Begin by removing the dest file is maybe not the safer approach :-) Here is a new try: begin by renaming the dest file to a new file.

------
# use maybe a PRNG instead of a dummy counter or tempfile
def _create_old_filename(filename):
   old = filename + '.old'
   index = 2
   while os.path.exists(old):
      old = filename + '-%s.old' % index
      index += 1
   return old

if os.name in ('nt', 'ce'):
   def atomic_rename(src, dst):
      if os.path.exists(dst):
         old = _create_old_filename(dst)
         rename(dst, old)
         rename(src, dst)
         unlink(old)
      else:
         rename(src, dst)
else:
   atomic_rename = os.rename
------

What can we do if "rename(src, dst)" fails?