Python for Formula Authors
This document explains how to successfully use Python in a Homebrew formula.
Homebrew draws a distinction between Python applications and Python libraries. The difference is that users generally do not care that applications are written in Python; it is unusual that a user would expect to be able to
import foo after installing an application. Examples of applications are
Python libraries exist to be imported by other Python modules; they are often dependencies of Python applications. They are usually no more than incidentally useful in a terminal. Examples of libraries are
py2cairo and the bindings that are installed by
Bindings are a special case of libraries that allow Python code to interact with a library or application implemented in another language.
Homebrew is happy to accept applications that are built in Python, whether the apps are available from PyPI or not. Homebrew generally won't accept libraries that can be installed correctly with
pip install foo. Bindings may be installed for packages that provide them, especially if equivalent functionality isn't available through pip.
Applications should unconditionally bundle all of their Python-language dependencies and libraries and should install any unsatisfied dependencies; these strategies are discussed in depth in the following sections.
Formulae for apps that require Python 3 should declare an unconditional dependency on
"python". These apps must work with the current Homebrew Python 3.x formula.
Applications that are compatible with Python 2 should use the Apple-provided system Python in
/usr/bin on systems that provide Python 2.7. To do this, declare:
depends_on "python@2" if MacOS.version <= :snow_leopard
No explicit Python dependency is needed on recent OS versions since
/usr/bin is always in
PATH for Homebrew formulae; on Leopard and older, the
PATH is used if it's at least version 2.7, or else Homebrew's Python 2.7.x is installed.
Applications should be installed into a Python virtualenv environment rooted in
libexec. This prevents the app's Python modules from contaminating the system site-packages and vice versa.
All of the Python module dependencies of the application (and their dependencies, recursively) should be declared as
resources in the formula and installed into the virtualenv, as well. Each dependency should be explicitly specified; please do not rely on
pip to perform automatic dependency resolution, for the reasons described here.
You can use homebrew-pypi-poet to help you write resource stanzas. To use it, set up a virtualenv and install your package and all its dependencies. Then,
pip install homebrew-pypi-poet into the same virtualenv. Running
poet some_package will generate the necessary resource stanzas. You can do this like:
# Install virtualenvwrapper brew install python python -m pip install virtualenvwrapper source $(brew --prefix)/bin/virtualenvwrapper.sh # Set up a temporary virtual environment mktmpenv # Install the package of interest as well as homebrew-pypi-poet pip install some_package homebrew-pypi-poet poet some_package # Destroy the temporary virtualenv you just created deactivate
Homebrew provides helper methods for instantiating and populating virtualenvs. You can use them by putting
include Language::Python::Virtualenv at the top of the
Formula class definition.
For most applications, all you will need to write is:
def install virtualenv_install_with_resources end
This is exactly the same as writing:
def install # Create a virtualenv in `libexec`. If your app needs Python 3, make sure that # `depends_on "python"` is declared, and use `virtualenv_create(libexec, "python3")`. venv = virtualenv_create(libexec) # Install all of the resources declared on the formula into the virtualenv. venv.pip_install resources # `pip_install_and_link` takes a look at the virtualenv's bin directory # before and after installing its argument. New scripts will be symlinked # into `bin`. `pip_install_and_link buildpath` will install the package # that the formula points to, because buildpath is the location where the # formula's tarball was unpacked. venv.pip_install_and_link buildpath end
Installing a formula with dependencies will look like this:
class Foo < Formula include Language::Python::Virtualenv url "..." resource "six" do url "https://pypi.python.org/packages/source/s/six/six-1.9.0.tar.gz" sha256 "e24052411fc4fbd1f672635537c3fc2330d9481b18c0317695b46259512c91d5" end resource "parsedatetime" do url "https://pypi.python.org/packages/source/p/parsedatetime/parsedatetime-1.4.tar.gz" sha256 "09bfcd8f3c239c75e77b3ff05d782ab2c1aed0892f250ce2adf948d4308fe9dc" end def install virtualenv_install_with_resources end end
You can also use the more verbose form and request that specific resources be installed:
def install venv = virtualenv_create(libexec) %w[six parsedatetime].each do |r| venv.pip_install resource(r) end venv.pip_install_and_link buildpath end
in case you need to do different things for different resources.
To add bindings for Python 3, please add
Build Python 2 bindings with the system Python by default (don't add an option) and they should be usable with any binary-compatible Python. If that isn't the case, it's an upstream bug; here's some advice for resolving it.
Bindings should follow the same advice for Python module dependencies as libraries; see below for more.
