How to determine if a directory is on same partition

Question

Say I have an input file, and a target directory. How do I determine if the input file is on the same hard-drive (or partition) as the target directory?

What I want to do is the copy a file if it's on a different, but move it if it's the same. For example:

target_directory = "/Volumes/externalDrive/something/"
input_foldername, input_filename = os.path.split(input_file)
if same_partition(input_foldername, target_directory):
    copy(input_file, target_directory)
else:
    move(input_file, target_directory)

Answer 1

In C, you would use stat() and compare the st_dev field. In python, os.stat should do the same.

import os
def same_partition(f1, f2):
    return os.stat(f1).st_dev == os.stat(f2).st_dev

Answer 2

Another way is the “better to ask forgiveness than permission” approach—just try to rename it, and if that fails, catch the appropriate OSError and try the copy approach. ie:

import errno
try:
    os.rename(source, dest):
except IOError, ex:
    if ex.errno == errno.EXDEV:
        # perform the copy instead.

This has the advantage that it will also work on Windows, where st_dev is always 0 for all partitions.

Note that if you actually want to copy and then delete the source file (ie. perform a move), rather than just copy, then shutil.move will already do what you want:

Help on function move in module shutil:

move(src, dst)
    Recursively move a file or directory to another location.

    If the destination is on our current filesystem, then simply use
    rename.  Otherwise, copy src to the dst and then remove src.