How to build py3-none-any wheels for a project with an optional C extension?

Question

msgpack includes an optional cython extension. Some users of the package want py3-none-any wheels of msgpack. I'm trying to figure out how to make it possible to build wheels both with and without the optional extension.

Answer 1

One possible solution is to use an environment variable in setup.py to decide whether to set ext_modules to an empty list of a list of setuptools.Extension

pyproject.toml

[build-system]
requires = ["setuptools", "wheel", "cython"]
build-backend = "setuptools.build_meta"

setup.py

from setuptools import setup, Extension
import os

if 'ONLY_PURE' in os.environ:
    ext_modules = []
else:
    module1 = Extension('helloworld', sources = ['helloworld.pyx'])
    ext_modules = [module1]
setup(ext_modules=ext_modules)

setup.cfg

[metadata]
name = mypackage
version = 0.0.1

[options]
py_modules = mypackage

mypackage.py

try:
    import helloworld
except ImportError:
    print('hello pure python')

helloworld.pyx

print("hello extension")

To build with extension:

$ pip install build
...
$ python -m build
...
$ ls dist/
mypackage-0.0.1-cp39-cp39-linux_x86_64.whl  mypackage-0.0.1.tar.gz

To build without extension

$ pip install build
...
$ ONLY_PURE='a nonempty string' python -m build
...
$ ls dist/
mypackage-0.0.1-py3-none-any.whl  mypackage-0.0.1.tar.gz