Explain pyproject.toml code

What this pyproject.toml sample shows

[build-system]

[build-system] tells pip what to install before it can build the package, here setuptools, and which backend does the build. pip install . read it, built the package and installed it: pip show invoicer then printed Version: 0.3.1 and the description as its Summary.

dependencies and extras

dependencies installs with the package: pip show invoicer listed Requires: python-dotenv, and python-dotenv was installed with it. [project.optional-dependencies] only installs when asked for: pytest came in with pip install ".[dev]" and not with plain pip install ..

[project.scripts]

Each line under [project.scripts] becomes a command. invoicer = "invoicer.cli:main" means: make a command called invoicer that runs the main function in invoicer/cli.py. After the install, typing invoicer printed invoicer 0.3.1: ready.

[tool.*] tables

Tables under [tool] belong to other tools, which read their own. pytest reads [tool.pytest.ini_options]: run in the project folder, pytest -q found the test in tests and printed 1 passed.

Good to know

• Run with Python 3.14 and pip 26.2 in a fresh virtual environment.
• requires-python is checked at install time. Changed to >=3.99, pip stopped with ERROR: Package 'invoicer' requires a different Python: 3.14.7 not in '>=3.99'.
• Names under [project] are checked. With dependencies misspelled as dependancies, the build failed with configuration error, saying project must not contain {'dependancies'}.

Watch out: A plain pip install . copies your code, so edits don't show until you install again. After changing cli.py, invoicer still printed invoicer 0.3.1: ready. Installed with pip install -e . instead, the same edit showed straight away: invoicer 0.3.1: edited.