Jon Chery
29ffb42898
---ci--- project: atelier phase: 0 milestone: v0.4 status: complete requirements: covered: [ATELIER-92, ATELIER-93, ATELIER-94, ATELIER-95, ATELIER-96, ATELIER-97, ATELIER-98, ATELIER-99, ATELIER-100, ATELIER-101, ATELIER-102, ATELIER-103, ATELIER-104, ATELIER-105, ATELIER-106, ATELIER-107, ATELIER-108, ATELIER-109, ATELIER-110, ATELIER-111, ATELIER-112, ATELIER-113, ATELIER-114, ATELIER-115, ATELIER-116, ATELIER-117] partial: [] ---/ci---
84 lines
3.2 KiB
Markdown
84 lines
3.2 KiB
Markdown
# Python — Language Application
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> How Atelier's domain principles apply in Python specifically. Derives from `domains/` docs.
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## Derived Docs
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- [py-types.md](py-types.md) — type hints + Pydantic, mypy/pyright, gradual typing.
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- [py-tooling.md](py-tooling.md) — ruff, mypy, poetry, uv, virtualenv discipline.
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- [py-async.md](py-async.md) — asyncio, anyio, cancellation, structured concurrency.
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- [py-testing.md](py-testing.md) — pytest, factory_boy, fixture discipline, parametrize.
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## Type System (C1 Correctness, Data P7 Type Fidelity)
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- **Type hints on every function:** `def get_user(id: UUID) -> User | None:`.
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- **`mypy --strict` or `pyright` in CI:** type check is not optional.
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- **No `Any` without justification:** `Any` disables the type checker. Use `object` + narrowing.
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- **Pydantic for runtime validation:** schemas validate and type at the boundary.
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```python
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from pydantic import BaseModel
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from uuid import UUID
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class UserCreate(BaseModel):
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email: str
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name: str
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# additionalProperties: false by default (extra='forbid')
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```
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## Error Handling (Errors P1 Errors are Data)
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- **Exceptions for exceptional cases,** not control flow. `raise` not `return None` for errors.
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- **Custom exception hierarchy:**
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```python
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class AppError(Exception): pass
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class ValidationError(AppError): pass
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class NotFoundError(AppError): pass
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```
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- **Never bare `except:`:** `except Exception as e:` (catch specific, not everything).
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- **Never `except: pass`:** log and re-raise or handle, never swallow (Errors P2).
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## Async (Concurrency P7, P8)
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- **`asyncio` for I/O-bound:** `async def`, `await`. Not threads for I/O.
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- **`anyio` for portability** if you may switch runtimes (trio compatibility).
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- **Timeout on every `await`:** `asyncio.wait_for(coro, timeout=5)`, not bare `await`.
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- **Cancellation propagated:** `asyncio.CancelledError` is not caught; it propagates.
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## Immutability (Concurrency P1)
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- **`frozen=True` dataclasses** for value objects:
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```python
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from dataclasses import dataclass
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@dataclass(frozen=True)
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class UserId:
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value: str
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```
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- **Tuples over lists** for fixed-length, immutable sequences.
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- **No in-place mutation of shared state:** return new objects.
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## Nullability (C1)
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- **`Optional[T]` is `T | None`:** explicit, must be checked.
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- **`None` is not "not found":** raise `NotFoundError` or return `Result`, not `None`.
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- **`assert` is for invariants,** not for runtime checks (stripped with `-O`).
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## Testing (Testing)
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- **pytest** with fixtures (factories, not shared state).
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- **`pytest --randomly`** to catch order-dependent tests (P2 Independence).
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- **`freezegun` for time:** no `datetime.now()` in tests; inject the clock.
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- **`factory_boy` or `pytest-factoryboy`** for realistic factories.
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## Observability (Observability P1)
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- **`structlog` or `python-json-logger`:** JSON logs, not `print`.
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- **`logging` with structured formatter:** every log has `request_id`, `user_id`, `event`.
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- **No secrets in logs:** `mask_secret()` helper, or `structlog` processors.
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## Tooling (DevOps P2)
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- **`ruff` for lint + format:** replaces flake8 + black + isort.
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- **`mypy --strict` in CI:** type check.
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- **`pip-tools` or `poetry` for lockfile:** pinned dependencies.
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- **`pip install --no-deps -r requirements.txt`:** reproducible install. |