How to Integrate CMake into setup.py for Building Python Extensions?

How to Integrate CMake in setup.py for setuptools Extension

Building Python Extensions and the Need for CMake

When developing Python extensions that integrate C libraries, CMake provides a comprehensive tool for managing build processes. However, the typical workflow involves manually compiling the libraries with CMake before running setup.py bdist_wheel. This can be inconvenient and time-consuming.

Invoking CMake in setup.py

To address this, it is possible to incorporate CMake into the setup.py build process. The key is to create a custom build_ext command that utilizes CMake to configure and build the extensions.

Customizing the build_ext Command

In the setup.py file, override the build_ext command class and register it in the command classes. Within your custom implementation, configure and invoke CMake to build the extension modules.

Sample Project and Setup Script

To demonstrate the concept, consider a simple project with a C extension (foo) and a Python module (spam.eggs). The setup.py script leverages a CMakeExtension class that encapsulates the extension without invoking the original build_ext. The build_cmake method handles CMake configuration and build steps.

<code class="python">import os
import pathlib

from setuptools import setup, Extension
from setuptools.command.build_ext import 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)

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

Testing and Validation

By building the project's wheel and installing it, you can verify if the library is successfully installed and functional. Running a wrapper function from the spam.eggs module should produce the expected output.

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