Python Architecture Standards

Targets Python 3.14. See STACK.md for pinned dependency versions.

1. Typing & Domain Safety

• Modern syntax: Built-in generics (list[str], dict[K, V], X | None). Never the legacy typing.List / typing.Optional.
• Deferred annotations (PEP 649, 3.14): Annotations are no longer eagerly evaluated — forward references no longer need quotes (def f(arg: NotYetDefined) works). Inspect via annotationlib.get_annotations(), not __annotations__ directly.
• Domain types: typing.NewType to separate distinct concepts (UserId vs OrderId).
• Constraints: Literal for string flags; Enum (with __str__) for states.
• Structured payloads: TypedDict over dict[str, Any] for known-shape mappings.
• Subclassing safety: typing.override decorator (3.12+) on every overriding method — mypy flags broken overrides.

2. Data Structures & Memory

• Immutability: Default to @dataclass(slots=True, frozen=True) for DTOs and value objects.
• Pydantic vs dataclass boundary: Pydantic only at application boundaries (API request/response, DB row parsing, config). Standard dataclasses for core domain logic — keeps the domain free of validation-framework coupling.
• Memory: __slots__ (explicit or via dataclass(slots=True)) on high-volume instances.

3. Interfaces & DI

• Protocols: typing.Protocol (structural typing) over deep abc.ABC inheritance. Define protocols where consumed.
• DI: Pass dependencies into __init__. Never instantiate external clients inside a class.
• State: No globals. contextvars only when request-scoped state is unavoidable.

4. Concurrency & Resources

• Asyncio discipline: Never block the event loop. Offload sync I/O or CPU work via asyncio.to_thread().
• Task groups: asyncio.TaskGroup for concurrent coroutines — handles cancellation and exception aggregation properly. Avoid bare asyncio.gather.
• Multiple interpreters (PEP 734, 3.14): Use concurrent.interpreters for CPU-bound parallelism — true multi-core without multiprocessing's overhead, no GIL contention.
• Introspection: Debug live async apps with python -m asyncio ps <PID> / pstree <PID> (3.14).
• Free-threaded builds (PEP 703): Be aware of the no-GIL variant. Design hot paths to avoid shared mutable state regardless of GIL presence.
• Resources: Wrap I/O in with / async with. Use contextlib for compositions.

5. Packages & Imports

• Imports: Three groups separated by blank lines — stdlib, third-party, local. Prefer absolute imports.
• __init__.py: Minimal. Use __all__ = [...] to declare the public API explicitly.
• Bundled resources: Use importlib.resources.files(__package__).joinpath("...").read_text() for embedded files (SQL, templates). Survives wheel and zipapp packaging — never use __file__-relative paths for shipped assets.

6. Errors & Testing

• Exceptions: A base custom exception per module. Always chain (raise NewError(...) from err). Never bare except:.
• Bracketless except (PEP 758, 3.14): except TimeoutError, ConnectionRefusedError: is now valid without parens when no as clause.
• Finally hazards (PEP 765, 3.14): return / break / continue inside finally now emits SyntaxWarning — refactor it out.
• Iterables: map(strict=True) (3.14) when consuming parallel iterables, matching zip(strict=True).
• Testing: pytest 9 with conftest.py fixtures. Never the legacy unittest module. pytest-asyncio for async tests.

7. Documentation

• Docstrings: Google style (Args, Returns, Raises).
• DRY: Don't repeat type info already in hints.
• Focus: Explain why (domain rules, edge cases), not what.

8. Stdlib defaults

Prefer stdlib when it covers the use case.

• pathlib.Path for all paths — never os.path strings. New in 3.14: Path.copy(), Path.move(), Path.copy_into(), Path.move_into() for recursive operations.
• compression.zstd (3.14) over gzip / bz2 for new payloads — gzip / bz2 / lzma / zlib are now re-exported under compression.*.
• importlib.resources for shipped files (see §5).
• contextlib for resource lifecycle composition.
• dataclasses for data containers (see §2).

9. Database access — SQL files + importlib.resources

Recommended pattern, not mandatory. Mirrors the Go sqlx + //go:embed philosophy: raw SQL in .sql files, loaded once at module import, executed via psycopg 3. No ORM by default — keeps queries auditable in git and gives editors full SQL syntax highlighting and linting.

from importlib.resources import files
import psycopg
from psycopg.rows import class_row

GET_USER_BY_ID = files(__package__).joinpath("queries/get_user_by_id.sql").read_text()

class UserRepo:
    def __init__(self, conn: psycopg.AsyncConnection) -> None:
        self._conn = conn

    async def get_by_id(self, user_id: int) -> User | None:
        async with self._conn.cursor(row_factory=class_row(User)) as cur:
            await cur.execute(GET_USER_BY_ID, (user_id,))
            return await cur.fetchone()

Layout:

src/myapp/userrepo/
├── __init__.py
├── repo.py
└── queries/
    ├── get_user_by_id.sql
    ├── insert_user.sql
    └── list_users.sql

• Driver: psycopg 3 — sync + async, server-side cursors, COPY, prepared statements.
• Migrations: alembic — versioned, works with raw SQL (no SQLAlchemy ORM required).
• Dynamic queries: Compose .sql fragments in Python; never concatenate user input — bind parameters.
• When an ORM is genuinely needed: SQLAlchemy 2.x (Core or ORM). Record the decision in an ADR.

10. Tooling

• Environment + packaging: uv — replaces pip, pip-tools, virtualenv, pyenv. Single binary, fast.
• Lint + format: ruff — replaces black, isort, flake8, pyupgrade. One config, one tool. Drop-in template: assets/ruff.toml — copy to your project root as ruff.toml (or fold into pyproject.toml under [tool.ruff]) and set known-first-party to your package name.
• Type checking: mypy --strict as the baseline. No Any-by-default escape hatches.
• Test: pytest 9 + pytest-asyncio for async paths.

Canonical libraries

See STACK.md for the full pinned list — pydantic, pydantic-settings, fastapi, uvicorn, httpx, pytest, pytest-asyncio, mypy, ruff, uv, typer, psycopg, alembic.