python-best-practices

Python Best Practices

Working in an Existing Codebase

When you spot something that violates these guidelines, flag it and explain why, but do not change it without explicit permission. Unsolicited refactors break working code, inflate diffs, and waste review time. State the issue, the impact, and stop there.

The Zen of Python

Run python -m this. The most actionable lines:

Explicit is better than implicit. Simple is better than complex. Flat is better than nested. Readability counts. Errors should never pass silently. There should be one — and preferably only one — obvious way to do it.

When in doubt, ask which option reads most clearly six months from now.

Project Setup — use the Astral stack

uv init my-project           # pyproject.toml + uv.lock + .venv
uv add httpx                 # runtime dep
uv add --dev pytest ruff ty  # dev deps
uv sync                      # install from lockfile (CI / fresh clone)
uv run pytest                # run inside managed env

Commit both pyproject.toml and uv.lock. Never run bare pip install.

uvx (uv tool run) runs a tool in a temporary isolated environment — useful for one-off or CI invocations without permanent installation: uvx ruff check, uvx ruff format, uvx ty check.

Linting & formatting — ruff: config lives in a dedicated ruff.toml at the project root. Always respect the rules defined there; do not apply or suggest rules outside of what the project has configured.

Type checking — ty (Astral, Rust-based, 10-100× faster than mypy): uvx ty check. Config lives in a dedicated ty.toml at the project root. Always respect the project's local ty config. Fall back to mypy --strict if you need plugins (Django ORM, SQLAlchemy).

Project layout: See project-layout.md.

Design Decisions

Follow existing codebase conventions first. Introduce a different pattern only when it clearly reduces complexity — and flag it rather than applying it silently.

Dataclasses vs Pydantic — ask: has this data been validated yet?

• No (HTTP body, config file, env vars, anything external) → pydantic.BaseModel
• Yes (internal domain objects, data that came from your own code) → @dataclass

Use frozen=True on value objects (coordinates, IDs, measurements).

Functions vs classes — default to functions. Add a class only when you need shared state across calls, a concrete interface implementation, or lifecycle management (__enter__/__exit__).

Composition over inheritance — build behaviour by combining small focused pieces rather than extending base classes. Inherit only for genuine "is-a" relationships with shared implementation; keep hierarchies shallow.

Return early. Handle error/edge cases at the top with guard clauses. Keep the happy path at the bottom, unnested.

Testing — pytest only

See testing.md for full guidelines. Short version: pytest only, no unittest.TestCase mixing, Arrange → Act → Assert, mock all external I/O.

Prefer Existing Libraries

Python's ecosystem is large. Before writing custom code, check if a well-maintained library already solves the problem. Prefer libraries that are:

• Part of the stdlib first
• Widely adopted (high download count, active maintenance, good issue tracker hygiene)
• Battle-tested over time — be cautious of libraries under a year old or with very low usage

Common reliable choices: httpx (HTTP), pydantic (validation), click/typer (CLI), structlog (logging), tenacity (retries), rich (terminal output), polars/pandas (data). Don't reimplement what these do well.

Idioms

• Prefer comprehensions over map/filter; use generator expressions when you only iterate once
• Always use with for resources (files, connections, locks)
• Unpack tuples by name (x, y = point) instead of indexing (point[0])
• Annotate every function signature — it documents intent and enables static analysis

Async

• Use async/await only for I/O-bound work. CPU-bound work belongs in a ProcessPoolExecutor
• Run independent I/O concurrently: asyncio.TaskGroup (Python 3.11+, preferred) or asyncio.gather
• Never call blocking code inside an async function — use async equivalents or loop.run_in_executor