bug in os.getgroups?

[Jeff Epler]

This is liely to be because /tmp is "sticky":
       The  `sticky'  bit  (S_ISVTX)  on a directory means that a
       file in that directory can be renamed or deleted  only  by
       the  owner of the file, by the owner of the
       directory, and by root.
(from my system's manpage for stat(2))

You aren't root, so you can't delete this file in /tmp, which is
typically a "sticky" directory.  It doesn't matter that the group
matches.

I can observe this behavior on my own system from the shell:
$ id 
uid=229(jepler) gid=15(man) groups=15(man),20(games),22(utmp),23(document)
$ ll blah
-rw-rw-r--    1 root     man             0 Jun 25 15:23 blah
$ rm blah
rm: cannot unlink `blah': Operation not permitted

Jeff