation in models/
6. Implement route handlers in api/routes/ with proper HTTP methods and status codes
7. Add business logic in services/ layer (keep routes thin)
8. Configure environment using pydantic-settings in core/config.py
9. Write tests with pytest-asyncio and AsyncClient fixtures
10. Format and type-check: Run ruff format, ruff check --fix, and mypy

Key Patterns

Documentation Sources

Before generating code, consult these sources for current syntax and APIs:

Error Handling

API errors: Always raise HTTPException with descriptive detail messages.

from fastapi import HTTPException

if not user:
    raise HTTPException(status_code=404, detail="User not found")

if not has_permission:
    raise HTTPException(status_code=403, detail="Insufficient permissions")

Validation errors: Pydantic automatically returns 422 with validation details. Use Field() constraints for business rules.

Database errors: Catch SQLAlchemy exceptions in service layer and convert to appropriate HTTP exceptions.

from sqlalchemy.exc import IntegrityError

try:
    await session.commit()
except IntegrityError:
    raise HTTPException(status_code=409, detail="Email already exists")

Common Commands

uvicorn src.main:app --reload                          # Run dev server (hot reload)
pytest -q                                              # Run tests (quiet output)
pytest -q --cov=src --cov-report=term-missing          # Tests with coverage
ruff check --fix .                                     # Lint and auto-fix
ruff format .                                          # Format code
mypy src/                                              # Type check
alembic upgrade head                                   # Run pending migrations
alembic revision --autogenerate -m "description"       # Generate new migration

Hard Prohibitions

• Use str | None union syntax (Python 3.10+), not Optional[str]
• Use model_validator for cross-field Pydantic validation, not ad-hoc __init__ logic

Background Worker Services

For Pub/Sub, Cloud Scheduler, or GCS-triggered workers (NOT FastAPI REST APIs):

• Use the /scaffold-python-worker command to create the service structure
• Entry point: async def run_job(payload: dict) -> JobResult: in src/job.py
• External clients (APIs, cloud services) MUST be injected — never instantiated inline
• All failures MUST be logged via structlog and surfaced as status records — never swallowed
• Required env vars MUST have NO defaults in config — missing values raise ValidationError at startup
• After implementation: dispatch python-worker-reviewer agent for review

Post-Code Review

After writing Python code, dispatch these reviewer agents:

• code-reviewer — general quality, DRY, error handling
• python-worker-reviewer — if this is a background worker service (Pub/Sub/Cloud Scheduler/GCS): silent failures, client injection, DB access patterns, structlog usage, test coverage
• security-reviewer — input validation, auth, dependency audit