How should I structure a Python package that contains Cython code

I’ve done this myself now, in a Python package simplerandom (BitBucket repo – EDIT: now github) (I don’t expect this to be a popular package, but it was a good chance to learn Cython).

This method relies on the fact that building a .pyx file with Cython.Distutils.build_ext (at least with Cython version 0.14) always seems to create a .c file in the same directory as the source .pyx file.

Here is a cut-down version of setup.py which I hope shows the essentials:

from distutils.core import setup
from distutils.extension import Extension

try:
    from Cython.Distutils import build_ext
except ImportError:
    use_cython = False
else:
    use_cython = True

cmdclass = {}
ext_modules = []

if use_cython:
    ext_modules += [
        Extension("mypackage.mycythonmodule", ["cython/mycythonmodule.pyx"]),
    ]
    cmdclass.update({'build_ext': build_ext})
else:
    ext_modules += [
        Extension("mypackage.mycythonmodule", ["cython/mycythonmodule.c"]),
    ]

setup(
    name="mypackage",
    ...
    cmdclass=cmdclass,
    ext_modules=ext_modules,
    ...
)

I also edited MANIFEST.in to ensure that mycythonmodule.c is included in a source distribution (a source distribution that is created with python setup.py sdist):

...
recursive-include cython *
...

I don’t commit mycythonmodule.c to version control ‘trunk’ (or ‘default’ for Mercurial). When I make a release, I need to remember to do a python setup.py build_ext first, to ensure that mycythonmodule.c is present and up-to-date for the source code distribution. I also make a release branch, and commit the C file into the branch. That way I have a historical record of the C file that was distributed with that release.