What is the easiest way to make an optional C extension for a python package?

Question

I've created a C extension that I'd like to enable in my Python package (using setuptools) only if a command line option is passed in. What is the easiest way to do this?

I can't seem to find any straightforward ways of going about this.

Answer 1

There's actually a distribute/setuptools feature called "Features" that can be used for this. It's explicitly designed to have setup.py do different things based on --with-xxx and --without-xxx command line options.

• This blog post gives a nice introduction, I can't find any better documentation at this time (besides the Distribute source - the Feature class and features keyword).
• The jinja project's setup.py uses Features for your exact purpose, it might be a good template to work from.
• The simplejson setup.py also does something similar, except that it's coded to always try to build the C-extension feature it defines, and fall back gracefully to pure-python when building fails; this may also be useful for your purpose.

Answer 2

ext_modules = []
if '--add-this' in sys.argv:
    ext_modules.append(Extension(...))
    sys.argv.remove('--add-this')
setup(...
      ext_modules = ext_modules
)

This is hacky, but might be easiest. A more advanced approach would be to extend the Distribution class to support a flag, say --with-modules and then customize ext_modules inside finalize_options.