Prevent package from being installed on old Python versions

The magicstack distribution on pypi is broken. It's failing because the source distribution doesn't contain a magicstack package even though the setup.py for the source distribution says it should.

As long as pypi contains a source distribution (e.g. .tar.gz, .zip), pip will download that if it can't find a matching binary distribution (e.g. .egg, .whl) for your version of python/os/architecture.

One option is to only upload binary distributions to PyPI (preferably wheels). The other option is to check the sys.version in your setup.py for compatible versions and raise an exception otherwise.

There is a correct way to do this, but unfortunately pip only started supporting it in version 9.0.0 (released 2016-11-02), and so users with older versions of pip will continue to download packages willy-nilly regardless of what Python version they're for.

In your setup.py file, pass setup() a python_requires argument that lists your package's supported Python versions as a PEP 440 version specifier. For example, if your package is for Python 3+ only, write:

setup(
    ...
    python_requires='>=3',
    ...
)

If your package is for Python 3.3 and up but you're not willing to commit to Python 4 support yet, write:

setup(
    ...
    python_requires='~=3.3',
    ...
)

If your package is for Python 2.6, 2.7, and all versions of Python 3 starting with 3.3, write:

setup(
    ...
    python_requires='>=2.6, !=3.0.*, !=3.1.*, !=3.2.*, <4',
    ...
)

And so on.

Once you've done that, you will need to upgrade your version of setuptools to at least 24.2.0 in order for the python_requires argument to be processed; earlier versions will just ignore it with a warning. All of your project's sdists and wheels built afterwards will then contain the relevant metadata that tells PyPI to tell pip what Python versions they're for.