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A number of unix systems expose a getgroupslist function to fetch the groups that a user is a member of. It would be nice if that function were exposed to python.
See issue7900 for more information on why that would be useful.

Did a bit of digging and found that getgrouplist signature differs on (at least) Linux and Mac OS:
 - http://www.kernel.org/doc/man-pages/online/pages/man3/getgrouplist.3.html
 -http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man3/getgrouplist.3.html 

I don't see how this difference is relevant for exposing the functionality in python:
Linux:
int getgrouplist(const char *user, gid_t group,
 gid_t *groups, int *ngroups);
MacOS:
int
 getgrouplist(const char *name, int basegid, int *groups, int *ngroups);
The only difference is that Linux uses gid_t and MacOS uses int. The requested posix.getgrouplist() will take a python string and a python integer and return a list of integers.

@Alexander, it was just a note, that implementation in posixmodule.c won't be the same across all flavours of Unix :)

Attached is a patch (against the latest revision, 87178) which adds the functionality to the posix module as well as adds a testcase for it.
I haven't added it to the os module, I'm not sure if that should be done.
I tested it on Linux & FreeBSD and it works as expected. Since I don't have an OS X system, I could not test it on there.

A slightly updated patch. Targeting for 3.3.

I can't say anything about the functionality, but the patch looks good to me (works under Linux and Solaris - the latter missing getgrouplist()).

Patch looks good, but this likely runs into the same problem as #7900: the user can be a member of more than NGROUPS_MAX on OSX.
I'll test this tomorrow (by creating a temporary user that is a member of more than 16 groups).

Looks good to me as well. Just a nit-pick: in python code base "sizeof" is not separated from the opening parenthesis. I understand the desire to distinguish "sizeof" from a function, but it is probably better to be consistent.

Ronald, does it have the same problem as #7900 on OS X or can I commit?

I haven't tested yet, but feel free to commit and I'll fix any OSX issues later.

New changeset 9ebee3211be9 by Ross Lagerwall in branch 'default':
Issue #9344: Add os.getgrouplist().
http://hg.python.org/cpython/rev/9ebee3211be9 

Thanks!

Nit: when a patch gets committed and the issue closed, the preferred resolution is "fixed" ;)
(see http://docs.python.org/devguide/triaging.html#resolution)

> Nit: when a patch gets committed and the issue closed, the preferred
> resolution is "fixed" ;)
> (see http://docs.python.org/devguide/triaging.html#resolution)
Sure, I will try remember that.

Actually, when it is a feature request rather than a bug fix we usually use 'accepted'. It doesn't matter a whole lot, though.