Add Extention to a Python Distribution before the build

setup.py

# This may help others to get started with there custom build process
#
# you find millions of post how to make a custom command and to run code
# but if you want to somewhat alter the way how extentions  are build or 
# not you may find yourself stuck ... like I was 

from distutils.command.build import build as _build
from distutils.core import setup, Extension

foo_module = Extension(name='foo',
                       sources=['foo/foomodule.c'])
                              
            
class FooBuildCommand(_build):

    user_options = _build.user_options + [
        ('with-foo', None,
         'build module with foo support'), ]

    def initialize_options(self):
        self.with_foo = None
        _build.initialize_options(self)

    def finalize_options(self):
        _build.finalize_options(self)

    def run(self):
        if self.with_foo:
            self.announce('build with foo support ...', 2)
            self.distribution.ext_modules.append(foo_module)
            self.distribution.packages.append('foo')
        _build.run(self)

# the big take away here is to define ext_module even if you dont have extentions !
# if you dont do this you will get an error in the custom command because ext_modules is None !

setup_dict = {'name': 'BAR',
              'version': '1.0',
              'license': 'LGPLv2.1',
              'author': 'Markus Rosjat',
              'author_email': 'markus.rosjat@gmail.com',
              'description': 'the awesome bar package',
              'packages': ['bar', ],
              'ext_modules': [],
              'cmdclass': {'build': FooBuildCommand, }}

setup(**setup_dict)