python - Extending setuptools extension to use CMake in setup.py?

Question

I'm writing a Python extension that links a C++ library and I'm using cmake to help with the build process. This means that right now, the only way I know how to bundle it, I have to first compile them with cmake before I can run setup.py bdist_wheel. There must be a better way.

I was wondering if it's possible (or anybody has tried) to invoke CMake as part of the setup.py ext_modules build process? I'm guessing there is a way to create a subclass of something but I'm not sure where to look.

I'm using CMake because it gives me so much more control for building c and c++ libraries extensions with complex build steps exactly as I want it. Plus, I can easily build Python extensions directly with cmake with the PYTHON_ADD_MODULE() command in the findPythonLibs.cmake. I just wish this was all one step.

Answer 1

What you basically need to do is to override the build_ext command class in your setup.py and register it in the command classes. In your custom impl of build_ext, configure and call cmake to configure and then build the extension modules. Unfortunately, the official docs are rather laconic about how to implement custom distutils commands (see Extending Distutils); I find it much more helpful to study the commands code directly. For example, here is the source code for the build_ext command.

Example project

I have prepared a simple project consisting out of a single C extension foo and a python module spam.eggs:

so-42585210/
├── spam
│   ├── __init__.py  # empty
│   ├── eggs.py
│   ├── foo.c
│   └── foo.h
├── CMakeLists.txt
└── setup.py

Files for testing the setup

These are just some simple stubs I wrote to test the setup script.

spam/eggs.py (only for testing the library calls):

from ctypes import cdll
import pathlib


def wrap_bar():
    foo = cdll.LoadLibrary(str(pathlib.Path(__file__).with_name('libfoo.dylib')))
    return foo.bar()

spam/foo.c:

#include "foo.h"

int bar() {
    return 42;
}

spam/foo.h:

#ifndef __FOO_H__
#define __FOO_H__

int bar();

#endif

CMakeLists.txt:

cmake_minimum_required(VERSION 3.10.1)
project(spam)
set(src "spam")
set(foo_src "spam/foo.c")
add_library(foo SHARED ${foo_src})

Setup script

This is where the magic happens. Of course, there is a lot of room for improvements - you could pass additional options to CMakeExtension class if you need to (for more info on the extensions, see Building C and C++ Extensions), make the CMake options configurable via setup.cfg by overriding methods initialize_options and finalize_options etc.

import os
import pathlib

from setuptools import setup, Extension
from setuptools.command.build_ext import build_ext as build_ext_orig


class CMakeExtension(Extension):

    def __init__(self, name):
        # don't invoke the original build_ext for this special extension
        super().__init__(name, sources=[])


class build_ext(build_ext_orig):

    def run(self):
        for ext in self.extensions:
            self.build_cmake(ext)
        super().run()

    def build_cmake(self, ext):
        cwd = pathlib.Path().absolute()

        # these dirs will be created in build_py, so if you don't have
        # any python sources to bundle, the dirs will be missing
        build_temp = pathlib.Path(self.build_temp)
        build_temp.mkdir(parents=True, exist_ok=True)
        extdir = pathlib.Path(self.get_ext_fullpath(ext.name))
        extdir.mkdir(parents=True, exist_ok=True)

        # example of cmake args
        config = 'Debug' if self.debug else 'Release'
        cmake_args = [
            '-DCMAKE_LIBRARY_OUTPUT_DIRECTORY=' + str(extdir.parent.absolute()),
            '-DCMAKE_BUILD_TYPE=' + config
        ]

        # example of build args
        build_args = [
            '--config', config,
            '--', '-j4'
        ]

        os.chdir(str(build_temp))
        self.spawn(['cmake', str(cwd)] + cmake_args)
        if not self.dry_run:
            self.spawn(['cmake', '--build', '.'] + build_args)
        # Troubleshooting: if fail on line above then delete all possible 
        # temporary CMake files including "CMakeCache.txt" in top level dir.
        os.chdir(str(cwd))


setup(
    name='spam',
    version='0.1',
    packages=['spam'],
    ext_modules=[CMakeExtension('spam/foo')],
    cmdclass={
        'build_ext': build_ext,
    }
)

Testing

Build the project's wheel, install it. Test the library is installed:

$ pip show -f spam
Name: spam
Version: 0.1
Summary: UNKNOWN
Home-page: UNKNOWN
Author: UNKNOWN
Author-email: UNKNOWN
License: UNKNOWN
Location: /Users/hoefling/.virtualenvs/stackoverflow/lib/python3.6/site-packages
Requires: 
Files:
  spam-0.1.dist-info/DESCRIPTION.rst
  spam-0.1.dist-info/INSTALLER
  spam-0.1.dist-info/METADATA
  spam-0.1.dist-info/RECORD
  spam-0.1.dist-info/WHEEL
  spam-0.1.dist-info/metadata.json
  spam-0.1.dist-info/top_level.txt
  spam/__init__.py
  spam/__pycache__/__init__.cpython-36.pyc
  spam/__pycache__/eggs.cpython-36.pyc
  spam/eggs.py
  spam/libfoo.dylib

Run the wrapper function from spam.eggs module:

$ python -c "from spam import eggs; print(eggs.wrap_bar())"
42