On some systems (specifically, Linux), the rename system call will throw
an EXDEV error if rename is used across filesystems.  It would be convenient for the user if os.rename were extended to handle this transparently (like most mv commands do).

The benefits of this. . .getting rid of code like the following:

try:
    os.rename('ff','/tmp/ff')
except:
    open('/tmp/ff','w').write(open('ff','r').read())
    os.unlink('ff')

Actually, the real benefit is that code (written by morons like myself) using os.rename will continue to work even after the administrator moves the target directory to another filesystem.

I took a quick look at posixmodule.c.  A quick hack changes posix_2str's
signature to the following:

PyObject *args
char *format
int (*func)(const char*, const char *)
int (*special_handler)(const char *, const char *)

and the inner function to:

if (res != 0)
    if ((! special_handler) || (*special_handler)(path1,path2))
         return posix_error()

Of course, then a smart copy routine (includes an unlink step).  The most unclear thing at this point is what to do with the errno.  Would a failure in
the errorhandler report the original errno or its own errno???

Personally, a more general solution would allow the user (python-level) to optionally pass in *their own* error handling function/method.