When running test_pathlib on my OSX (10.8) laptop I get these errors:

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ERROR: test_group (test.test_pathlib.PathTest)
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Traceback (most recent call last):
  File "/Users/guido/cpython/Lib/test/test_pathlib.py", line 1332, in test_group
    name = grp.getgrgid(gid).gr_name
KeyError: 'getgrgid(): gid not found: 849048494'

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ERROR: test_group (test.test_pathlib.PosixPathTest)
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Traceback (most recent call last):
  File "/Users/guido/cpython/Lib/test/test_pathlib.py", line 1332, in test_group
    name = grp.getgrgid(gid).gr_name
KeyError: 'getgrgid(): gid not found: 849048494'

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This is probably some kind of OSX quirk where st_gid is set to a group that's not in /etc/groups, because (except briefly during the reboot process) on OS X the /etc configuration files for users/groups do not provide the whole truth.

Sadly I don't know what API to use to get the group name.   "ls -l" doesn't seem to know either.  My home dir has that gid too, so it's not an isolated anomaly. It may well be some kind of corporate thing (it's not really "my" laptop. :-)

Well, the KeyError is legitimate then (it's even in the docs :-)).
The test will probably have to be skipped in this case.

You should be able to find the group and group name using the "Directory Utility" app.  The easiest way to launch it is from a shell with:

open "/System/Library/CoreServices/Directory Utility.app"

There is a command line utility, dscl, but it is much more painful.

New changeset b58b58948c27 by Antoine Pitrou in branch 'default':
Issue #19742: fix a test_pathlib failure when a file owner or group isn't in the system database
http://hg.python.org/cpython/rev/b58b58948c27

It should be fixed now (i.e. the test is skipped).