I've been using rarfile with in-memory streams and it worked great so far. However it seems this API behavior isn't consistent: Diaoul/subliminal#644
In-memory streams don't work with solid archives, I think it's up to rarfile to maintain a consistent API and save the file to a temporary location instead of failing. Of course writing on the filesystem is no easy decision so I suggest to give a configuration option to disable this behavior.
Support solid archives from in-memory file object
Those need to be written fully out to tempfile.