How to find the mountpoint a file resides on?

For example, I've got a file with the following path:

/media/my_mountpoint/path/to/file.txt

I've got the whole path and want to get:

/media/my_mountpoint

How can I do this? Preferably in Python and without using external libraries / tools. (Both are not a requirement.)

You may either call the mount command and parse its output to find the longest common prefix with your path, or use the stat system call to get the device a file resides on and go up the tree until you get to a different device.

In Python, stat may be used as follows (untested and may have to be extended to handle symlinks and exotic stuff like union mounts):

def find_mount_point(path):
    path = os.path.abspath(path)
    orig_dev = os.stat(path).st_dev

    while path != '/':
        dir = os.path.dirname(path)
        if os.stat(dir).st_dev != orig_dev:
            # we crossed the device border
            break
        path = dir
    return path

Edit: I didn't know about os.path.ismount until just now. This simplifies things greatly.

def find_mount_point(path):
    path = os.path.abspath(path)
    while not os.path.ismount(path):
        path = os.path.dirname(path)
    return path