How to mount and unmount Linux filesystems using ctypes, mount and umount

I have a python script (running as root) that should be able to mount and unmount a USB flashdrive filesystem. Ive done some research and I found this answer on StackOverflow question which uses ctypes. However. the answer specifies how to mount, so I tried to create a similar function to unmount the device. So, I have it all:

import os
import ctypes
def mount(source, target, fs, options=''):
    ret = ctypes.CDLL('libc.so.6', use_errno=True).mount(source, target, fs, 0, options)
    if ret < 0:
        errno = ctypes.get_errno()
        raise RuntimeError("Error mounting {} ({}) on {} with options '{}': {}".
                           format(source, fs, target, options, os.strerror(errno)))

def unmount(device, options=0):
    ret = ctypes.CDLL('libc.so.6', use_errno=True).umount2(device, options)
    if ret < 0:
        errno = ctypes.get_errno()
        raise RuntimeError("Error umounting {} with options '{}': {}".format(device, options, os.strerror(errno)))

      

        
        
        
      

    

However, try unmount command with parameter "0" or "1", for example:

unmount('/dev/sdb', 0)

      

        
        
        
      

    

or

unmount('/dev/sdb', 1)

      

        
        
        
      

    

gives the following error:

Traceback (most recent call last):
  File "./BuildAndInstallXSystem.py", line 265, in <module>
    prepare_root_device()
  File "./BuildAndInstallXSystem.py", line 159, in prepare_root_device
    unmount('/dev/sdb', 0)
  File "./BuildAndInstallXSystem.py", line 137, in unmount
    raise RuntimeError("Error umounting {} with options '{}': {}".format(device, options, os.strerror(errno)))
RuntimeError: Error umounting /dev/sdb with options '0': Device or resource busy

      

        
        
        
      

    

while running with 2 as an option:

unmount('/dev/sdb', 2)

      

        
        
        
      

    

shuts down ALL of my filesystems, including "/", causing the system to crash.

All of this still applies even if I replace the device number with a specific partition:

/dev/sdb -> /dev/sdb1

      

        
        
        
      

    

What am I doing wrong?