Python-for-Formula-Authors.md

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 ansible and jrnl.

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 from a Terminal.app command line. Examples of libraries are py2cairo and the bindings that are installed by protobuf --with-python.

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. Libraries that can be pip-installed but have several Homebrew dependencies may be appropriate for the homebrew/python tap. Bindings may be installed for packages that provide them, especially if equivalent functionality isn't available through pip.

Running setup.py

Homebrew provides a helper method, Language::Python.setup_install_args, which returns 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)

where prefix is the destination prefix (usually libexec or prefix).

Python module dependencies

In general, applications should unconditionally bundle all of their dependencies and libraries and should install any unsatisfied dependencies; these strategies are discussed in depth in the following sections.

In the rare instance that this proves impractical, you can specify a Python module as an external dependency using this syntax:

depends_on "numpy" => :python

Or if the import name is different from the module name:

depends_on "MacFSEvents" => [:python, "fsevents"]

If you submit a formula with this syntax to core, you may be asked to rewrite it as a Requirement.

Applications

ansible.rb and jrnl.rb are good examples of applications that follow this advice.

Python declarations

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 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 python in PATH is used if it's at least version 2.7, or else Homebrew's python is installed.

Formulae for apps that require Python 3 should declare an unconditional dependency on :python3, which will cause the formula to use the first python3 discovered in PATH at install time (or install Homebrew's if there isn't one). These apps must work with the current Homebrew python3 formula.

Installing

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 setup.py or 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 on the Formula class definition, above def install.

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 :python3` 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
  # `link_scripts` takes a look at the virtualenv's bin directory before and
  # after executing the block which is passed into it. If the block caused any
  # new scripts to be written to the virtualenv's bin directory, link_scripts
  # will symlink those scripts into the path given as its argument (here, the
  # formula's `bin` directory in the Cellar.)
  # `pip_install buildpath` will install the package that the formula points to,
  # because buildpath is the location where the formula's tarball was unpacked.
  venv.link_scripts(bin) { venv.pip_install buildpath }
end

Example

Installing a formula with dependencies will look like this:

class Foo < Formula
  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

  include Language::Python::Virtualenv

  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.

Bindings

To add an option to a formula to build Python bindings, use depends_on :python => :recommended and install the bindings conditionally on build.with? "python" in your install method.

Python bindings should be optional because if the formula is bottled, any :recommended or mandatory dependencies on :python are always resolved by installing the Homebrew python formula, which will upset users that prefer to use the system Python. This is because we cannot generally create a binary package that works against both versions of Python.

Dependencies

Bindings should follow the same advice for Python module dependencies as libraries; see below for more.

Installing bindings

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.

If the configure and make scripts do not want to install into the Cellar, sometimes you can:

1. Call ./configure --without-python (or a similar named option)
2. cd into the directory containing the Python bindings
3. Call setup.py with system and Language::Python.setup_install_args (as described above)

Sometimes we have to inreplace a Makefile to use our prefix for the Python bindings. (inreplace is one of Homebrew's helper methods, which greps and edits text files on-the-fly.)

Libraries

Python declarations

Libraries should declare a dependency on :python or :python3 as appropriate, which will respectively cause the formula to use the first python or python3 discovered in PATH at install time. If a library supports both Python 2.x and Python 3.x, the :python dependency should be :recommended (i.e. built by default) and the :python3 dependency should be :optional. Python 2.x libraries must function when they are installed against either the system Python or Homebrew Python.

Formulae that declare a dependency on :python will always be bottled against Homebrew's python, since we cannot in general build binary packages that can be imported from both Pythons. Users can add --build-from-source after brew install to compile against whichever python is in PATH.

Installing

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 prefix.

Dependencies

The dependencies of libraries must be installed so that they are importable. The principle of minimum surprise suggests that installing a Homebrew library should not alter the other libraries in a user's sys.path. The best way to achieve this is to only install dependencies if they are not already installed. To minimize 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 prefix site-packages.

The matplotlib formula in homebrew/science deploys this strategy.

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 setup.py users.

What is --single-version-externally-managed?

--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.

easy_install does a few things that we need to avoid:

• fetches and installs dependencies
• upgrades dependencies in sys.path in-place
• writes .pth and site.py files 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.py uses 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 Language::Python.setup_install_args.

--prefix vs --root

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 setup.py-based setuptools, --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 --prefix with --root=/, which should work with either setuptools or distutils-based setup.py's but is kinda ugly.

pip vs. setup.py

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.