python-dev

name	python-dev
description	Python development. Use for Python, FastAPI, Pydantic, asyncpg, pytest, pandas, SQLAlchemy.
user-invocable	true
allowed-tools	Glob, Read, Grep, Bash, Task, Skill, Edit, Write, AskUserQuestion

Python Development

Announce: "I'm using the python-dev skill. Following the 4-phase workflow."

MANDATORY: Read and Follow the Shared Workflow

You MUST read workflow.md NOW and follow ALL 4 phases defined there. The sections below provide Python-specific inputs for each phase.

Read patterns.md during Phase 1.

If you lost workflow.md from context: Re-read ../../shared/workflow.md NOW before continuing.

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Python-Specific Configuration

Verification Commands (Phase 1 plan + Phase 3 verify)

WORKFLOW STATE Verification line: pytest tests/ -x -q, ruff check, ruff format --check

Phase 3 Tier A commands:

• pytest tests/ -x -q — default suite (unit + golden); excludes e2e and integration markers
• pytest tests/ -x -q -m e2e — e2e replay tests (no DB)
• pytest tests/ -x -q -m integration — integration tests (requires real DB)
• pytest tests/ -x -q -m '' — everything
• ruff check . — linting
• ruff format --check . — formatting
• pre-commit run --files <touched> — full hook chain (ruff + ruff-format + check-toml/yaml/merge-conflict + commitizen)

Phase 3 Tier B additional MCP verifications:

• PostgreSQL MCP → Query DB state before/after
• Legacy DB MCP → Query legacy databases for data verification

Implementation Rules (Phase 3)

• Model structure — Domain-driven app/model/<domain>/{models.py, requests.py, responses.py}; cross-cutting types in model/common/. Separate *Request from *Response; never merge. When a response carries both DB-mapped fields AND computed/Python-only fields, split into a *DbRow base (SELECT columns only) + a *Model subclass that adds enrichment and @computed_field properties. Query builders and aggregation utilities receive the DB-row base. Composition over deep inheritance.
• Settings — BaseSettings hierarchy, SecretStr for sensitive values, @lru_cache(maxsize=1) on the get_settings accessor. Tests that mutate env call get_settings.cache_clear().
• Error handling — One global Exception handler in create_app + a per-domain-error handler in app/exception_handlers.py. Domain errors raised straight from service/repository; never wrapped in HTTPException. Handler detail strings hardcoded — never str(exc).
• Async hygiene — except asyncio.CancelledError: raise separately from except Exception. Bounded asyncio.Queue for background work. asyncio.gather for independent queries. Heavy SDK clients constructed once at lifespan; blocking calls wrapped in asyncio.to_thread at adapter seams.
• Minimize complexity — generators for large data, dict lookups over list scans, function length ~50-60 lines (soft guide; extract a helper past that).

Staff Review Configuration (Phase 4)

• Patterns file path: Path to this skill's patterns.md

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Python-Specific Rules

• Type hints are mandatory — use Pydantic and strict typing per patterns.md.
• All imports at module top — never inside a function or method.
• Tests are required during implementation, not after — RED → GREEN → REFACTOR for every behavior.
• No positive claim without running pytest tests/ -x -q.
• Pydantic model fields with non-trivial types, defaults, or validators MUST have WHY comments explaining the rationale (data source format, business rule, cross-system constraint).
• One global FastAPI Exception handler in create_app; never per-endpoint try / except → HTTPException.
• TestClient(app, raise_server_exceptions=False) for any test that exercises the global handler.
• Domain metadata (schema names, plant codes, environment-specific strings) lives on an Enum property — never hardcoded in N places.

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Quality Checklist (Python-Specific)

Add these to the shared workflow's verification checklist:

• Python 3.14 (built-in list[str] / dict[str, int], PEP 604 str | None; no from __future__ import annotations)
• No from typing import List, Dict, Optional — use built-ins
• Pydantic for all structured data; BaseSettings for configuration; SecretStr for sensitive values
• Request/Response Pydantic models separated; *DbRow base + *Model subclass where DB-mapped and computed fields coexist
• json_schema_extra used ONLY for OpenAPI examples — never for internal flags
• One global Exception handler + per-domain handlers in app/exception_handlers.py; routes/services raise domain errors, never HTTPException
• Sanitized 500 detail — never leak str(exc) to clients
• except asyncio.CancelledError re-raised; never swallowed in except BaseException
• Middleware order declared in one list and pinned by a test
• @lru_cache(maxsize=1) on get_settings; tests call cache_clear() after env mutations
• Heavy SDK clients constructed once at lifespan; blocking calls wrapped in asyncio.to_thread at adapter seams
• Repository returns typed Pydantic models, not raw asyncpg.Record
• Parameterized SQL ($1, $2) — no f-string interpolation of user input
• Independent DB queries run concurrently (asyncio.gather)
• No fetch-then-filter — JOIN or IN (subquery) in one round trip
• asyncio_mode="auto" in pyproject.toml; markers golden, e2e, integration
• TestClient(app, raise_server_exceptions=False) for tests that exercise the global handler
• MagicMock(spec=Class) + AsyncMock for async methods; autouse fixture clears app.dependency_overrides
• DataFrames copied before mutation; no iterrows()
• ruff check . passes; ruff format --check . passes
• Pre-commit (ruff + ruff-format + check-toml/yaml/merge-conflict + commitizen) passes on touched files
• pytest tests/ -x -q passes (or alternative verification documented)