If the bindings are installed by invoking a
setup.py, do something like:
cd "source/python" do system "python", *Language::Python.setup_install_args(prefix) end
If the configure script takes a
--with-python flag, it usually will not need extra help finding Python.
make scripts do not want to install into the Cellar, sometimes you can:
./configure --without-python(or a similar named option)
cdinto the directory containing the Python bindings
Language::Python.setup_install_args(as described above)
Sometimes we have to edit a
Makefile on-the-fly to use our prefix for the Python bindings using Homebrew's
inreplace helper method.
Libraries built for Python 3 should include
depends_on "python", which will bottle against Homebrew's Python 3.x. Python 2.x libraries must function when they are installed against either the system Python or brewed Python.
Python 2 libraries do not need a
depends_on "python@2" declaration; they will be built with the system Python, but should still be usable with any other Python 2.7. If this is not the case, it's an upstream bug; here's some advice for resolving it.
Libraries may be installed to
libexec and added to
sys.path by writing a
.pth file (named like "homebrew-foo.pth") to the
prefix site-packages. This simplifies the ensuing drama if
pip is accidentally used to upgrade a Homebrew-installed package and prevents the accumulation of stale .pyc files in Homebrew's site-packages.
Most formulae presently just install to
The dependencies of libraries must be installed so that they are importable. To minimise the potential for linking conflicts, dependencies should be installed to
libexec/<vendor> and added to
sys.path by writing a second
.pth file (named like "homebrew-foo-dependencies.pth") to the
Further down the rabbit hole
Additional commentary that explains why Homebrew does some of the things it does.
setuptools vs. distutils vs. pip
Distutils is a module in the Python standard library that provides developers a basic package management API. Setuptools is a module distributed outside the standard library that extends distutils. It is a convention that Python packages provide a
setup.py that calls the
setup() function from either distutils or setuptools.
Setuptools provides the
easy_install command, which is an end-user package management tool that fetches and installs packages from PyPI, the Python Package Index.
pip is another, newer end-user package management tool, which is also provided outside the standard library. While pip supplants
easy_install, pip does not replace the other functionality of the setuptools module.
Distutils and pip use a "flat" installation hierarchy that installs modules as individual files under site-packages while
easy_install installs zipped eggs to site-packages instead.
Distribute (not to be confused with distutils) is an obsolete fork of setuptools. Distlib is a package maintained outside the standard library which is used by pip for some low-level packaging operations and is not relevant to most
In the event that a formula needs to interact with
setup.py instead of calling
pip, Homebrew provides a helper method,
Language::Python.setup_install_args, which returns useful arguments for invoking
setup.py. Your formula should use this instead of invoking
setup.py explicitly. The syntax is:
system "python", *Language::Python.setup_install_args(prefix)
prefix is the destination prefix (usually
--single-version-externally-managed ("SVEM") is a setuptools-only argument to
setup.py install. The primary effect of SVEM is to use distutils to perform the install instead of using setuptools'
easy_install does a few things that we need to avoid:
- fetches and installs dependencies
- upgrades dependencies in
site.pyfiles which aren't useful for us and cause link conflicts
Setuptools requires that SVEM is used in conjunction with
--record, which provides a list of files that can later be used to uninstall the package. We don't need or want this because Homebrew can manage uninstallation but since setuptools demands it we comply. The Homebrew convention is to call the record file "installed.txt".
Detecting whether a
setup() from setuptools or distutils is difficult, but we always need to pass this flag to setuptools-based scripts.
pip faces the same problem that we do and forces
setup() to use the setuptools version by loading a shim around
setup.py that imports setuptools before doing anything else. Since setuptools monkey-patches distutils and replaces its
setup function, this provides a single, consistent interface. We have borrowed this code and use it in
setup.py accepts a slightly bewildering array of installation options. The correct switch for Homebrew is
--prefix, which automatically sets the
--install-foo family of options using sane POSIX-y values.
--root is used when installing into a prefix that will not become part of the final installation location of the files, like when building a .rpm or binary distribution. When using a
--root has the side effect of activating
--single-version-externally-managed. It is not safe to use
--root with an empty
--prefix because the
root is removed from paths when byte-compiling modules.
It is probably safe to use
--root=/, which should work with either setuptools or distutils-based
setup.py's but is kinda ugly.
PEP 453 makes a recommendation to downstream distributors (us) that sdist tarballs should be installed with
pip instead of by invoking
setup.py directly. We do not do this because Apple's Python distribution does not include pip, so we can't assume that pip is available. We could do something clever to work around Apple's piplessness but the value proposition is not yet clear.