def _convert_pyx_sources_to_lang(self):
        """
        Replace sources with .pyx extensions to sources with the target
        language extension. This mechanism allows language authors to supply
        pre-converted sources but to prefer the .pyx sources.
        """
        if _have_cython():
            # the build has Cython, so allow it to compile the .pyx files
            return
        lang = self.language or ''
        target_ext = '.cpp' if lang.lower() == 'c++' else '.c'
        sub = functools.partial(re.sub, '.pyx$', target_ext)
        self.sources = list(map(sub, self.sources))

        Replace sources with .pyx extensions to sources with the target
        language extension. This mechanism allows language authors to supply
        pre-converted sources but to prefer the .pyx sources.
        
product/gradle-plugin/src/main/python/setuptools/extension.py