error: each element of 'ext_modules' option must be an Extension instance or 2-tuple

Originally reported by: floppym (Bitbucket: floppym, GitHub: floppym)

On Gentoo Linux, we are running into a problem building pyzmq with recent versions of setuptools.

https://bugs.gentoo.org/show_bug.cgi?id=532708

We get this error:

error: each element of 'ext_modules' option must be an Extension instance or 2-tuple

From what I can tell, it has to do with the order in which distutils.extenstion.Extension and setuptools are imported. setuptools replaces the Extension module with its own implementation. If this happens after setup.py has imported distutils.extension.Extension, it causes this problem.

Here is a reduced test case:

% cat setup.py
from distutils.core import setup
from distutils.extension import Extension
import setuptools

setup(ext_modules=[
        Extension('demo', ['demo.c'])
])
% python3.4 setup.py build
running build
running build_ext
error: each element of 'ext_modules' option must be an Extension instance or 2-tuple

The obvious workaround is to always import setuptools first. However, it is not always obvious where setuptools is being imported.

For pyzmq, it seems like setuptools is being imported indirectly; setup.py does not import it anywhere. By trial and error, it looks like importing nose brings in setuptools.

• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/309

Start by reproducing the reduced setup.py example with Python 3.4 and the reported setuptools import order. Then inspect how setup.py, pyzmq, and nose cause setuptools and distutils.extension.Extension to be imported, and verify that the build accepts ext_modules without the reported error.