Inconsistency between os.getgroups and os.system('groups') after os.setgroups()

[jeff]

Run this test program as root:

import os

print "before:", os.getgroups()
os.system("groups")
os.setgroups([])
print "after:", os.getgroups()
os.system("groups")

After the os.setgroups, os.getgroups says that the process is not in any groups, just as you would expect.  However the groups command run using os.system says that the process is in the root group.  It appears that the new process started by os.system augments the group membership specified in the os.setgroups command with the group of the actual user of the original process (which is root).  I can suppress membership in the root group only by doing os.setgid and os.setuid before the os.system call (in which case I wind up in the group of the new user instead of root), but I have to be able to get back to root privilege so I can't use setgid and setuid.  How do I run a program from a Python script running as root such that the group membership of the process running the program does not include root?