Marking Cython as a Build Dependency?

Question

There is a Python package with a setup.py that reads thusly:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

setup(
  name = 'fastahack',
  ext_modules=[
    Extension("fastahack.cfastahack",
              sources=["fastahack/cfastahack.pyx", "lib/Fasta.cpp", "lib/split.cpp"],
              libraries=["stdc++"],
              include_dirs=["lib/"],
              language="c++"),
    ],
    package_data = {'lib': ['*.pyx', "*.c", "*.h", "README.rst"]},
    package_dir = {"fastahack": "fastahack"},
    cmdclass = {'build_ext': build_ext},
    packages = ['fastahack', 'fastahack.tests'],
    author = "Brent Pedersen",
    author_email="[email protected]",
    #test_suite='nose.collector'
)

This setup.py can't be imported if Cython is not installed. As far as I know, importing setup.py is how tools like pip figure out the dependencies of a package. I want to set up this package so that it could be uploaded to PyPI, with the fact that it depends on Cython noted, so that Cython will be downloaded and installed when you try to "pip install fastahack", or when you try to "pip install" directly from the Git repository.

How would I package this module so that it installs correctly from the Internet when Cython is not installed? Always using the latest version of Cython would be a plus.

Answer 1

My standard template for setup.py:

have_cython = False
try:
    from Cython.Distutils import build_ext as _build_ext
    have_cython = True
except ImportError:
    from distutils.command.build_ext import build_ext as _build_ext

if have_cython:
    foo  = Extension('foo', ['src/foo.pyx'])
else:
    foo  = Extension('foo', ['src/foo.c'])

setup (
   ...
   ext_modules=[foo],
   cmdclass={'build_ext': build_ext}

And don't forget to provide extention .c files with package - that will allow users to build module without installing cython.

Answer 2

You can specify Cython as a build dependency using PEP-518 project specification.

In the file pyproject.toml (in the same directory as setup.py) insert:

[build-system]
requires = ["setuptools", "wheel", "Cython"]

Cython will then be installed before building your package.

Note that (currently) you need to pass --no-use-pep517 to pip install if you are installing your package locally as editable (ie with --editable or -e)