When I tested the following code with python 2.5 on linux, I got an exception:

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Python 2.5 (r25:51908, Jan 24 2007, 12:48:15) 
[GCC 4.1.0 (SUSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import tarfile
>>> f = open("test.tar")
>>> tarfile.open(fileobj=f).getnames()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.5/tarfile.py", line 1019, in open
    return func(name, "r", fileobj)
  File "/usr/local/lib/python2.5/tarfile.py", line 1077, in gzopen
    pre, ext = os.path.splitext(name)
  File "/usr/local/lib/python2.5/posixpath.py", line 92, in splitext
    i = p.rfind('.')
AttributeError: 'NoneType' object has no attribute 'rfind'
>>> 
---------------------

However, the test with latest python (built from latest code in svn) passes.

----------------------
Python 2.6a0 (trunk:54698M, Apr  5 2007, 16:18:31) 
[GCC 4.1.0 (SUSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import tarfile
>>> f = open("test.tar")
>>> tarfile.open(fileobj=f).getnames()
['wikipediafs-0.2', 'wikipediafs-0.2/AUTHORS', 'wikipediafs-0.2/ChangeLog', 'wikipediafs-0.2/COPYING', 'wikipediafs-0.2/COPYRIGHT', 'wikipediafs-0.2/VERSION', 'wikipediafs-0.2/README', 'wikipediafs-0.2/setup.py', 'wikipediafs-0.2/site', 'wikipediafs-0.2/site/index.htm', 'wikipediafs-0.2/site/index_fr.htm', 'wikipediafs-0.2/src', 'wikipediafs-0.2/src/wikipediafs', 'wikipediafs-0.2/src/wikipediafs/WikipediaFS.py', 'wikipediafs-0.2/src/wikipediafs/WikipediaArticleList.py', 'wikipediafs-0.2/src/wikipediafs/WikipediaUser.py', 'wikipediafs-0.2/src/wikipediafs/WikipediaArticle.py', 'wikipediafs-0.2/src/wikipediafs/__init__.py', 'wikipediafs-0.2/src/wikipediafs/logger.py', 'wikipediafs-0.2/src/wikipediafs/version.py', 'wikipediafs-0.2/src/mount.wikipediafs', 'wikipediafs-0.2/TODO']
>>
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Which platform did you test on? You may want to give the latest version a try as well. 

Thanks,
Raghu.