tarfile.TarFile.open, if passed a fileobj argument for
a valid plain tar or tar.gz file, might fail to open
the file, and instead unexpectedly raise "ValueError:
no support for external file objects".

"open" iterates over TarFile.OPEN_METH, which is a
dictionary, to try find a method that works for a given
file.  If the internal, undefined, ordering of that
dict returns the "bz2" key before the right one for a
given file, and the fileobj argument is passed, TarFile
will attempt to call bz2open with the fileobj method,
which then raises "ValueError: no support for external
file objects", which then is not caught.

The end result is that it's possible to pass a valid
tar or tar.gz file to TarFile.open in the fileobj
argument, but get a ValueError from bz2open instead of
a TarFile object.

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I just submitted patch #1488881 that would solve your
problem as a side-effect. Thanks for your report.

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Fixed with commit of said patch.