Python Architect

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).
• Enums: Default to Enum (with __str__ overridden) for closed sets of domain states — members are distinct identities, not interchangeable with raw primitives, which catches accidental comparisons against arbitrary strings/ints. Reach for StrEnum (3.11+) when members must interoperate directly with strings — JSON payloads, query params, f-strings — without a .value call at every site. Reach for IntEnum when members must support arithmetic or ordering against plain integers (HTTP status codes, priority levels, wire values from an external system). Both trade Enum's identity-safety for primitive compatibility — reach for them only when that interop is a real requirement, not by default.
• Constraints: Literal for a narrow, function-local set of string flags that doesn't warrant a full Enum.
• Structured payloads: TypedDict over dict[str, Any] for known-shape mappings (see §3 for the broader dict/tuple-avoidance principle).
• Subclassing safety: typing.override decorator (3.12+) on every overriding method — mypy flags broken overrides.

2. Generators & Iterators

• Return Iterator[T] / write a generator when: the sequence is large or unbounded, the consumer might short-circuit (break, early return), or it's backed by a cursor (DB pagination, paginated HTTP APIs, file streaming). Laziness avoids materializing the whole sequence in memory.
• Return list[T] when: the result is small, bounded, and the caller almost always consumes the whole thing — don't wrap it in a generator just to look idiomatic.
• Async generators: AsyncIterator[T] / async def ... yield for streaming I/O (paginated API clients, chunked reads) — pairs with async for. See §5 for asyncio discipline.
• itertools: default toolkit for lazy composition (chain, islice, groupby, pairwise) over manual index bookkeeping.
• Anti-pattern: collecting a generator into a list immediately after producing it (list(gen())) just to satisfy a type checker — return the concrete type the caller actually needs instead of round-tripping through both.

3. Data Structures & Memory

• Immutability: Default to @dataclass(slots=True, frozen=True) for DTOs and value objects.
• Mutable defaults: Never use a mutable literal ([], {}, set()) as a function parameter default or a bare dataclass field default — it's shared across every call/instance. Use None and assign inside the function body, or field(default_factory=list) on dataclasses. Enforced by ruff's B006/B008 (bugbear, see §11).
• 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.
• Typed models over dicts/tuples: Prefer a dataclass / NamedTuple / TypedDict (see §1) to dict[str, Any] or a raw tuple for anything with a stable shape — attribute access catches typos and missing fields that dict keys and tuple indices can't. Reserve bare dicts/tuples for genuinely dynamic or anonymous data (arbitrary JSON blobs, zip() output consumed immediately, coordinate pairs).
• Ordering: @dataclass(order=True) for value objects that need comparison operators — avoid hand-rolled __lt__/__gt__/__le__/__ge__ chains. For one-off custom sort keys, pass a plain function to sorted(key=...) rather than implementing a full ordering protocol.
• Memory: __slots__ (explicit or via dataclass(slots=True)) on high-volume instances.

4. 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.

5. 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.

6. 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.

7. Errors & Testing

• Exceptions: A base custom exception per module. Always chain (raise NewError(...) from err). Never bare except:.
• Exception groups (PEP 654, 3.11+): asyncio.TaskGroup (§5) raises ExceptionGroup when child tasks fail — catch with except* (e.g. except* TimeoutError:), never a bare except Exception, or concurrent failures from separate tasks collapse into one swallowed exception.
• 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.
• Integration tests against Docker dependencies: testcontainers-python — spins up real Postgres/Redis/Kafka/etc. containers per test run instead of mocking the driver or relying on a shared dev instance. Mark these with a dedicated pytest marker (e.g. @pytest.mark.integration) and exclude by default so pytest stays fast.

8. 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.

9. 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 §6).
• contextlib for resource lifecycle composition.
• dataclasses for data containers (see §3).

10. 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.

11. Tooling

• Environment + packaging: uv — replaces pip, pip-tools, virtualenv, pyenv. Single binary, fast. Commit uv.lock; run uv sync --frozen in CI.
• 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. Run ruff check and ruff format --check on every commit and in CI; treat warnings as errors.
  • Correctness & bugs: F (pyflakes), B (bugbear, incl. B006/B008 mutable defaults — see §3, and B904 exception chaining — see §7), ASYNC (asyncio anti-patterns — see §5), RUF (ruff-specific, e.g. RUF012 mutable class defaults).
  • Security: S (flake8-bandit) — SQL/command injection, hardcoded secrets, weak crypto. Test files relax S101/S105-S107 via per-file-ignores since asserts and fixture creds are expected there.
  • Typing discipline: ANN (typed signatures — the mypy --strict baseline), TC (TYPE_CHECKING guards, with runtime-evaluated-base-classes carved out for Pydantic/Settings — see §3), PYI (stub-file quality).
  • Modernization: UP (pyupgrade — see §1), FA (future annotations), FURB (refurb), PERF (perflint).
  • Style & structure: I (isort), N (pep8-naming), C4 (comprehensions), SIM (simplify), RET/RSE (control-flow and raise style), PIE, PTH (pathlib over os.path — see §9), ISC, TID, A (no builtin shadowing).
  • Complexity & size: PL (pylint subset), thresholds tuned in [lint.pylint] (max-args = 7, max-branches = 12, max-returns = 6, max-statements = 50) — split functions instead of suppressing.
  • Test style: PT (flake8-pytest-style) — fixture/mark parenthesis conventions tuned in [lint.flake8-pytest-style] (see §7).
  • Docs & dead code: D (pydocstyle, Google convention — see §8), ERA (eradicate — no commented-out code).
  • Test-file relaxations: per-file-ignores drops ANN, D, S101, PLR2004, SLF001, INP001 under tests/** — type hints and docstrings on test functions add noise without value; asserts, magic numbers, and private-member access are the point of a test.
  • Auto-fix guardrails: fixable = ["ALL"], but unfixable excludes ERA, F401, F841 — never let --fix silently delete commented-out code or unused imports/locals; those need a human decision.
• Type checking: mypy --strict as the baseline — no Any-by-default escape hatches. Drop-in template: assets/mypy.ini — copy to your project root (or fold [mypy] into pyproject.toml's [tool.mypy]) and set packages to your package name.
  • Beyond --strict: warn_unreachable, warn_redundant_casts, warn_unused_ignores, strict_equality, extra_checks — catch dead branches, stale # type: ignore comments, and cross-type equality bugs that --strict alone misses.
  • Per-module overrides: relax disallow_untyped_defs under tests.* (fixtures and @pytest.mark.parametrize routinely defeat full inference); ignore_errors under migrations.* (Alembic-generated, not hand-typed); scope ignore_missing_imports to named untyped dependencies instead of a blanket override, which would silently swallow first-party import typos too.
  • CI parity: run ruff check, ruff format --check, and mypy as three separate, mandatory CI gates — a formatting fix should never ride along with a type fix in the same commit.
• Test: pytest 9 + pytest-asyncio for async paths. pytest-cov for coverage gating in CI.

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.