Index: configure.in
===================================================================
--- configure.in	(revision 87729)
+++ configure.in	(working copy)
@@ -2536,7 +2536,7 @@
 # checks for library functions
 AC_CHECK_FUNCS(alarm accept4 setitimer getitimer bind_textdomain_codeset chown \
  clock confstr ctermid execv fchmod fchown fork fpathconf ftime ftruncate \
- gai_strerror getgroups getlogin getloadavg getpeername getpgid getpid \
+ gai_strerror getgrouplist getgroups getlogin getloadavg getpeername getpgid getpid \
  getpriority getresuid getresgid getpwent getspnam getspent getsid getwd \
  initgroups kill killpg lchmod lchown lstat mbrtowc mkfifo mknod mktime \
  mremap nice pathconf pause plock poll pthread_init \
Index: Doc/library/os.rst
===================================================================
--- Doc/library/os.rst	(revision 87729)
+++ Doc/library/os.rst	(working copy)
@@ -219,6 +219,17 @@
    Availability: Unix.
 
 
+.. function:: getgrouplist(user, group)
+
+   Return list of group ids that *user* belongs to. If *group* is not in the
+   list, it is included; typically, *group* is specified as the group ID
+   field from the password record for *user*.
+
+   Availability: Unix.
+
+   .. versionadded:: 3.3
+
+
 .. function:: getgroups()
 
    Return list of supplemental group ids associated with the current process.
Index: Lib/test/test_posix.py
===================================================================
--- Lib/test/test_posix.py	(revision 87729)
+++ Lib/test/test_posix.py	(working copy)
@@ -373,6 +373,21 @@
                 os.chdir(curdir)
                 support.rmtree(base_path)
 
+    @unittest.skipUnless(hasattr(posix, 'getgrouplist'), "test needs posix.getgrouplist()")
+    @unittest.skipUnless(hasattr(pwd, 'getpwuid'), "test needs pwd.getpwuid()")
+    @unittest.skipUnless(hasattr(os, 'getuid'), "test needs os.getuid()")
+    def test_getgrouplist(self):
+        with os.popen('id -G') as idg:
+            groups = idg.read().strip()
+
+        if not groups:
+            raise unittest.SkipTest("need working 'id -G'")
+
+        self.assertEqual(
+            set([int(x) for x in groups.split()]),
+            set(posix.getgrouplist(pwd.getpwuid(os.getuid())[0],
+                pwd.getpwuid(os.getuid())[3])))
+
     def test_getgroups(self):
         with os.popen('id -G') as idg:
             groups = idg.read().strip()
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 87729)
+++ Modules/posixmodule.c	(working copy)
@@ -4240,7 +4240,71 @@
     return PyLong_FromPid(getpid());
 }
 
+#ifdef HAVE_GETGROUPLIST
+PyDoc_STRVAR(posix_getgrouplist__doc__,
+"getgrouplist(user, group) -> list of groups to which a user belongs\n\n\
+Returns a list of groups to which a user belongs.\n\n\
+    user: username to lookup\n\
+    group: base group id of the user");
 
+static PyObject *
+posix_getgrouplist(PyObject *self, PyObject *args)
+{
+#ifdef NGROUPS_MAX
+#define MAX_GROUPS NGROUPS_MAX
+#else
+    /* defined to be 16 on Solaris7, so this should be a small number */
+#define MAX_GROUPS 64
+#endif
+
+    const char *user;
+    int i, ngroups;
+    PyObject *list;
+#ifdef __APPLE__
+    int *groups, basegid;
+#else
+    gid_t *groups, basegid;
+#endif
+    ngroups = MAX_GROUPS;
+
+    if (!PyArg_ParseTuple(args, "si", &user, &basegid))
+        return NULL;
+
+#ifdef __APPLE__
+    groups = PyMem_Malloc(ngroups * sizeof (int));
+#else
+    groups = PyMem_Malloc(ngroups * sizeof (gid_t));
+#endif
+    if (groups == NULL)
+        return PyErr_NoMemory();
+
+    if (getgrouplist(user, basegid, groups, &ngroups) == -1) {
+        PyMem_Del(groups);
+        return posix_error();
+    }
+
+    list = PyList_New(ngroups);
+	if (list == NULL) {
+        PyMem_Del(groups);
+        return NULL;
+    }
+
+    for (i = 0; i < ngroups; i++) {
+        PyObject *o = PyLong_FromUnsignedLong((unsigned long)groups[i]);
+        if (o == NULL) {
+            Py_DECREF(list);
+            PyMem_Del(groups);
+            return NULL;
+        }
+        PyList_SET_ITEM(list, i, o);
+    }
+
+    PyMem_Del(groups);
+
+    return list;
+}
+#endif
+
 #ifdef HAVE_GETGROUPS
 PyDoc_STRVAR(posix_getgroups__doc__,
 "getgroups() -> list of group IDs\n\n\
@@ -7872,6 +7936,9 @@
 #ifdef HAVE_GETGID
     {"getgid",          posix_getgid, METH_NOARGS, posix_getgid__doc__},
 #endif /* HAVE_GETGID */
+#ifdef HAVE_GETGROUPLIST
+    {"getgrouplist",    posix_getgrouplist, METH_VARARGS, posix_getgrouplist__doc__},
+#endif
 #ifdef HAVE_GETGROUPS
     {"getgroups",       posix_getgroups, METH_NOARGS, posix_getgroups__doc__},
 #endif
Index: pyconfig.h.in
===================================================================
--- pyconfig.h.in	(revision 87729)
+++ pyconfig.h.in	(working copy)
@@ -278,6 +278,9 @@
 /* Define this if you have flockfile(), getc_unlocked(), and funlockfile() */
 #undef HAVE_GETC_UNLOCKED
 
+/* Define to 1 if you have the `getgrouplist' function. */
+#undef HAVE_GETGROUPLIST
+
 /* Define to 1 if you have the `getgroups' function. */
 #undef HAVE_GETGROUPS