Setuptools platform specific dependencies

Question

Is there any way to tell setuptools or distribute to require a package on a specific platform?

In my specific case, I'm using readline, which comes as part of the standard library on Unix systems, but on Windows I need the pyreadline module to replace that functionality (cf. this question). If I just put it in the requirements It also installs on Unix systems where it's completely useless.

Answer 1

While the answer given by Martijn Pieters was totally valid at the time, Python packaging has changed a lot since then.

The preferred format to distribute packages is using wheels*. Using wheels it is not possible to run Python code during installation.

Wheel use metadata version two as specified in PEP 0427. Environment markers can be used to specify platform specific dependencies.

Setuptools allows to specify these environment markers as extras_require keys. The following example script depends on pyreadline for Windows systems and on pyxdg for Linux distributions.

#!/usr/bin/env python
from setuptools import setup

setup(
    name='spam',
    version='0.0.1',
    extras_require={
        ':sys_platform == "win32"': [
            'pyreadline'
        ],
        ':"linux" in sys_platform': [
            'pyxdg'
        ]
    })

*Also release an sdist, so platforms which can't use wheel can still install your package.

Answer 2

When I first wrote my answer here, in 2013, we didn't yet have PEP 496 – Environment Markers and PEP 508 – Dependency specification for Python Software Packages. Now that we do, the answer is: put environment markers in your setup_requires:

setup_requires = [
    'foo',
    'bar',
    'pyreadline; sys_platform == "win32"',
]

setup(
    # ...
    setup_requires=setup_requires,
)

This is supported as of setuptools 20.6.8, released in May 2016 (support was introduced in version 20.5 but was briefly disabled in intervening releases).

Note that setuptools will use easy_install to install those requirements when it is being executed, which is hard to configure for when using pip to install the project.

It may better to not use setuptools to handle build-time dependencies, and use a pyproject.toml file following the recommendations from PEP 518 – Specifying Minimum Build System Requirements for Python Projects. Using the PEP 518 build-system with built-time dependencies, means creating a pyproject.toml file that looks something like this:

[build-system]
requires = [
    "setuptools",
    "wheel",
    "foo",
    "bar",
    "pyreadline; sys_platform == "win32",
]

That's the same list as setup_requires but with setuptools and wheel added. This syntax is supported by pip as of version 10.0.0, released in March 2018.

My old answer, from 2013, follows.

---

setup.py is simply a python script. You can create dynamic dependencies in that script:

import sys

setup_requires = ['foo', 'bar']

if sys.platform() == 'win32':
    setup_requires.append('pyreadline')

setup(
    # ...
    setup_requires=setup_requires,
)