for Mac, do `eject`

eject.py

#!/usr/bin/env python                                                                                                       
# -*- coding: utf-8 -*-                                                                                                     

import argparse
import subprocess


def _main():
    parser = argparse.ArgumentParser()
    args = parser.parse_args()

    eject_status = subprocess.check_call(["drutil", "eject"])
    print eject_status

if __name__ == '__main__':
    _main()