This whole thing is a mess.  Great analysis, by the way.   I started a
similar experiment some time ago, but gave up when I discovered that a
member of the admin group can read any file regardless of permissions
settings.  Your solution is probably the best we can do for 2.7, but
for 3.x I still believe it is better to add os.getgrouplist and
document that on OSX users should use it in preference of
os.getgroups.  (A better explanation of the difference between the two
will probably be even better.)

On Thu, Jul 8, 2010 at 4:52 PM, Ronald Oussoren <report@bugs.python.org> wrote:
..
> The behaviour w.r.t. secondairy groups is rather different than what I'd expect on a Unix system.