How to bypass the 0-255 range limit for sys.exit() in python?

In python (on a Linux system), I'm launching a command using os.system() and retrieving the return code. If that return code is different from 0, I would like to make the program exit with that same return code. So I wrote:

ret = os.system(cmd)
if ret != 0:
   print "exit with status %s" % ret
   sys.exit(ret)

When the return code is lower than 256, it works fine, but when it's greater than 255, the exit code used is 0. How can I make sys.exit() accept codes greater than 255?

Edit: the limit is actually 255

In fact, the ret variable receives 256, but sys.exit() fails to use it, so the program returns 0 instead. When I launch cmd manually, I see that it returns 1, not 256.