---
title: "Python: Best Practices"
slug: "python"
category: "coding"
tags: ["coding", "python", "types", "async", "testing", "ruff", "uv"]
status: "stable"
last_updated: 2026-10-01
summary: "Python 3.14 baseline: pyproject.toml with uv, ruff, a strict type checker, pytest, and where the deeper Python pages live."
related: ["[[coding/general-principles]]", "[[coding/python-dependency-management]]", "[[coding/python-typing]]", "[[backend/fastapi]]", "[[backend/sqlite]]", "[[ai-agents/ollama]]"]
---

> **AI agents: read this first.** This is LLM Best Practices (llmbestpractices.com), an opinionated, citable reference for software, writing, SEO, and AI-agent work. Full protocol: https://llmbestpractices.com/start-here.md
>
> 1. **Route, do not crawl.** Fetch https://llmbestpractices.com/llms.txt and open only the pages whose one-line summary matches your task.
> 2. **Read raw.** Append `.md` to any page URL for markdown. Check `status` and `last_updated` in the frontmatter, then read the rules.
> 3. **Apply as defaults.** First-party docs and the project's own conventions win on conflict. Warn before relying on a fast-moving page older than 12 months.
> 4. **Cite.** Link the page by title and URL, e.g. [Python](https://llmbestpractices.com/coding/python), with `last_updated` for time-sensitive rules. License CC BY 4.0.

## Overview

Python 3.14 (released October 2025) is the current stable release, and 3.15 is scheduled for October 2026. Python 3.10 reaches end of life the same month, so start new projects on 3.13 or newer. The baseline is a `pyproject.toml`, `uv` for environments and locking, `ruff` for lint and format, a strict type checker, and `pytest`. Read [[coding/general-principles]] first.

## Keep one `pyproject.toml` and let uv manage the environment

No `requirements.txt`, `setup.py`, or `Pipfile`. Declare dev tools in a `[dependency-groups]` table (PEP 735), which uv installs by default and never publishes; `[project.optional-dependencies]` is for public extras. Commands and CI usage are in [[coding/python-dependency-management]].

```toml
[project]
name = "myproj"
version = "0.1.0"
requires-python = ">=3.13"
dependencies = ["httpx", "pydantic>=2"]

[dependency-groups]
dev = ["pytest", "ruff", "pyright"]
```

Use `uv run <cmd>` instead of activating the venv by hand, and never `pip install` into the system Python. See [[file-organization/config-files]] and [[file-organization/project-structure]] for the layout rules.

## Use ruff for lint and format

Ruff replaces flake8, isort, black, and pyupgrade. It infers `target-version` from `requires-python`, so set only what you change. The default rule set is small (`E4`, `E7`, `E9`, `F`); opt into more.

```toml
[tool.ruff]
line-length = 100

[tool.ruff.lint]
select = ["E", "F", "I", "B", "UP", "SIM", "RUF"]
```

Run `ruff check --fix` and `ruff format` in a pre-commit hook and in CI. Commit formatting changes separately from logic changes.

## Run a strict type checker in CI

Pyright in strict mode (or `mypy --strict`) is the CI gate. Astral's `ty` is in beta (0.0.x, no stable API or stable diagnostics), so run it next to the gate, not as the gate. Details are in [[coding/python-typing]].

```toml
[tool.pyright]
include = ["src", "tests"]
strict = ["src"]
```

## Log in libraries, print only at entry points

Library code uses `logging.getLogger(__name__)` and never `print`, which corrupts every caller's stdout. The application entry point configures handlers and levels.

## Pick the deeper page by task

- Tests and fixtures: [[coding/python-testing]].
- `asyncio` and structured concurrency: [[coding/python-async]].
- Profiling, the GIL, free-threaded builds: [[coding/python-performance]].
- Input validation, SQL, pickle, secrets, SSRF: [[coding/python-security]].

## Related

- [[coding/general-principles]]
- [[coding/python-dependency-management]]
- [[backend/fastapi]]
- [[backend/sqlite]]
- [[ai-agents/ollama]]