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It looks like if you pass a "fileobj" argument to "gettarinfo", it assumes it can use the "name" as a text string.
>>> import tarfile
>>> with tarfile.open("/dev/null", "w") as tar, open("/bin/sh", "rb") as file: tar.gettarinfo(fileobj=file)
... 
<TarInfo 'bin/sh' at 0x7f13cc937f20>
>>> with tarfile.open("/dev/null", "w") as tar, open(b"/bin/sh", "rb") as file: tar.gettarinfo(fileobj=file)
... 
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/media/disk/home/proj/python/cpython/Lib/tarfile.py", line 1767, in gettarinfo
 arcname = arcname.replace(os.sep, "/")
TypeError: expected bytes, bytearray or buffer compatible object
>>> with tarfile.open("/dev/null", "w") as tar, open(0, "rb", closefd=False) as file: tar.gettarinfo(fileobj=file)
... 
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/media/disk/home/proj/python/cpython/Lib/tarfile.py", line 1766, in gettarinfo
 drv, arcname = os.path.splitdrive(arcname)
 File "Lib/posixpath.py", line 133, in splitdrive
 return p[:0], p
TypeError: 'int' object is not subscriptable
In my case, my code always sets the final TarInfo.name attribute later on, so the initial name does not matter. Perhaps at least the documentation should say that "fileobj.name" must be a real unencoded file name string unless "arcname" is also given. My workaround was to add a dummy arcname argument, a bit like this:
# Explicit dummy name to avoid using file name of bytes
tarinfo = self.tar.gettarinfo(fileobj=file, arcname="")
# . . .
tarinfo.name = "{}/{}".format(self.pkgname, name)

Agreed, the documentation should be modified to say "(using os.fstat on its file descriptor, and its 'name' attribute if arcname is not specified").

Over in Issue 22468, I posted a documentation patch which includes wording to address this bug.

New changeset 94a94deaf06a by Martin Panter in branch '3.5':
Issues #22468, #21996, #22208: Clarify gettarinfo() and TarInfo usage
https://hg.python.org/cpython/rev/94a94deaf06a
New changeset 9d5217aaea13 by Martin Panter in branch '2.7':
Issues #22468, #21996, #22208: Clarify gettarinfo() and TarInfo usage
https://hg.python.org/cpython/rev/9d5217aaea13