brew/docs/Homebrew-and-Python.md

# Python

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 provides formulae to brew Python 3.x.

Homebrew provided a `python@2` formula until the end of 2019, at which point it was removed due to the Python 2 deprecation.

**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.x
Homebrew provides a formula for Python 3.x (`python@3.x`).

**Important:** Homebrew does not guarantee that Python 3, once installed, remains the same version.
In particular, Homebrew may choose to upgrade the minor or patch version of Python 3, which may in
turn break any custom virtual environments that use the Homebrewed Python. Python developers
who expect virtual environment stability should consider using a Python version manager rather than
the Homebrewed Python.

The executables are organised as follows:

* `python3` points to Homebrew's Python 3.x (if installed)
* `pip3` points to Homebrew's Python 3.x's pip (if installed)

Unversioned symlinks for `python`, `python-config`, `pip` etc. are installed here:

```sh
$(brew --prefix)/opt/python/libexec/bin
```

## Setuptools, Pip, etc.
The Python formulae install [pip](https://pip.pypa.io/) (as `pip3`) and [Setuptools](https://pypi.python.org/pypi/setuptools).

Setuptools can be updated via pip3, without having to re-brew Python:

```sh
python3 -m pip install --upgrade setuptools
```

Similarly, pip3 can be used to upgrade itself via:

```sh
python3 -m pip install --upgrade pip
```

## `site-packages` and the `PYTHONPATH`
The `site-packages` is a directory that contains Python modules (especially bindings installed by other formulae). Homebrew creates it here:

```sh
$(brew --prefix)/lib/pythonX.Y/site-packages
```

So, for Python 3.y.z, you'll find it at `/usr/local/lib/python3.y/site-packages`.

Python 3.y also searches for modules in:

* `/Library/Python/3.y/site-packages`
* `~/Library/Python/3.y/lib/python/site-packages`

Homebrew's `site-packages` directory is first created if (1) any Homebrew formula with Python bindings are installed, or (2) upon `brew install python`.

### Why here?
The reasoning for this location is to preserve your modules between (minor) upgrades or re-installations of Python. Additionally, Homebrew has a strict policy never to write stuff outside of the `brew --prefix`, so we don't spam your system.

## Homebrew-provided Python bindings
Some formulae provide Python bindings.

**Warning!** Python may crash (see [Common Issues](Common-Issues.md)) if you `import <module>` from a brewed Python if you ran `brew install <formula_with_python_bindings>` against the system Python. If you decide to switch to the brewed Python, then reinstall all formulae with Python bindings (e.g. `pyside`, `wxwidgets`, `pygtk`, `pygobject`, `opencv`, `vtk` and `boost-python`).

## Policy for non-brewed Python bindings
These should be installed via `pip install <package>`. To discover, you can use `pip search` or <https://pypi.python.org/pypi>.

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

## Brewed Python modules
For brewed Python, modules installed with `pip3` or `python3 setup.py install` will be installed to the `$(brew --prefix)/lib/pythonX.Y/site-packages` directory (explained above). Executable Python scripts will be in `$(brew --prefix)/bin`.

The system Python may not know which compiler flags to set in order to build bindings for software installed in Homebrew so you may need to run:

```sh
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.
Homebrew will still install Python modules into Homebrew's `site-packages` and *not* into the virtual environment's site-package.

Virtualenv has a `--system-site-packages` switch to allow "global" (i.e. Homebrew's) `site-packages` to be accessible from within the virtualenv.

## Why is Homebrew's Python being installed as a dependency?
Formulae that declare an unconditional dependency on the `"python"` formula are bottled against Homebrew's Python 3.x and require it to be installed.