How can you streamline Python extension builds using CMake within setup.py?

Subclassing Build Extensions with CMake

Introduction:

Extending Python with C code requires careful orchestration between compiling and linking steps. To simplify this process, CMake provides a convenient way to configure and build such extensions. However, integrating CMake with setup.py has posed challenges for developers. This article aims to address these challenges by delving into extending setuptools to seamlessly use CMake in setup.py builds.

The Need for Integration:

Traditionally, Python extensions are compiled and linked prior to running setup.py, creating a tedious two-step process. By leveraging CMake, developers gain fine-grained control over complex build sequences. Moreover, CMake's Python extension support allows for effortless compilation directly from setup.py, eliminating the need for intermediate steps.

Customizing Build Extensions:

To integrate CMake into setup.py, a custom extension class is necessary. This subclass, CMakeExtension, overrides the default build behavior to invoke CMake commands instead.

Example Integration:

Consider a simple project structure that includes a C extension (foo) and a Python module (spam.eggs). The CMakeLists.txt and setup.py files play crucial roles in the build process:

# CMakeLists.txt
project(spam)
set(src "spam")
set(foo_src "spam/foo.c")
add_library(foo SHARED ${foo_src})

# setup.py
from setuptools import setup, Extension
from setuptools.command.build_ext import build_ext_orig

class CMakeExtension(Extension):
    def __init__(self, name):
        super().__init__(name, sources=[])

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

The CMakeExtension class registers itself as a special extension that only calls CMake during the build process, while the customized build_ext overrides the default behavior to invoke CMake.

Testing the Build:

Building the project involves running setup.py sdist bdist_wheel. The resulting wheel distribution can be installed and tested using pip show to verify the Python extension library (libfoo.dylib) is present. Calling the wrapper function in spam.eggs confirms that the extension works as expected.

Conclusion:

Extending setuptools with CMake in setup.py offers a streamlined approach to building Python extensions that rely on CMake for complex build sequences. It enables developers to integrate these two tools seamlessly, reducing manual steps and enhancing the development workflow.

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