def compile_python_files(self, dir):
        '''
        Compile the python files (recursively) for the python files inside
        a given folder.

        .. note:: python2 compiles the files into extension .pyo, but in
            python3, and as of Python 3.5, the .pyo filename extension is no
            longer used...uses .pyc (https://www.python.org/dev/peps/pep-0488)
        '''
        args = [self.ctx.hostpython]
        if self.ctx.python_recipe.name == 'python3':
            args += ['-OO', '-m', 'compileall', '-b', '-f', dir]
        else:
            args += ['-OO', '-m', 'compileall', '-f', dir]
        subprocess.call(args)