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docs: improve Python docs, add section about PEP 668
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Remove comment about Python@2 that is now long gone
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iMichka committed Feb 15, 2024
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This page describes how Python is handled in Homebrew for users. See [Python for Formula Authors](Python-for-Formula-Authors.md) for advice on writing formulae to install packages written in Python.

Homebrew should work with any [CPython](https://stackoverflow.com/questions/2324208/is-there-any-difference-between-cpython-and-python) and defaults to the macOS system Python.
Homebrew will install the necessary Python 3 version that is needed to make your packages work. Python 2 (or 1) is not supported.

Homebrew provides formulae to brew Python 3.y. A `python@2` formula was provided until the end of 2019, at which point it was removed due to the Python 2 deprecation.
## Python 3

**Important:** If you choose to use a Python which isn't either of these two (system Python or brewed Python), the Homebrew team cannot support any breakage that may occur.

## Python 3.y

Homebrew provides formulae for maintained releases of Python 3.y (`[email protected]`).
Homebrew provides formulae for the newest and maintained releases of Python 3 (`[email protected]`) (https://devguide.python.org/versions/).
We keep older `[email protected]` versions according to our [versioned formulae guidelines](https://docs.brew.sh/Versions).

**Important:** Python may be upgraded to a newer version at any time. Consider using a version
manager such as `pyenv` if you require stability of minor or patch versions for virtual environments.
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$(brew --prefix python)/libexec/bin
```

**Warning!** The executables do not always point to the latest Python 3 version, as there is always a delay between the newest Python 3 release and the homebrew-core repository switching to the newest version.

## Setuptools, pip, etc.

The Python formulae install [pip](https://pip.pypa.io/) (as `pip3`) and [Setuptools](https://pypi.org/project/setuptools/).

Setuptools can be updated via `pip`, without having to re-brew Python:
Setuptools can be updated via `pip`, without having to reinstall brewed Python:

```sh
python3 -m pip install --upgrade setuptools
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These should be installed via `pip install <package>`. To discover, you can use <https://pypi.org/search>.

Starting with Python 3.12, we highly recommend you to use a separate virtualenv for this (see the section about [PEP 668](https://peps.python.org/pep-0668/#marking-an-interpreter-as-using-an-external-package-manager) below).

**Note:** macOS's system Python does not provide `pip`. Follow the [pip documentation](https://pip.pypa.io/en/stable/installation/) to install it for your system Python if you would like it.

## Brewed Python modules
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CFLAGS="-I$(brew --prefix)/include" LDFLAGS="-L$(brew --prefix)/lib" pip install <package>
```

## Virtualenv

**Warning!** When you `brew install` formulae that provide Python bindings, you should **not be in an active virtual environment.**

Activate the virtualenv *after* you've brewed, or brew in a fresh terminal window. This will ensure Python modules are installed into Homebrew's `site-packages` and *not* into that of the virtual environment.
Activate the virtualenv *after* you have installed your package with brew, or install brew's packages in a fresh terminal window. This will ensure Python modules are installed into Homebrew's `site-packages` and *not* into that of the virtual environment.

## PEP 668 ([email protected]) and virtualenvs

Starting with [email protected], Homebrew follows [PEP 668](https://peps.python.org/pep-0668/#marking-an-interpreter-as-using-an-external-package-manager).

If you wish to install a non-brew-packaged Python package (from PyPI for example):

* create a virtual environment using `python3 -m venv path/to/venv`. Then use `path/to/venv/bin/python` and `path/to/venv/bin/pip`.
* or use `pipx install xyz`, which will manage a virtual environment for you.
You can install `pipx` by running `brew install pipx`.
When you use `pipx` to install a Python application, it will always use a virtual environment for you.

Virtualenv has a `--system-site-packages` switch to allow "global" (i.e. Homebrew's) `site-packages` to be accessible from within the virtualenv.
It is possible to install some Python packages as formulae, by using `brew install xyz`. We do not recommend using these formulae and recommend you install them with pip using a virtualenv. These syste-wide Hombrew Python formulae are often Homebrew-specific formulae that are useful as dependencies for other Homebrew formulae. It is not recommended to rely on them.

## Why is Homebrew's Python being installed as a dependency?

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