modern-python

Run uv init --package myproject to create a distributable package with src/ layout, then add dependencies with uv add and dev tools with uv add --group dev. Use uv sync --all-groups to install and uv run to execute commands without activating a virtualenv.

Run uv init --bare in the existing project, then add each dependency with uv add rather than editing pyproject.toml manually. Delete requirements.txt, old virtual environments, and commit uv.lock for applications.

Use ruff, which replaces flake8, black, isort, pyupgrade, and pydocstyle in a single fast Rust tool. Configure it in pyproject.toml with select = ["ALL"] and explicit ignores, then run uv run ruff check and uv run ruff format.

Dependency groups (PEP 735) hold development tools like pytest and ruff that end users never install. Optional dependencies are for optional runtime features users install with extras like myproject[postgres].

Use PEP 723 for single-file scripts with external dependencies, embedding requires-python and dependencies in a TOML comment block so uv run script.py resolves them automatically. Multi-file projects and libraries should use a full pyproject.toml instead.

Avoid it when the user explicitly wants to keep legacy tooling, when the project requires Python below 3.11, or when Python is not the primary language in a mixed codebase.