A TarFile object constructor accepts a fileobj argument. When this argument is set and it has a "mode" property, tarfile._mode is blindly copied from there. Otherwise, mode is set using the mode argument in the constructor.
This usually works, but in the case where fileobj is a gzip.GzipFile, the "mode" property is not a string like "rb" or "wb" but an integer, which is not expected in TarFile.
This has not been noticed before probably because the TarFile._mode property is usually unused internally, but in my case it's a problem when using it together with tarfile multivolume mode provided in issue #18321.