how to determine if files are on same or different file systems

[Skip Montanaro]

    Kamus> It would be nice if I could write a function that would determine
    Kamus> if the source and destination are on the same file system or not,
    Kamus> and thus use rename or copy appropriately, or if there is already
    Kamus> such a built-in function.

You might want to check out os.path.ismount().  Using it, you can walk up
the two paths toward the root until you hit elements of bot paths where
os.path.ismount() returns True.  If the two mount points are the same, the
two original paths are on the same file system.  Here's some sample code.
It doesn't help much on my Mac because it essentially has just a single disk
partition.  On my web server (Linux w/ multiple partitions), it seems to
work okay:

    % df
    Filesystem           1k-blocks      Used Available Use% Mounted on
    /dev/hda1               248847     93239    142758  40% /
    /dev/hda6             14333167   9393563   4194190  70% /backup
    none                    385572         0    385572   0% /dev/shm
    /dev/hda5              9867356   7702853   1652433  83% /home
    /dev/hda7              1981000   1568913    309675  84% /usr
    /dev/hda8               248847    120886    115111  52% /var
    % python samefs.py 
    ('/', '/usr') 0
    ('/bin', '/etc') 1
    ('/etc/passwd', '/var/log') 0
    ('/Applications', '/Volumes Kisses In The Rain') 0

Skip

import os

def samefs(path1, path2):
    if not (os.path.exists(path1) and os.path.exists(path2)):
        return False

    while path1 != os.path.dirname(path1):
        if os.path.ismount(path1):
            break
        path1 = os.path.dirname(path1)

    while path2 != os.path.dirname(path2):
        if os.path.ismount(path2):
            break
        path2 = os.path.dirname(path2)

    return path1 == path2

print ("/", "/usr"), samefs("/", "/usr")
print ("/bin", "/etc"), samefs("/bin", "/etc")
print ("/etc/passwd", "/var/log"), samefs("/etc/passwd", "/var/log")
print ("/Applications", "/Volumes Kisses In The Rain"), \
      samefs("/Applications", "/Volumes Kisses In The Rain")