$ cat >test.py <<EOF
import tarfile
import sys

with tarfile.open(sys.argv[1], mode="r|*") as f:
  while True:
    info = f.next()
    if not info:
      break
EOF
$ tar cf test.tar -T /dev/null
$ python2.7 test.py test.tar
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    info = f.next()
  File "/usr/lib/python2.7/tarfile.py", line 2319, in next
    self.fileobj.seek(self.offset)
  File "/usr/lib/python2.7/tarfile.py", line 555, in seek
    raise StreamError("seeking backwards is not allowed")
tarfile.StreamError: seeking backwards is not allowed
$ python3.4 test.py test.tar
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    info = f.next()
  File "/usr/lib/python3.4/tarfile.py", line 2244, in next
    self.fileobj.seek(self.offset)
  File "/usr/lib/python3.4/tarfile.py", line 518, in seek
    raise StreamError("seeking backwards is not allowed")
tarfile.StreamError: seeking backwards is not allowed

I have reconfirmed that the above still happens using a top of tree 2.7.9+ build.

2232     def next(self):
2233         """Return the next member of the archive as a TarInfo object, when
2234            TarFile is opened for reading. Return None if there is no more
2235            available.
2236         """
2237         self._check("ra")
2238         if self.firstmember is not None:
2239             m = self.firstmember
2240             self.firstmember = None
2241             return m
2242 
2243         # Read the next block.
2244         self.fileobj.seek(self.offset)
2245         tarinfo = None

raise a StreamError at #2244, It should catch this Error and return None. I would like to post a patch to fix it.

a simple fix

Reproduced on 3.11.