# Python — Language Application

> How Atelier's domain principles apply in Python specifically. Derives from `domains/` docs.

## Derived Docs

- [py-types.md](py-types.md) — type hints + Pydantic, mypy/pyright, gradual typing.
- [py-tooling.md](py-tooling.md) — ruff, mypy, poetry, uv, virtualenv discipline.
- [py-async.md](py-async.md) — asyncio, anyio, cancellation, structured concurrency.
- [py-testing.md](py-testing.md) — pytest, factory_boy, fixture discipline, parametrize.

## Type System (C1 Correctness, Data P7 Type Fidelity)

- **Type hints on every function:** `def get_user(id: UUID) -> User | None:`.
- **`mypy --strict` or `pyright` in CI:** type check is not optional.
- **No `Any` without justification:** `Any` disables the type checker. Use `object` + narrowing.
- **Pydantic for runtime validation:** schemas validate and type at the boundary.

```python
from pydantic import BaseModel
from uuid import UUID

class UserCreate(BaseModel):
    email: str
    name: str
    # additionalProperties: false by default (extra='forbid')
```

## Error Handling (Errors P1 Errors are Data)

- **Exceptions for exceptional cases,** not control flow. `raise` not `return None` for errors.
- **Custom exception hierarchy:**
```python
class AppError(Exception): pass
class ValidationError(AppError): pass
class NotFoundError(AppError): pass
```
- **Never bare `except:`:** `except Exception as e:` (catch specific, not everything).
- **Never `except: pass`:** log and re-raise or handle, never swallow (Errors P2).

## Async (Concurrency P7, P8)

- **`asyncio` for I/O-bound:** `async def`, `await`. Not threads for I/O.
- **`anyio` for portability** if you may switch runtimes (trio compatibility).
- **Timeout on every `await`:** `asyncio.wait_for(coro, timeout=5)`, not bare `await`.
- **Cancellation propagated:** `asyncio.CancelledError` is not caught; it propagates.

## Immutability (Concurrency P1)

- **`frozen=True` dataclasses** for value objects:
```python
from dataclasses import dataclass
@dataclass(frozen=True)
class UserId:
    value: str
```
- **Tuples over lists** for fixed-length, immutable sequences.
- **No in-place mutation of shared state:** return new objects.

## Nullability (C1)

- **`Optional[T]` is `T | None`:** explicit, must be checked.
- **`None` is not "not found":** raise `NotFoundError` or return `Result`, not `None`.
- **`assert` is for invariants,** not for runtime checks (stripped with `-O`).

## Testing (Testing)

- **pytest** with fixtures (factories, not shared state).
- **`pytest --randomly`** to catch order-dependent tests (P2 Independence).
- **`freezegun` for time:** no `datetime.now()` in tests; inject the clock.
- **`factory_boy` or `pytest-factoryboy`** for realistic factories.

## Observability (Observability P1)

- **`structlog` or `python-json-logger`:** JSON logs, not `print`.
- **`logging` with structured formatter:** every log has `request_id`, `user_id`, `event`.
- **No secrets in logs:** `mask_secret()` helper, or `structlog` processors.

## Tooling (DevOps P2)

- **`ruff` for lint + format:** replaces flake8 + black + isort.
- **`mypy --strict` in CI:** type check.
- **`pip-tools` or `poetry` for lockfile:** pinned dependencies.
- **`pip install --no-deps -r requirements.txt`:** reproducible install.