如何构造包含Cython代码的Python包

I’d like to make a Python package containing some Cython code. I’ve got the the Cython code working nicely. However, now I want to know how best to package it.

For most people who just want to install the package, I’d like to include the .c file that Cython creates, and arrange for setup.py to compile that to produce the module. Then the user doesn’t need Cython installed in order to install the package.

But for people who may want to modify the package, I’d also like to provide the Cython .pyx files, and somehow also allow for setup.py to build them using Cython (so those users would need Cython installed).

How should I structure the files in the package to cater for both these scenarios?

The Cython documentation gives a little guidance. But it doesn’t say how to make a single setup.py that handles both the with/without Cython cases.

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.

Adding to Craig McQueen’s answer: see below for how to override the sdist command to have Cython automatically compile your source files before creating a source distribution.

That way your run no risk of accidentally distributing outdated C sources. It also helps in the case where you have limited control over the distribution process e.g. when automatically creating distributions from continuous integration etc.

from distutils.command.sdist import sdist as _sdist

...

class sdist(_sdist):
    def run(self):
        # Make sure the compiled Cython files in the distribution are up-to-date
        from Cython.Build import cythonize
        cythonize(['cython/mycythonmodule.pyx'])
        _sdist.run(self)
cmdclass['sdist'] = sdist

http://docs.cython.org/en/latest/src/userguide/source_files_and_compilation.html#distributing-cython-modules

It is strongly recommended that you distribute the generated .c files as well as your Cython sources, so that users can install your module without needing to have Cython available.

It is also recommended that Cython compilation not be enabled by default in the version you distribute. Even if the user has Cython installed, he probably doesn’t want to use it just to install your module. Also, the version he has may not be the same one you used, and may not compile your sources correctly.

This simply means that the setup.py file that you ship with will just be a normal distutils file on the generated .c files, for the basic example we would have instead:

from distutils.core import setup
from distutils.extension import Extension
 
setup(
    ext_modules = [Extension("example", ["example.c"])]
)

The easiest is to include both but just use the c-file? Including the .pyx file is nice, but it’s not needed once you have the .c file anyway. People who want to recompile the .pyx can install Pyrex and do it manually.

Otherwise you need to have a custom build_ext command for distutils that builds the C file first. Cython already includes one. http://docs.cython.org/src/userguide/source_files_and_compilation.html

What that documentation doesn’t do is say how to make this conditional, but

try:
     from Cython.distutils import build_ext
except ImportError:
     from distutils.command import build_ext

Should handle it.

Including (Cython) generated .c files are pretty weird. Especially when we include that in git. I’d prefer to use setuptools_cython. When Cython is not available, it will build an egg which has built-in Cython environment, and then build your code using the egg.

A possible example: https://github.com/douban/greenify/blob/master/setup.py

Update(2017-01-05):

Since setuptools 18.0, there’s no need to use setuptools_cython. Here is an example to build Cython project from scratch without setuptools_cython.

This is a setup script I wrote which makes it easier to include nested directories inside the build. One needs to run it from folder within a package.

Givig structure like this:

__init__.py
setup.py
test.py
subdir/
      __init__.py
      anothertest.py

setup.py

from setuptools import setup, Extension
from Cython.Distutils import build_ext
# from os import path
ext_names = (
    'test',
    'subdir.anothertest',       
) 

cmdclass = {'build_ext': build_ext}
# for modules in main dir      
ext_modules = [
    Extension(
        ext,
        [ext + ".py"],            
    ) 
    for ext in ext_names if ext.find('.') < 0] 
# for modules in subdir ONLY ONE LEVEL DOWN!! 
# modify it if you need more !!!
ext_modules += [
    Extension(
        ext,
        ["/".join(ext.split('.')) + ".py"],     
    )
    for ext in ext_names if ext.find('.') > 0]

setup(
    name='name',
    ext_modules=ext_modules,
    cmdclass=cmdclass,
    packages=["base", "base.subdir"],
)
#  Build --------------------------
#  python setup.py build_ext --inplace

Happy compiling ;)

The simple hack I came up with:

from distutils.core import setup

try:
    from Cython.Build import cythonize
except ImportError:
    from pip import pip

    pip.main(['install', 'cython'])

    from Cython.Build import cythonize


setup(…)

Just install Cython if it could not be imported. One should probably not share this code, but for my own dependencies it’s good enough.

All other answers either rely on

• distutils
• importing from Cython.Build, which creates a chicken-and-egg problem between requiring cython via setup_requires and importing it.

A modern solution is to use setuptools instead, see this answer (automatic handling of Cython extensions requires setuptools 18.0, i.e., it’s available for many years already). A modern standard setup.py with requirements handling, an entry point, and a cython module could look like this:

from setuptools import setup, Extension

with open('requirements.txt') as f:
    requirements = f.read().splitlines()

setup(
    name='MyPackage',
    install_requires=requirements,
    setup_requires=[
        'setuptools>=18.0',  # automatically handles Cython extensions
        'cython>=0.28.4',
    ],
    entry_points={
        'console_scripts': [
            'mymain = mypackage.main:main',
        ],
    },
    ext_modules=[
        Extension(
            'mypackage.my_cython_module',
            sources=['mypackage/my_cython_module.pyx'],
        ),
    ],
)

The easiest way I found using only setuptools instead of the feature limited distutils is

from setuptools import setup
from setuptools.extension import Extension
try:
    from Cython.Build import cythonize
except ImportError:
    use_cython = False
else:
    use_cython = True

ext_modules = []
if use_cython:
    ext_modules += cythonize('package/cython_module.pyx')
else:
    ext_modules += [Extension('package.cython_module',
                              ['package/cython_modules.c'])]

setup(name='package_name', ext_modules=ext_modules)

I think I found a pretty good way of doing this by providing a custom build_ext command. The idea is the following:

1. I add the numpy headers by overriding finalize_options() and doing import numpy in the body of the function, which nicely avoids the problem of numpy not being available before setup() installs it.

2. If cython is available on the system, it hooks into the command’s check_extensions_list() method and by cythonizes all out-of-date cython modules, replacing them with C extensions that can later handled by the build_extension() method. We just provide the latter part of the functionality in our module too: this means that if cython is not available but we have a C extension present, it still works, which allows you to do source distributions.

Here’s the code:

import re, sys, os.path
from distutils import dep_util, log
from setuptools.command.build_ext import build_ext

try:
    import Cython.Build
    HAVE_CYTHON = True
except ImportError:
    HAVE_CYTHON = False

class BuildExtWithNumpy(build_ext):
    def check_cython(self, ext):
        c_sources = []
        for fname in ext.sources:
            cname, matches = re.subn(r"(?i)\.pyx$", ".c", fname, 1)
            c_sources.append(cname)
            if matches and dep_util.newer(fname, cname):
                if HAVE_CYTHON:
                    return ext
                raise RuntimeError("Cython and C module unavailable")
        ext.sources = c_sources
        return ext

    def check_extensions_list(self, extensions):
        extensions = [self.check_cython(ext) for ext in extensions]
        return build_ext.check_extensions_list(self, extensions)

    def finalize_options(self):
        import numpy as np
        build_ext.finalize_options(self)
        self.include_dirs.append(np.get_include())

This allows one to just write the setup() arguments without worrying about imports and whether one has cython available:

setup(
    # ...
    ext_modules=[Extension("_my_fast_thing", ["src/_my_fast_thing.pyx"])],
    setup_requires=['numpy'],
    cmdclass={'build_ext': BuildExtWithNumpy}
    )