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Why overcomplicate things?
import io, tarfile
with tarfile.open("foo.tar", mode="w") as tar:
 b = "hello world!".encode("utf-8")
 t = tarfile.TarInfo("helloworld.txt")
 t.size = len(b) # this is crucial
 tar.addfile(t, io.BytesIO(b))
My answer to issue10369 was never supposed to be used as a reference on how to add file-like objects to a TarFile. I posted it as a simpler but equivalent version of the code of the original poster, which is why it looks "hackish".
I think the documentation on TarFile.gettarinfo() is rather clear on how to use it (i.e. that it needs a file object with a valid file descriptor). Also, I think that the code above is intuitive and simple.