brew/docs/Homebrew-and-Python.md

Python

This page describes how Python is handled in Homebrew for users. See Python for Formula Authors for advice on writing formulae to install packages written in Python.

Homebrew should work with any CPython and defaults to the macOS system Python.

Homebrew provides formulae to brew 3.x and a more up-to-date Python 2.7.x.

Important: If you choose to install 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 or Python 2.x

Homebrew provides one formula for Python 2.7.x and another for Python 3.x.

To install Python 3.X with Homebrew:

brew install python or brew install python3

To (optionally) install Python 2.X with Homebrew:

brew install python2 or brew install python@2

For Python 2.X only: if you want to be able to call python2 from the path, add the following line to your ~/.bash_profile (or equivalent). For more information, see the formula's caveats.

export PATH="/usr/local/opt/python@2/libexec/bin:$PATH"

The executables are organized as follows so that Python 2 and Python 3 can both be installed without conflict:

• python and python3 point to Homebrew's Python 3.x (if installed) otherwise the macOS system Python
• python2 points to Homebrew's Python 2.7.x (if installed)
• pip and pip3 point to Homebrew's Python 3.x's pip (if installed)
• pip2 points to Homebrew's Python 2.7.x's pip (if installed)

(Wondering which one to choose?)

Setuptools, Pip, etc.

The Python formulae install pip (as pip or pip2) and Setuptools.

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

python -m pip install --upgrade setuptools

Similarly, pip can be used to upgrade itself via:

python -m pip install --upgrade pip

Note on pip install --user

The normal pip install --user is disabled for brewed Python. This is because of a bug in distutils, because Homebrew writes a distutils.cfg which sets the package prefix.

A possible workaround (which puts executable scripts in ~/Library/Python/<X>.<Y>/bin) is:

python -m pip install --user --install-option="--prefix=" <package-name>

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:

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

So, for Python 3.6.x, you'll find it at /usr/local/lib/python3.6/site-packages.

Python 3.6 also searches for modules in:

• /Library/Python/3.6/site-packages
• ~/Library/Python/3.6/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. Sometimes a --with-python or --with-python@2 option has to be passed to brew install in order to build the Python bindings. (Check with brew options <formula>.)

Warning! Python may crash (see Common Issues) 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: System Python does not provide pip. Follow the pip documentation to install it for your system Python if you would like it.)

Brewed Python modules

For brewed Python, modules installed with pip or python 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:

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" or "python@2" formulae are bottled against Homebrew's Python 3.x or 2.7.x and require it to be installed.