Actually, looking closer at the module, perhaps you just need to set the errorlevel=1 option:

>>> with tarfile.open("truncated.tar", errorlevel=1) as tar:...     tar.extractall("test-dir")
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/home/proj/python/cpython/Lib/tarfile.py", line 1996, in extractall
    numeric_owner=numeric_owner)
  File "/home/proj/python/cpython/Lib/tarfile.py", line 2038, in extract
    numeric_owner=numeric_owner)
  File "/home/proj/python/cpython/Lib/tarfile.py", line 2108, in _extract_member
    self.makefile(tarinfo, targetpath)
  File "/home/proj/python/cpython/Lib/tarfile.py", line 2154, in makefile
    copyfileobj(source, target, tarinfo.size)
  File "/home/proj/python/cpython/Lib/tarfile.py", line 242, in copyfileobj
    raise OSError("end of file reached")
OSError: end of file reached